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E4B10

AMATEUR PRACTICES · Measurement technique and limitations: instrument accuracy and performance limitations; probes; techniques to minimize errors; measurement of Q; instrument calibration; S parameters; vector network analyzers; RF signals

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Which of the following methods measures intermodulation distortion in an SSB transmitter?

  1. AModulate the transmitter using two RF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
  2. BModulate the transmitter using two AF signals having non-harmonically related frequencies and observe the RF output with a spectrum analyzer
  3. CModulate the transmitter using two AF signals having harmonically related frequencies and observe the RF output with a peak reading wattmeter
  4. DModulate the transmitter using two RF signals having harmonically related frequencies and observe the RF output with a logic analyzer
Why IMD is tested with a standard two-tone test: feed two audio tones of equal amplitude at non-harmonically related frequencies (classically something like 700 Hz and 1900 Hz) into the microphone input, and look at the RF output on a spectrum analyzer. Because an SSB transmitter is a linear amplifier chain, any nonlinearity mixes the two tones and creates extra products at 2f1-f2, 2f2-f1, and so on, which show up as sidebands spaced around the two wanted tones. The tones must not be harmonically related so the distortion products land on distinct frequencies instead of hiding on top of each other or on harmonics.
Watch out Injecting two RF signals is the way you test a receiver or amplifier front end for IMD, not how you modulate an SSB transmitter, whose input is audio. A peak reading wattmeter only shows total power and cannot separate the distortion products from the wanted tones.
Two AUDIO tones in, spectrum analyzer on the RF out, and keep the tones non-harmonic so the products stand alone.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of E4B

  1. E4B01 Which of the following factors most affects the accuracy of a frequency counter?
  2. E4B02 What is the significance of voltmeter sensitivity expressed in ohms per volt?
  3. E4B03 Which S parameter is equivalent to forward gain?
  4. E4B04 Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?
  5. E4B05 What three test loads are used to calibrate an RF vector network analyzer?
  6. E4B06 How much power is being absorbed by the load when a directional power meter connected between a transmitter a…
  7. E4B07 What do the subscripts of S parameters represent?
  8. E4B08 Which of the following can be used to determine the Q of a series-tuned circuit?
  9. E4B09 Which of the following can be measured by a two-port vector network analyzer?
  10. E4B10 Which of the following methods measures intermodulation distortion in an SSB transmitter?
  11. E4B11 Which of the following can be measured with a vector network analyzer?
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