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G8B09

SIGNALS AND EMISSIONS · - Frequency changing; bandwidths of various modes; deviation; intermodulation

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Why is it good to match receiver bandwidth to the bandwidth of the operating mode?

  1. AIt is required by FCC rules
  2. BIt minimizes power consumption in the receiver
  3. CIt improves impedance matching of the antenna
  4. DIt results in the best signal-to-noise ratio
Why Receiver noise power is proportional to the bandwidth you let through, so a filter wider than the signal admits extra noise and adjacent interference while adding no more desired signal. Matching the filter to the mode's bandwidth (about 2.4 kHz for SSB, 500 Hz or less for CW, a few hundred Hz for many digital modes) passes all of the signal and rejects the rest, giving the best signal-to-noise ratio. Too narrow a filter is also bad because it chops off part of the signal and distorts it.
Watch out FCC rules set limits on transmitted bandwidth and symbol rates, not on how wide your receiver filter must be, so the 'required by rules' choice is wrong; receiver filtering has nothing to do with antenna impedance or power consumption.
Noise comes in with bandwidth: wider filter = more noise, same signal. Fit the filter to the mode.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of G8B

  1. G8B01 Which mixer input is varied or tuned to convert signals of different frequencies to an intermediate frequency…
  2. G8B02 What is the term for interference from a signal at twice the IF frequency from the desired signal?
  3. G8B03 What is another term for the mixing of two RF signals?
  4. G8B04 What is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the …
  5. G8B05 Which intermodulation products are closest to the original signal frequencies?
  6. G8B06 What is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency?
  7. G8B07 What is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 M…
  8. G8B08 Why is it important to know the duty cycle of the mode you are using when transmitting?
  9. G8B09 Why is it good to match receiver bandwidth to the bandwidth of the operating mode?
  10. G8B10 What is the relationship between transmitted symbol rate and bandwidth?
  11. G8B11 What combination of a mixer's Local Oscillator (LO) and RF input frequencies is found in the output?
  12. G8B12 What process combines two signals in a non-linear circuit to produce unwanted spurious outputs?
  13. G8B13 Which of the following is an odd-order intermodulation product of frequencies F1 and F2?
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