Hint: A FILTER - PROCESSES

Modulation is the process that combines the audio signal with the radio frequency. In AM (amplitude modulation) this encodes all of the information about the audio signal in the sidebands, at which point the carrier frequency serves no further purpose.

A balanced modulator first modulates and then also suppresses the now superfluous carrier signal.

The output of the balanced modulator gets passed to the filter, which removes one of the sidebands. That single sideband is then forwarded on to the mixer which will produce a signal with the desired frequency.

For more info see Wikipedia: Single-sideband modulation - 3.1 Bandpass filtering

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Hint: A BALANCED MODULATOR is used to combine signals from the CARRIER OSCILLATOR and SPEECH AMPLIFIER and then send the result to the filter in a typical single sideband phone transmitter.

The balanced modulator "removes" the signal at the carrier frequency and leaves the two side-band signals to be sent on for filtering.

For more info see Wikipedia: Single-sideband modulation

Warning - The COMBINE hint may be misleading as both G7C02 and G7C04 use the word 'combine' and have differing answers:

Carrier Oscillator + speech amp -> balanced mod. -> filter

BFO + IF amp -> product detector -> AF amp

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The mixer is the circuit used in a superheterodyne receiver to process signals from the RF amplifier and local oscillator and send the result to the IF filter.

The mixer combines the two input frequencies, f1 and f2 and creates a new common intermediate frequency which is the new carrier wave.

Also produced are two new sideband signals which are the input frequency sum (f1 + f2) which becomes the upper sideband and the input frequency difference (f1 - f2) which becomes the lower sideband frequency. The signal is then sent to the filter for band selection.

For more info see Wikipedia: Single-sideband modulation, Frequency Mixer

Hint: Remember that an Oscillator “Mixes” things

Mixer Use Memory Table

All these are types of mixers.

• supermixer = superheterodyne receiver + mixer
• transmit balanced SSB = balanced modulator generates SSB
• SSB produces voice = product detector demodulates SSB
  [KQ4AEY]

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Hint: Think BFO = BYPRODUCT

The product detector is the circuit used to combine the signals from the intermediate frequency (IF) amplifier and beat frequency oscillator (BFO). The product detector is a type of frequency mixer that acts as a demodulator for SSB signals. The result is then sent to the audio frequency (AF) amplifier in the single-sideband receiver.

Silly tip: BFO sounds like UFO and we all want to "Detect(or)" those.

For more info see Wikipedia: Product detector

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Variable frequency with the stability of a crystal oscillator is an advantage of a transceiver controlled by a direct digital synthesizer (DDS). The DDS can be used over a wide range of frequencies and can be precisely controlled to create finely stepped sine wave forms while producing very little phase noise.

For more info see Wikipedia: Direct digital synthesizer

Silly Hint: A synthesizer in a music instrument creates variable frequencies

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(B). The impedance of a low-pass filter as compared to the impedance of the transmission line into which it is inserted should be about the same.

Just like any filter, a low-pass filter needs to appear to pass all signals with frequencies of the intended passband, while rejecting signals of all other frequencies. To accomplish this, it needs to match the transmission line for the passband frequencies and exhibit high impedance for all others. This way, at passband frequencies, the filter will appear to be part of the transmission line, with close to the same impedance.

Silly Hint: A quarterback usually throws a low-pass just so not to get sacked and lose yards; maintain status quo (keep it the same)

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You might not need an RF amplifier, because the signal could be strong enough. That eliminates two of the distractors.

You also might not need an audio amplifier at the end of the process (the sound might be loud enough already). That eliminates the third distractor.

Per wikipedia, a superheterodyne receiver is:

a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency. —wikipedia

The oscillator is used to generate the frequency that will be combined with the signal.

The mixer actually combines the signal and the generated frequency in order to create the intermediate frequencies.

The detector takes the output of the mixer and demodulates it.

Silly HINT: The correct answer is the only choice that does not include an amplifier.

For more info see Wikipedia: Superheterodyne receiver

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Hint: to CONVERT is to DISCRIMINATE

The frequency discriminator will demodulate the audio signal by converting signal frequency variations to amplitude variations.

Note that product detector is a tempting answer, but this is used in AM, not FM, receivers.

The mixer comes before the Intermediate Frequency (IF) amplifier, not after it.

A phase inverter makes no sense.

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(D). A Digital Signal Processor (DSP) IF filter uses all of the components listed for processing the signal, so all of these choices are correct. The DSP converts the analog signal to a digital format (analog to digital converter), then uses software for producing a modulated, filtered, clear signal (using a digital processor chip). The last step is to then convert the digital signal back to analog (digital to analog converter.)

For more info see Wikipedia: Digital Signal Processing (DSP)

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(B). Digital signal processor filtering is accomplished by converting the signal from analog to digital and using digital processing. Once converted to digital format the DSP can remove unwanted noise, convert, or decode the signal in any number of ways. The signal is then returned from digital to analog form.

For more info see Wikipedia: Digital Signal Processor

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Hint. A software defined radio (SDR) does MOST FUNCTIONS by software.

(A). A software defined radio (SDR) is a radio in which most major signal processing functions are performed by software. Software handles the functions of mixing, modulating/demodulating, amplifying the signals rather than larger analog systems. Also notice that two of the other answers/distractors don't even have the word software in them.

For more info see Wikipedia: software-defined radio (SDR)

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