Transmitters.
Subelement L09
Section L09
The output tuning controls on a transmitter power amplifier with an adjustable PI network:
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allow switching to different antennas
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reduce the possibility of cross-modulation in adjunct receivers
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are involved with frequency multiplication in the previous stage
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Correct Answerallow efficient transfer of power to the antenna
Tube power amplifiers always include a matching network to match the high impedance of the tube to the antenna system impedance. As always, impedance match is all about maximum power transfer.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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The purpose of using a centre-tap return connection on the secondary of transmitting tube's filament transformer is to:
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obtain optimum power output
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Correct Answerprevent modulation of the emitted wave by the alternating current filament supply
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reduce the possibility of harmonic emissions
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keep the output voltage constant with a varying load
When the cathode is simply the filament (a "directly-heated cathode"), the voltage drop across the filament (e.g., 6.3 volts AC) is in series with the cathode DC reference voltage: as an example, while one side of the filament might be at a certain DC voltage, the other extremity is at some other value, a value influenced by the AC voltage impressed on the filament. Electron flow is affected by an AC variation, hum results.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, the input signal is applied to:
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Correct Answerthe cathode
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the plate
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the control grid
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the filament leads
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, the plate is connected to the pi-network through a:
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electrolytic capacitor
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Correct Answerblocking capacitor
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by-pass capacitor
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tuning capacitor
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, the plate is connected to a radio frequency choke. The other end of the radio frequency choke connects to the:
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ground
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B- (bias)
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Correct AnswerB+ (high voltage)
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filament voltage
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, the cathode is connected to a radio frequency choke. The other end of the radio frequency choke connects to the:
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Correct AnswerB- (bias)
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ground
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filament voltage
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B+ (high voltage)
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, the secondary winding of a transformer is connected directly to the vacuum tube. This transformer provides:
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Screen voltage
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Correct Answerfilament voltage
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B- (bias)
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B+ (high voltage)
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, what would be the approximate B+ voltage required for an output of 400 watts at 400 mA with approximately 50 percent efficiency?
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500 volts
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3000 volts
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1000 volts
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Correct Answer2000 volts
400 watts out at 50% efficiency supposes that 800 watts DC are needed. Power is voltage times current ; thus, voltage is power divided by current ; 800 watts divided by 0.4 ampere = 2000 volts.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a grounded grid amplifier using a triode vacuum tube, each side of the filament is connected to a capacitor whose other end is connected to ground. These are:
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electrolytic capacitors
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blocking capacitors
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Correct Answerby-pass capacitors
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tuning capacitors
A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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After you have opened a VHF power amplifier to make internal tuning adjustments, what should you do before you turn the amplifier on?
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Remove all amplifier shielding to ensure maximum cooling
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Correct AnswerBe certain all amplifier shielding is fastened in place
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Make sure that the power interlock switch is bypassed so you can test the amplifier
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Be certain no antenna is attached so that you will not cause any interference
Harmonics produced in an early stage of a transmitter may be reduced in a later stage by:
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greater input to the final stage
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transistors instead of tubes
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Correct Answertuned circuit coupling between stages
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larger value coupling capacitors
Key words: STAGES. Resonant circuits in the coupling between stages help convey only the operating frequency. Larger coupling capacitors would pass the harmonics more readily.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a simple 2 stage CW transmitter circuit, the oscillator stage and the class C amplifier stage are inductively coupled by a RF transformer. Another role of the RF transformer is to:
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provide the necessary feedback for oscillation
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act as part of a balanced mixer
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Correct Answerbe part of a tuned circuit
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act as part of a pi filter
A simple two-stage CW transmitter comprises an oscillator and a Class-C power amplifier. A transformer at the output of the oscillator serves the dual purpose of tuned circuit and coupling to the next stage. The DC supply to the final amplifier is bypassed to ground with a capacitor and decoupled through an RF choke so RF is kept out of the supply.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a simple 2 stage CW transmitter, current to the collector of the transistor in the class C amplifier stage flows through a radio frequency choke (RFC) and a tapped inductor. The RFC, on the tapped inductor side, is also connected to grounded capacitors. The purpose of the RFC and capacitors is to:
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Correct Answerform a low-pass filter
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provide negative feedback
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form a key-click filter
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form a RF-tuned circuit
A simple two-stage CW transmitter comprises an oscillator and a Class-C power amplifier. A transformer at the output of the oscillator serves the dual purpose of tuned circuit and coupling to the next stage. The DC supply to the final amplifier is bypassed to ground with a capacitor and decoupled through an RF choke so RF is kept out of the supply.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In a simple 2 stage CW transmitter, the transistor in the second stage would act as:
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the master oscillator
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an audio oscillator
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Correct Answera power amplifier
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a frequency multiplier
A simple two-stage CW transmitter comprises an oscillator and a Class-C power amplifier. A transformer at the output of the oscillator serves the dual purpose of tuned circuit and coupling to the next stage. The DC supply to the final amplifier is bypassed to ground with a capacitor and decoupled through an RF choke so RF is kept out of the supply.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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An advantage of keying the buffer stage in a transmitter is that:
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Correct Answerchanges in oscillator frequency are less likely
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key clicks are eliminated
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the radiated bandwidth is restricted
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high RF voltages are not present
Keying a subsequent stage provides the oscillator with a fairly constant load (isolation) and allows it to run continuously for better stability.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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As a power amplifier is tuned, what reading on its grid current meter indicates the best neutralization?
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Minimum grid current
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Maximum grid current
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A maximum change in grid current as the output circuit is changed
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Correct AnswerA minimum change in grid current as the output circuit is changed
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What does a neutralizing circuit do in an RF amplifier?
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It eliminates AC hum from the power supply
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It reduces incidental grid modulation
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It controls differential gain
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Correct AnswerIt cancels the effects of positive feedback
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the reason for neutralizing the final amplifier stage of a transmitter?
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To cut off the final amplifier during standby periods
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To keep the carrier on frequency
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Correct AnswerTo eliminate parasitic oscillations
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To limit the modulation index
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Parasitic oscillations are usually generated due to:
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a mismatch between power amplifier and transmission line
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Correct Answeraccidental resonant frequencies in the power amplifier
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harmonics from some earlier multiplier stage
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excessive drive or excitation to the power amplifier
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Parasitic oscillations would tend to occur mostly in:
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Correct AnswerRF power output stages
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high gain audio output stages
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high voltage rectifiers
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mixer stages
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Why is neutralization necessary for some vacuum-tube amplifiers?
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To reduce grid-to-cathode leakage
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To cancel AC hum from the filament transformer
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To reduce the limits of loaded Q
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Correct AnswerTo cancel oscillation caused by the effects of interelectrode capacitance
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Parasitic oscillations in an RF power amplifier may be caused by:
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excessive harmonic production
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Correct Answerlack of neutralization
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overdriven stages
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poor voltage regulation
Undesired positive feedback in an RF amplifier causes parasitic oscillations: the amplifier becomes an oscillator. Inter-electrode capacitance (e.g., plate-to-grid), coupling from output to input, stray inductance or capacitance can start up oscillations. Neutralization is the process of cancelling positive-feedback paths. To test a tube amplifier for parasitic oscillations, connect nothing to the input and output terminals, apply DC power, monitor grid and plate current while slowly varying the controls on the output tuning network; if grid current develops or plate current changes, oscillations are present.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What type of signal does a balanced modulator produce?
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Full carrier
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Single sideband, suppressed carrier
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Correct AnswerDouble sideband, suppressed carrier
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FM with balanced deviation
One method of producing SSB is a Balanced Modulator followed by a filter. The modulator takes in a fixed-frequency RF signal and mixes it with audio from the speech amplifier. The modulator is said to be balanced because the two original inputs are not present at the output: carrier suppression has taken place. Present, however, are a lower and upper sideband. A subsequent filter selects one of the sidebands to complete the creation of a single sideband suppressed-carrier signal. Note that there is no RF output when no audio is applied.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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How can a single-sideband phone signal be produced?
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By using a loop modulator followed by a mixer
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By using a reactance modulator followed by a mixer
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Correct AnswerBy using a balanced modulator followed by a filter
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By driving a product detector with a DSB signal
One method of producing SSB is a Balanced Modulator followed by a filter. The modulator takes in a fixed-frequency RF signal and mixes it with audio from the speech amplifier. The modulator is said to be balanced because the two original inputs are not present at the output: carrier suppression has taken place. Present, however, are a lower and upper sideband. A subsequent filter selects one of the sidebands to complete the creation of a single sideband suppressed-carrier signal. Note that there is no RF output when no audio is applied.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Carrier suppression in a single-sideband transmitter takes place in:
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the frequency multiplier stage
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Correct Answerthe balanced modulator stage
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the carrier decouple stage
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the mechanical filter
One method of producing SSB is a Balanced Modulator followed by a filter. The modulator takes in a fixed-frequency RF signal and mixes it with audio from the speech amplifier. The modulator is said to be balanced because the two original inputs are not present at the output: carrier suppression has taken place. Present, however, are a lower and upper sideband. A subsequent filter selects one of the sidebands to complete the creation of a single sideband suppressed-carrier signal. Note that there is no RF output when no audio is applied.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Transmission with SSB, as compared to conventional AM transmission, results in:
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a greater bandpass requirement in the receiver
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3 dB gain in the transmitter
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Correct Answer6 dB gain in the transmitter and 3 dB gain in the receiver
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6 dB gain in the receiver
Under noisy conditions, SSB can bring up to a 9 dB improvement over an AM signal of the same peak power. In AM, the Peak Envelope Power present in one of the two sidebands equals one fourth the carrier power: e.g., a 100-watt AM transmitter only packs 25 watts PEP in each sideband. SSB concentrates all the available power in one sideband alone: a 4 to 1 improvement or 6 dB. Using half the bandwidth on SSB reception, permits taking in only half of the noise at the receiver, an additional 3 dB improvement.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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The peak power output of a single-sideband transmitter, when being tested by a two-tone generator is:
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one-half of the RF peak output power of any of the tones
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one-quarter of the RF peak output power of any of the tones
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Correct Answertwice the RF power output of any of the tones
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equal to the RF peak output power of any of the tones
A two-tone test permits verifying the linearity of an SSB transmitter. The test requires a generator producing two low-distortion non-harmonically related audio sine waves of equal amplitude. The frequencies must fall within the normal transmitter audio passband: e.g., 700 and 1900 Hz. A sample of the transmitter's output is observed on an oscilloscope while the tones are fed into the microphone input. Feeding an SSB transmitter with two equal-amplitude steady audio tones produces two equal-amplitude RF signals: total power is thus twice the power present in each RF signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What kind of input signal is used to test the amplitude linearity of a single-sideband phone transmitter while viewing the output on an oscilloscope?
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An audio-frequency square wave
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Normal speech
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Correct AnswerTwo audio-frequency sine waves
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An audio-frequency sine wave
A two-tone test permits verifying the linearity of an SSB transmitter. The test requires a generator producing two low-distortion non-harmonically related audio sine waves of equal amplitude. The frequencies must fall within the normal transmitter audio passband: e.g., 700 and 1900 Hz. A sample of the transmitter's output is observed on an oscilloscope while the tones are fed into the microphone input. Feeding an SSB transmitter with two equal-amplitude steady audio tones produces two equal-amplitude RF signals: total power is thus twice the power present in each RF signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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When testing the amplitude linearity of a single-sideband transmitter what audio tones are fed into the microphone input and on what kind of kind of instrument is the output observed?
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Two harmonically related tones are fed in, and the output is observed on an oscilloscope
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Two harmonically related tones are fed in, and the output is observed on a distortion analyzer
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Two non-harmonically related tones are fed in, and the output is observed on a distortion analyzer
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Correct AnswerTwo non-harmonically related tones are fed in, and the output is observed on an oscilloscope
A two-tone test permits verifying the linearity of an SSB transmitter. The test requires a generator producing two low-distortion non-harmonically related audio sine waves of equal amplitude. The frequencies must fall within the normal transmitter audio passband: e.g., 700 and 1900 Hz. A sample of the transmitter's output is observed on an oscilloscope while the tones are fed into the microphone input. Feeding an SSB transmitter with two equal-amplitude steady audio tones produces two equal-amplitude RF signals: total power is thus twice the power present in each RF signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What audio frequencies are used in a two-tone test of the linearity of a single-sideband phone transmitter?
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Correct AnswerAny two audio tones may be used, but they must be within the transmitter audio passband, and should not be harmonically related
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20 Hz and 20 kHz tones must be used
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1200 Hz and 2400 Hz tones must be used
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Any two audio tones may be used, but they must be within the transmitter audio passband, and must be harmonically related
A two-tone test permits verifying the linearity of an SSB transmitter. The test requires a generator producing two low-distortion non-harmonically related audio sine waves of equal amplitude. The frequencies must fall within the normal transmitter audio passband: e.g., 700 and 1900 Hz. A sample of the transmitter's output is observed on an oscilloscope while the tones are fed into the microphone input. Feeding an SSB transmitter with two equal-amplitude steady audio tones produces two equal-amplitude RF signals: total power is thus twice the power present in each RF signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What measurement can be made of a single-sideband phone transmitter's amplifier by performing a two-tone test using an oscilloscope?
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Correct AnswerIts linearity
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Its frequency deviation
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Its percent of carrier phase shift
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Its percent of frequency modulation
A two-tone test permits verifying the linearity of an SSB transmitter. The test requires a generator producing two low-distortion non-harmonically related audio sine waves of equal amplitude. The frequencies must fall within the normal transmitter audio passband: e.g., 700 and 1900 Hz. A sample of the transmitter's output is observed on an oscilloscope while the tones are fed into the microphone input. Feeding an SSB transmitter with two equal-amplitude steady audio tones produces two equal-amplitude RF signals: total power is thus twice the power present in each RF signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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How much is the carrier suppressed below peak output power in a single-sideband phone transmission?
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At least 60 dB
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Correct AnswerAt least 40 dB
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No more than 20 dB
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No more than 30 dB
"Most well-designed balanced modulators can provide between 30 and 50 dB of carrier suppression. ...The filter roll-off can be used to obtain an additional 20 dB of carrier suppression." (ARRL Handbook 1985)
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is meant by "flat topping" in a single-sideband phone transmission?
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The transmitter's carrier is properly suppressed
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Correct AnswerSignal distortion caused by excessive drive
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Signal distortion caused by insufficient collector current
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The transmitter's automatic level control is properly adjusted
Flattening of the peaks is an extreme form of distortion where the output of the transmitter is incapable of reproducing the original pattern of the audio input on voice peaks. This is generally caused by excessive audio input to the transmitter: too much audio causes the amplifier stage to exceed its linear operation range. The purpose of the ALC (Automatic Level Control) is to prevent overdrive.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Maintaining the peak RF output of a SSB transmitter at a relatively constant level requires a circuit called the:
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Correct Answerautomatic level control (ALC)
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automatic gain control (AGC)
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automatic output control (AOC)
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automatic volume control (AVC)
Automatic Level Control (ALC) serves to prevent overdriving an amplifier. The ALC circuit samples the envelope (peak) of the RF output to develop a DC control voltage used to control the gain of an earlier stage. AGC and AVC are receiver circuits.
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Speech compression associated with SSB transmission implies:
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a lower signal-to-noise ratio
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circuit level instability
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Correct Answerfull amplification of low level signals and reducing or eliminating amplification of high level signals
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full amplification of high level signals and reducing or eliminating signals amplification of low level
Audio compression maintains a high voice level despite variations in the voice signal incoming from a microphone. To produce a high average output without exceeding a certain peak value, low level signals need to be amplified while high level signals are passed along with little or no gain. [ compression is the automatic reduction of gain as the signal level increases beyond a pre-set level known as the threshold. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Which principle is not associated with analog signal processing?
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Clipping
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Correct AnswerFrequency division
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Compression
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Bandwidth limiting
Key words: NOT ASSOCIATED WITH ANALOG. Compression, bandwidth limiting and clipping can all be performed as analog processes. Frequency division requires a numerical computation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Which of the following is not a method used for peak limiting, in a signal processor?
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AF clipping
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Correct AnswerFrequency clipping
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RF clipping
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Compression
The expression "peak limiting" entails limiting the amplitude. Compression, AF clipping and RF clipping are valid operations. There is no such thing as frequency clipping.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the undesirable result of AF clipping in a speech processor?
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Reduction in peak amplitude
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Correct AnswerIncreased harmonic distortion
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Reduced average power
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Increased average power
Audio frequency clipping abruptly stops voltage excursions at a certain level. This gives the audio a square wave appearance; square waves are rich in harmonics. AF clippers need to be followed by a low-pass filter to prevent harmonics from entering modulation stages. You may also eliminate the bad answers: "reduction in peak amplitude" is the object of clipping, "increased average power" is a result of clipping (softer passages are no longer dwarfed by the peaks), "increased average power" simply contradicts the previous answer.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Which description is not correct? You are planning to build a speech processor for your transceiver. Compared to AF clipping, RF clipping:
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is more expensive to implement
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is more difficult to implement
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Correct Answeris easier to implement
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has less distortion
Working at radio-frequencies is evidently more difficult and thus more expensive than dealing with audio frequencies. RF clipping is generally presumed to induce less distortion because any harmonics generated through clipping automatically end up outside the passband of subsequent filters. At audio frequencies, harmonics of the lower speech frequencies fall within the audio passband and can muddle the audio signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Automatic Level Control (ALC) is another name for:
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AF clipping
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Correct AnswerRF compression
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AF compression
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RF clipping
Clipping places a hard limit on voltage swings. Compression is a reduction in gain when signal exceed a certain threshold. The ALC circuit samples the envelope (peak) of the RF output and produces a DC control voltage used to control the gain of an earlier stage when the output reaches a certain level.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In an FM phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency, what is the modulation index, when the modulating frequency is 1000 Hz?
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0.3
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3000
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1000
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Correct Answer3
Deviation = the amount of frequency shift, at a given instant, from the centre carrier frequency (e.g., plus or minus 5 kHz). Modulation Index = the ratio of deviation to modulating frequency for a particular audio frequency (both being expressed in the same units): e.g., 3 kHz deviation with 1 kHz audio represents a Modulation Index of 3. Deviation Ratio = the ratio of maximum deviation to the maximum modulating frequency: e.g. maximum deviation of 5 kHz with a highest modulating frequency of 3 kHz is a Deviation Ratio of 1.66 .
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the modulation index of an FM phone transmitter producing an instantaneous carrier deviation of 6 kHz when modulated with a 2 kHz modulating frequency?
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Correct Answer3
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0.333
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2000
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6000
Deviation = the amount of frequency shift, at a given instant, from the centre carrier frequency (e.g., plus or minus 5 kHz). Modulation Index = the ratio of deviation to modulating frequency for a particular audio frequency (both being expressed in the same units): e.g., 3 kHz deviation with 1 kHz audio represents a Modulation Index of 3. Deviation Ratio = the ratio of maximum deviation to the maximum modulating frequency: e.g. maximum deviation of 5 kHz with a highest modulating frequency of 3 kHz is a Deviation Ratio of 1.66 .
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the deviation ratio of an FM phone transmitter having a maximum frequency swing of plus or minus 5 kHz and accepting a maximum modulation rate of 3 kHz?
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Correct Answer1.66
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60
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0.16
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0.6
Deviation = the amount of frequency shift, at a given instant, from the centre carrier frequency (e.g., plus or minus 5 kHz). Modulation Index = the ratio of deviation to modulating frequency for a particular audio frequency (both being expressed in the same units): e.g., 3 kHz deviation with 1 kHz audio represents a Modulation Index of 3. Deviation Ratio = the ratio of maximum deviation to the maximum modulating frequency: e.g. maximum deviation of 5 kHz with a highest modulating frequency of 3 kHz is a Deviation Ratio of 1.66 .
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the deviation ratio of an FM phone transmitter having a maximum frequency swing of plus or minus 7.5 kHz and accepting a maximum modulation rate of 3.5 kHz?
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0.47
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47
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0.214
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Correct Answer2.14
Deviation = the amount of frequency shift, at a given instant, from the centre carrier frequency (e.g., plus or minus 5 kHz). Modulation Index = the ratio of deviation to modulating frequency for a particular audio frequency (both being expressed in the same units): e.g., 3 kHz deviation with 1 kHz audio represents a Modulation Index of 3. Deviation Ratio = the ratio of maximum deviation to the maximum modulating frequency: e.g. maximum deviation of 5 kHz with a highest modulating frequency of 3 kHz is a Deviation Ratio of 1.66 .
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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When the transmitter is not modulated, or the amplitude of the modulating signal is zero, the frequency of the carrier is called its:
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modulating frequency
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Correct Answercentre frequency
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frequency deviation
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frequency shift
Centre Frequency is the transmitter output frequency in the absence of modulation. Frequency deviation and frequency shift both are synonyms for the offset in carrier frequency caused by modulation at a given instant. Modulating frequency relates to the audio frequency used for modulation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In an FM transmitter system, the amount of deviation from the centre frequency is determined solely by the:
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amplitude and the frequency of the modulating frequency
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modulating frequency and the amplitude of the centre frequency
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Correct Answeramplitude of the modulating frequency
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frequency of the modulating frequency
In Frequency Modulation, the amplitude of the modulation is translated into the importance of the deviation, the modulation frequency is reflected in the rhythm of the deviation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Any FM wave with single-tone modulation has:
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Correct Answeran infinite number of sideband frequencies
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two sideband frequencies
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four sideband frequencies
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one sideband frequency
Unlike AM where a single modulating frequency creates only a pair of side frequencies (one on each side of the carrier), FM creates an infinite number of side frequency pairs; the Modulation Index influences the amplitude of the side frequencies through a mathematical relation known as a Bessel Function. The number of side frequencies with significant amplitude determines the required bandwidth. For certain Modulation Index values, there is zero energy at the centre frequency; the energy is then totally found in the side frequencies.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Some types of deviation meters work on the principle of:
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Correct Answera carrier null and multiplying the modulation frequency by the modulation index
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detecting the frequencies in the sidebands
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the amplitude of power in the sidebands
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a carrier peak and dividing by the modulation index
Certain Modulation Index values cause nulls at the centre carrier frequency: e.g., the Bessel function returns zero for the carrier component at Modulation Indices of 2.4048, 5.5201 or 8.6537 . Detecting a carrier null permits determining deviation as Modulation Index times modulating frequency. For example, with a tone of 905 hertz and deviation set at 4996 hertz (nearly 5 kHz), a null in the carrier will be observed because 4996 Hz deviation for a tone of 905 Hz is a Modulation Index of 5.52 . An all-mode receiver with a sharp filter permits observing the carrier component. The procedure could be used to set a transmitter or calibrate a home-made deviation meter.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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When using some deviation meters, it is important to know:
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pass-band of the IF filter
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Correct Answermodulating frequency and the modulation index
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modulation index
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modulating frequency
Certain Modulation Index values cause nulls at the centre carrier frequency: e.g., the Bessel function returns zero for the carrier component at Modulation Indices of 2.4048, 5.5201 or 8.6537 . Detecting a carrier null permits determining deviation as Modulation Index times modulating frequency. For example, with a tone of 905 hertz and deviation set at 4996 hertz (nearly 5 kHz), a null in the carrier will be observed because 4996 Hz deviation for a tone of 905 Hz is a Modulation Index of 5.52 . An all-mode receiver with a sharp filter permits observing the carrier component. The procedure could be used to set a transmitter or calibrate a home-made deviation meter.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the significant bandwidth of an FM-phone transmission having a +/- 5-kHz deviation and a 3-kHz modulating frequency?
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8 kHz
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5 kHz
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3 kHz
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Correct Answer16 kHz
Carson's Rule permits estimating the bandwidth of an FM signal: bandwidth equals twice the sum of deviation + modulating frequency, in this example, 5 + 3 = 8, twice 8 = 16. [ Mathematician and engineer John R. Carson (1887-1940) had predicted the approximate bandwidth of an FM signal circa 1922. ]
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a +/- 5-kHz deviation, 146.52-MHz FM-phone transmitter?
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+/- 12 kHz
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+/- 5 kHz
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+/- 41.67 Hz
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Correct Answer+/- 416.7 Hz
In this example, the frequency multiplication ratio between oscillator and output is 12 ( 146.52 divided by 12.21 = 12 ). Hence, the oscillator needs only be shifted by 416.7 Hz, i.e., 5000 Hz divided by 12.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What type of circuit varies the tuning of an amplifier tank circuit to produce FM signals?
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Correct AnswerA phase modulator
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A balanced modulator
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A double balanced mixer
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An audio modulator
Two methods exist to produce Frequency Modulation. The Direct Method supposes forcing deviation directly on the oscillator; deviation is then multiplied along with the oscillator frequency up to the operating frequency. Phase Modulation is an indirect method whereby the phase of the signal is affected (i.e., retarding/advancing) in step with the modulation by varying a reactance on a stage other than the oscillator.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What audio shaping network is added at an FM transmitter to attenuate the lower audio frequencies?
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Correct AnswerA pre-emphasis network
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An audio prescaler
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A heterodyne suppressor
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A de-emphasis network
With direct FM, deviation is independent of modulating frequency, actual deviation is determined solely by the modulating amplitude. With Phase Modulation, deviation depends on the amount of phase shift and its rapidity, increasing modulating frequency results in proportionally more deviation even if amplitude is held constant. Because commercial standards were based on Phase Modulation, an FM transmitter requires an artificial boost in high frequency response so that PM and FM sound the same at the receiver. A pre-emphasis network tailors the frequency response in the FM transmitter. De-emphasis is employed in the receiver to restore a flat audio response.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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The characteristic difference between a phase modulator and a frequency modulator is:
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the centre frequency
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de-emphasis
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frequency inversion
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Correct Answerpre-emphasis
With direct FM, deviation is independent of modulating frequency, actual deviation is determined solely by the modulating amplitude. With Phase Modulation, deviation depends on the amount of phase shift and its rapidity, increasing modulating frequency results in proportionally more deviation even if amplitude is held constant. Because commercial standards were based on Phase Modulation, an FM transmitter requires an artificial boost in high frequency response so that PM and FM sound the same at the receiver. A pre-emphasis network tailors the frequency response in the FM transmitter. De-emphasis is employed in the receiver to restore a flat audio response.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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In most modern FM transmitters, to produce a better sound, a compressor and a clipper are placed:
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Correct Answerbetween the audio amplifier and the modulator
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between the multiplier and the PA
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between the modulator and the oscillator
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in the microphone circuit, before the audio amplifier
In this context, compression and clipping are AUDIO processes aimed at maintaining high average deviation without exceeding a given peak value. Two answers can be readily scratched as they pertain to radiofrequency (RF) paths. The microphone circuit is not suitable as the audio level at that point is too low for a simple clipper.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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Three important parameters to be verified in an FM transmitter are:
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distortion, bandwidth and sideband power
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modulation, pre-emphasis and carrier suppression
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frequency stability, de-emphasis and linearity
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Correct Answerpower, frequency deviation and frequency stability
Stability is paramount in all transmitters, frequency deviation ultimately determines bandwidth while linearity (absence of distortion) minimizes out-of-channel emissions. Carrier Suppression is a concern with SSB, pre-emphasis (FM transmitter) and de-emphasis (FM receiver) are simple resistor-capacitor networks.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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