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E7E

PRACTICAL CIRCUITS

Modulation and demodulation: reactance, phase, and balanced modulators; detectors; mixers

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E7E011 of 11

Which of the following can be used to generate FM phone signals?

Why FM is produced by varying the frequency of an oscillator in step with the audio. A reactance modulator places a voltage-variable reactance (a varactor or a transistor stage made to look like a capacitor or inductor) across the oscillator's tank circuit, so the audio voltage pulls the resonant frequency up and down. The deviation follows the audio amplitude and the rate follows the audio frequency, which is exactly frequency modulation. The low-level oscillator signal is then multiplied and amplified to the operating frequency.
Watch out A balanced modulator suppresses the carrier and outputs double sideband, the first step toward SSB, not FM. Modulating the final amplifier is how plate or collector amplitude modulation (AM) is done, not frequency modulation.
FM starts at the oscillator: reactance modulator changes frequency. Balanced modulator = SSB. Modulated final = AM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E022 of 11

What is the function of a reactance modulator?

Why A reactance modulator presents a variable reactance, typically an effective capacitance (often a varactor diode whose junction capacitance changes with applied audio voltage), across an oscillator's tank circuit. Since the oscillator frequency depends on L and C, varying that capacitance at an audio rate swings the carrier frequency or phase, producing FM or PM. Angle modulation comes from changing the resonant elements, not the signal amplitude.
Watch out The choices built on varying a resistance are wrong because resistance is not reactance; loading a tank with changing resistance alters amplitude and Q rather than shifting the resonant frequency. AM is produced by varying gain or supply level in an amplifier stage, not by a reactance modulator.
Reactance means L or C. Change C, change frequency: reactance modulator gives FM/PM, never AM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E033 of 11

What is a frequency discriminator?

Why A frequency discriminator is the classic FM demodulator: a tuned phase-shift network (as in the Foster-Seeley discriminator or ratio detector) converts instantaneous frequency deviation into a proportional amplitude, which is then rectified to recover the audio. Its S-shaped response curve gives zero output at center frequency and positive or negative output as the carrier swings above or below it. Modern receivers often use a quadrature detector or PLL for the same job, but the discriminator is the named textbook circuit.
Watch out The choice calling it an FM generator has it backwards: creating FM is the job of a reactance modulator or a directly modulated VCO, while the discriminator is on the receive side.
Discriminator DEtects, it tells frequencies apart on the receive end; reactance modulator makes FM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E044 of 11

What is one way to produce a single-sideband phone signal?

Why A balanced modulator multiplies the audio and carrier and cancels the carrier itself, leaving double sideband suppressed carrier (DSB-SC). A sharp crystal or mechanical filter then removes one of the two sidebands, leaving single sideband. This is the classic filter method of SSB generation; the other standard approach is the phasing method, which uses two balanced modulators fed with signals 90 degrees apart so one sideband cancels.
Watch out A reactance modulator varies the frequency or phase of an oscillator, so it produces FM or PM, not SSB. A product detector is a receiving stage that recovers audio from an SSB or DSB signal, not a generator.
Balanced modulator kills the carrier, filter kills a sideband. Reactance modulator means FM, not SSB.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E055 of 11

What is added to an FM speech channel to boost the higher audio frequencies?

Why FM detectors produce noise that rises with audio frequency (the triangular noise spectrum), so the high end of the audio band would come out noisiest. To fix this, the transmitter runs the speech through a pre-emphasis network, a simple high-pass shaping circuit (about 6 dB per octave, typically a 75 microsecond time constant) that lifts the higher audio frequencies before modulation. The receiver then applies the mirror-image de-emphasis, restoring flat response while pushing the high-frequency noise back down.
Watch out A de-emphasis network is the matching circuit at the receiving end, cutting the highs back down; it attenuates rather than boosts. The other two choices are not real circuit names.
PRE-emphasis comes first (transmit, boost); DE-emphasis comes after (receive, cut).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E066 of 11

Why is de-emphasis used in FM communications receivers?

Why In a phase modulator the frequency deviation produced rises in proportion to the modulating audio frequency, so a PM transmitter automatically emphasizes the highs by about 6 dB per octave (the same curve as the 75 microsecond pre-emphasis network used ahead of true FM modulators). A receiver detector recovers that boosted audio, so a de-emphasis network with the complementary falling response is needed to restore a flat, natural-sounding baseband. That is what lets one receiver design work with both frequency-modulated and phase-modulated transmitters.
Watch out Rolling off the highs does cut high-frequency hiss and some impulse noise energy, which makes the noise-reduction answer tempting, but that is a side benefit; the design reason the network exists is to undo the transmitter's pre-emphasis so PM and FM sound alike.
Pre-emphasis at the transmitter (or built into PM), de-emphasis at the receiver: the two curves must cancel.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E077 of 11

What is meant by the term "baseband" in radio communications?

Why Baseband refers to the original information signal before it is shifted up to a radio frequency, for example the roughly 300 Hz to 3000 Hz audio of a voice channel, or the video or data stream feeding a modulator. Modulation translates that baseband spectrum onto a carrier, and the detector in the receiver translates it back down to baseband. So the term describes the message signal's own frequency range, not anything about the RF carrier.
Watch out The choice about the lowest band the radio covers plays off the word "base" meaning lowest, but baseband is about where the modulating information lives, not about the tuning range of the equipment.
Baseband = the message at its birth frequency, before the carrier carries it anywhere.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E088 of 11

What are the principal frequencies that appear at the output of a mixer?

Why A mixer is a nonlinear (multiplying) stage, and multiplying two sine waves at f1 and f2 produces new products at f1+f2 and f1-f2. Some of each original input also leaks through to the output, so the principal components you see are f1, f2, f1+f2 and f1-f2. A filter after the mixer then selects the one product you want, which is why a 14.2 MHz signal mixed with a 5.745 MHz LO can be converted to a 8.455 MHz IF (the difference) or 19.945 MHz (the sum).
Watch out Doubling and quadrupling the input frequency is what a frequency multiplier or a harmonic generator does, not a mixer; the 1.414 and 0.707 numbers come from peak/RMS voltage relationships and have nothing to do with mixing products.
Mixers add and subtract: sum and difference, plus the two originals leaking through.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E099 of 11

What occurs when the input signal levels to a mixer are too high?

Why A mixer is a nonlinear device that ideally produces just the sum and difference of the two input frequencies. When drive levels exceed the mixer's linear operating range, higher-order terms in its transfer characteristic come into play, so harmonics of the inputs mix as well and produce many additional unwanted output frequencies. These spurious products (often written as mF1 +/- nF2) land in the passband as birdies, intermod and images, degrading receiver dynamic range.
Watch out Terms like mixer blanking and automatic limiting are not mixer behaviors at all; limiting belongs to FM IF stages, and AGC voltage responds to detected signal level in the IF chain, not to mixer overdrive.
Overdrive a mixer and it stops being a two-tone multiplier: too much level means more products, not fewer.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E1010 of 11

How does a diode envelope detector function?

Why An envelope detector recovers AM audio by half-wave rectifying the modulated RF with a diode, then feeding the pulses to an RC low-pass filter. The capacitor charges to the peak of each RF cycle and discharges slowly through the resistor, so the output voltage follows the amplitude envelope of the carrier while the RF itself is filtered away. The RC time constant must be long compared with one RF cycle but short compared with the highest audio frequency.
Watch out Zener breakdown describes voltage regulation, not detection, and the diode in an envelope detector is forward biased on signal peaks, not operated in reverse breakdown. Sensing reactance versus frequency describes a varactor, used in FM modulators rather than AM detection.
Envelope detector = rectify then smooth: diode chops the RF, RC follows the audio outline.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E7E1111 of 11

Which type of detector is used for demodulating SSB signals?

Why SSB has no carrier, so the receiver must supply one. A product detector is essentially a mixer: it multiplies the incoming SSB signal by a locally generated carrier (the BFO) at the suppressed carrier frequency, and the difference product falls in the audio range. This same circuit demodulates CW as well, which is why SSB/CW receivers share a product detector and BFO.
Watch out A discriminator is an FM detector, converting frequency deviation into audio; phase detectors and phase comparators are PLL building blocks that output a voltage proportional to phase difference, not recovered SSB audio.
Product detector = product (multiplication) of signal and BFO. Discriminator is for FM, product is for SSB/CW.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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