Frequency Modulation Questions and Answers

Q: The Armstrong method generates FM indirectly by

Armstrong's method phase modulates a crystal-controlled carrier at low level, then multiplies the frequency to raise the small deviation to the value needed. It gives excellent stability because the crystal is never pulled. Varactor pulling is the direct method it replaces. Amplitude modulation followed by limiting produces a constant-amplitude signal with no frequency deviation. And mixing translates frequency without modulating it.

Q: Frequency multiplication in an Armstrong transmitter multiplies

Multiplication scales the carrier and the deviation by the same factor, while the message frequency is untouched, so the modulation index is multiplied too. That is exactly why the method works: a phase modulator can only produce a small index, and multiplication raises it to the value FM broadcasting needs. Multiplying only one of the two would be physically impossible. And the message frequency is a property of the audio, which no radio-frequency operation can alter.

Q: An integrator placed before a phase modulator produces

Because frequency is the derivative of phase, integrating the message before a phase modulator yields frequency modulation, and differentiating before a frequency modulator yields phase modulation. That equivalence is why the two are treated as one family. Amplitude modulation requires a quite different modulator. Uniformly wider phase deviation would need a gain change rather than an integrator. And an integrator attenuates the high frequencies, so it is the opposite of pre-emphasis.

Q: Automatic frequency control in an FM transmitter compares the oscillator with

The loop mixes the modulated oscillator down against a crystal reference and feeds the error back to hold the centre frequency, with a time constant long enough not to cancel the wanted modulation. That last detail is the subtle part of the design. Deviation is set by the modulator's sensitivity. Power control is a separate loop. And no transmitter has access to a distant receiver's oscillator. The loop lets a drifting oscillator behave like a crystal-controlled one.

Q: The Foster-Seeley discriminator responds to

It uses the phase relationship that develops between primary and secondary of a tuned transformer as the frequency departs from resonance, converting that into a differential output. It is linear over a useful range but responds to amplitude as well, so it needs a limiter ahead of it. Amplitude alone carries no FM information. Envelope rate of change is not what it senses. And two rectified slopes describes the balanced slope detector, a distinct and cruder arrangement.

Q: A phase-locked loop demodulates FM by

The loop forces its oscillator to follow the input frequency, so the control voltage is a faithful copy of the message. It is highly linear, needs no critical tuned circuits and is easily integrated, which is why it dominates modern receivers. Rectification recovers an envelope, not a frequency. Multiplying by a fixed carrier is a coherent AM detector. And zero-crossing counting is a different technique, workable but noisier at low signal levels.

Q: Which FM detector is most easily integrated onto a single chip?

The phase-locked loop needs no tuned transformers and few external components, so it integrates readily and holds its alignment without adjustment. All three alternatives depend on carefully tuned inductive components that resist integration and drift with age and temperature. That practical difference, rather than any theoretical advantage, is what displaced the discriminator family from consumer equipment.

Q: A quadrature detector recovers FM by

The signal is multiplied by a copy of itself passed through a network giving ninety degrees at the centre frequency, so the product varies with frequency deviation. It uses one tuned circuit and integrates well, which made it common in television sound stages. Rectifying sidebands is not a detection method. A crystal comparison would give frequency error rather than the message. And the envelope of a limited signal is constant by construction.

41.

The Armstrong method generates FM indirectly by

A.Mixing two oscillators with a fixed difference
B.Phase modulating at low level and then multiplying
C.Pulling a crystal oscillator with a varactor
D.Amplitude modulating and then limiting

Answer: B

Armstrong's method phase modulates a crystal-controlled carrier at low level, then multiplies the frequency to raise the small deviation to the value needed. It gives excellent stability because the crystal is never pulled. Varactor pulling is the direct method it replaces. Amplitude modulation followed by limiting produces a constant-amplitude signal with no frequency deviation. And mixing translates frequency without modulating it.

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42.

Frequency multiplication in an Armstrong transmitter multiplies

A.The deviation only
B.The message frequency along with the deviation
C.Both the carrier frequency and the deviation
D.The carrier frequency only

Answer: C

Multiplication scales the carrier and the deviation by the same factor, while the message frequency is untouched, so the modulation index is multiplied too. That is exactly why the method works: a phase modulator can only produce a small index, and multiplication raises it to the value FM broadcasting needs. Multiplying only one of the two would be physically impossible. And the message frequency is a property of the audio, which no radio-frequency operation can alter.

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43.

An integrator placed before a phase modulator produces

A.Amplitude modulation
B.A wider phase deviation at all frequencies
C.Pre-emphasis of the high audio frequencies
D.Frequency modulation

Answer: D

Because frequency is the derivative of phase, integrating the message before a phase modulator yields frequency modulation, and differentiating before a frequency modulator yields phase modulation. That equivalence is why the two are treated as one family. Amplitude modulation requires a quite different modulator. Uniformly wider phase deviation would need a gain change rather than an integrator. And an integrator attenuates the high frequencies, so it is the opposite of pre-emphasis.

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44.

Automatic frequency control in an FM transmitter compares the oscillator with

A.A stable reference and corrects any drift
B.The message amplitude to set the deviation
C.The antenna current to hold the power constant
D.The receiver's local oscillator over the air

Answer: A

The loop mixes the modulated oscillator down against a crystal reference and feeds the error back to hold the centre frequency, with a time constant long enough not to cancel the wanted modulation. That last detail is the subtle part of the design. Deviation is set by the modulator's sensitivity. Power control is a separate loop. And no transmitter has access to a distant receiver's oscillator. The loop lets a drifting oscillator behave like a crystal-controlled one.

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45.

A slope detector demodulates FM by

A.Locking a voltage-controlled oscillator to the input
B.Using the sloping side of a tuned circuit response
C.Comparing the phase of two tuned circuits
D.Counting zero crossings over a fixed interval

Answer: B

Tuning a resonant circuit off centre puts the signal on the slope of its response, so frequency deviation becomes amplitude variation that an ordinary envelope detector can recover. It is the simplest FM detector and the least linear. Phase comparison between two circuits is the Foster-Seeley discriminator. Zero-crossing counting is a digital-style detector. And locking an oscillator is the phase-locked loop method, which is the most linear of the four.

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46.

The Foster-Seeley discriminator responds to

A.The rate of change of the envelope
B.The difference between two rectified slopes
C.The phase difference between two tuned circuits
D.The amplitude of the input signal only

Answer: C

It uses the phase relationship that develops between primary and secondary of a tuned transformer as the frequency departs from resonance, converting that into a differential output. It is linear over a useful range but responds to amplitude as well, so it needs a limiter ahead of it. Amplitude alone carries no FM information. Envelope rate of change is not what it senses. And two rectified slopes describes the balanced slope detector, a distinct and cruder arrangement.

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47.

A ratio detector differs from a Foster-Seeley discriminator in that it

A.Requires no tuned circuits
B.Responds to phase rather than frequency
C.Needs no limiter because FM has constant amplitude
D.Provides some amplitude limiting itself

Answer: D

The ratio detector's diode arrangement makes its output depend on the ratio of two voltages rather than their difference, so amplitude variation largely cancels and a separate limiter may be unnecessary. It still uses tuned circuits. Both detectors work through phase to sense frequency. And the last option misstates the reason: FM has constant amplitude at the transmitter, but noise and fading make it vary by the time it reaches the receiver, which is exactly why limiting matters.

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48.

A phase-locked loop demodulates FM by

A.Tracking the input with a voltage-controlled oscillator
B.Rectifying the input and smoothing it
C.Multiplying the input by a fixed carrier
D.Counting the zero crossings of the limited signal

Answer: A

The loop forces its oscillator to follow the input frequency, so the control voltage is a faithful copy of the message. It is highly linear, needs no critical tuned circuits and is easily integrated, which is why it dominates modern receivers. Rectification recovers an envelope, not a frequency. Multiplying by a fixed carrier is a coherent AM detector. And zero-crossing counting is a different technique, workable but noisier at low signal levels.

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49.

Which FM detector is most easily integrated onto a single chip?

A.The balanced slope detector
B.The phase-locked loop
C.The Foster-Seeley discriminator
D.The ratio detector

Answer: B

The phase-locked loop needs no tuned transformers and few external components, so it integrates readily and holds its alignment without adjustment. All three alternatives depend on carefully tuned inductive components that resist integration and drift with age and temperature. That practical difference, rather than any theoretical advantage, is what displaced the discriminator family from consumer equipment.

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50.

A quadrature detector recovers FM by

A.Comparing the input against a crystal reference
B.Differentiating the envelope of the limited signal
C.Multiplying the signal by a version shifted ninety degrees
D.Rectifying two sidebands separately

Answer: C

The signal is multiplied by a copy of itself passed through a network giving ninety degrees at the centre frequency, so the product varies with frequency deviation. It uses one tuned circuit and integrates well, which made it common in television sound stages. Rectifying sidebands is not a detection method. A crystal comparison would give frequency error rather than the message. And the envelope of a limited signal is constant by construction.

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