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STEP 04 / 06 · ABOUT 6 MIN

Match mode and filter

Frequency alone is not enough. Demodulation and the passed frequency range must fit.

AM, FM and SSB recover information in different ways. With SSB you select the upper sideband, USB, or the lower sideband, LSB. A correctly placed filter does not replace the right mode. Conversely, the correct mode cannot fully recover a signal cut off by the filter.

Our USB exercise signal lies between −2.7 and −0.3 kHz relative to the centre of the displayed span. Its reference tuning is −3.0 kHz. The lab places the lower USB filter edge 0.3 kHz above tuning. USB, −3.0 kHz and a 2.4 kHz filter cover the displayed extent. These values belong to the learning model, not to a recommended transmit frequency.

Narrow the filter and observe the clipped part. Then widen it: extra bandwidth can pass unnecessary noise and neighbouring signals. Find a width that sufficiently covers the wanted signal. The lab assesses geometric overlap; it does not decode speech.

Your turn.

Try USB at −3.0 kHz in the lab. Compare filter widths of 1.0, 2.4 and 8.0 kHz. Watch signal coverage, the noise indicator and neighbouring signals. Then return to the question.

Try it: a USB filter that is too narrow →Read more: modulation →
UNDERSTAND & APPLY

Check your understanding

Why is the widest filter not automatically the best choice?

Sources & further documentation

Your learning path— / 6
  1. Understand the receive chain
  2. Connect frequency and wavelength
  3. Read the waterfall
  4. Match mode and filter
  5. Use gain deliberately
  6. Tune your own signal

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