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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Silly Hint: Average IQ (of a person) is 90

Another silly hint: In the pool of questions, this is G7C 09; the answer is 90, so there's an easy relation in the question.

I & Q signals are used as components of a quadrature signal. By definition, they are separated by 90 degrees. You just need to memorize this. It may help to remember they are QUADrature, think of dividing a signal into 4 parts: 360 degrees divided by 4 is 90 degrees.

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I and Q signals are the In-phase and Quadrature components of a signal. They provide enough data for software to be able to decode many different types of modulation.

It is debatable whether "all types" of modulation can be created, but certainly a great many different types can be. That is the power of a Software Defined Radio!

Addressing the distractors:

• The need for high resolution analog-to-digital converters is eliminated - No, actually the higher the resolution on the A2D converters the more you can do with the SDR as those are what allow converting the provided signals (analog) into a digital form!

• Minimum detectable signal level is reduced - No, there isn't any real impact on that, though it's conceivable that you may be able to make better use of what you have with good enough software.

• Converting the signal from digital to analog creates mixing products - No, there is no reason that D2A converters should create mixing products.

Memory hints:

• A person with high IQ can create ALL kinds of things

• Software requires processing

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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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A low pass filter passes the low frequencies and rejects the high frequencies. "Cutoff" indicates the point at which frequencies are rejected.

Note that "roll off" describes the shape of the filter's curve between the pass band and stop band. It isn't a specific frequency, it is more likely to be described as a rate of change like dB/decade.

Hint: A quarterback also throws a “Lowpass” to “cut-off” any sacks

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Notch depth describes the ability to accept a signal in the center of a band pass filter.

Rolloff describes the steepness of the edge of a filter between the pass band and stop band.

Insertion loss describes the loss across the band by the insertion of the filter into the circuit.

Ultimate rejection describes the ability to reject signals outside the pass band.

Mnemonics:

• A maximum reject is an ultimate reject.
• The question and answer both have reject.
• Ultimate means "as far as possible", the filter rejects signals far from its "passband".

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Hint: a bandwidth has UPPER and LOWER limits

"Upper and lower half-power" is correct. These might also be referred to as the upper and lower cutoff or -3dB frequencies.

Don't be tricked by: "Cutoff and Rolloff". Rolloff describes the shape of the edge of the filter's transfer function, not a specific frequency.

Half-Power Point, aka Half-Power Frequencies

The half-power point is the point at which the output power has dropped to half of its peak value; that is, at a level of approximately -3 dB.

In filters, optical filters, and electronic amplifiers, the half-power point is also known as half-power bandwidth and is a commonly used definition for the cutoff frequency.

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Hint: To be “inside” is to “insert”

A filter's pass band is the band that allows the signal to pass through it. However, even in the pass band the filter does attenuate the signal some. This base level of attenuation from having the filter inserted into the signal path is called the "Insertion loss".

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Hint: synthesizer = variable frequency in a musical instrument

A Direct Digital Synthesizer is a signal generator that directly generates waveforms with a digital to analog converter. It has many possible applications so for this question it helps to use the process of elimination.

A. A high-stability variable frequency oscillator in a transceiver - YES, this is definitely possible.

B. A digital voltmeter - NO, this does not generate a signal.

C. A digital mode interface between a computer and a transceiver - NO, this is not a signal generator.

D. A high-sensitivity radio direction finder - NO, this is not a signal generator.

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