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What is an Audio Peaking Filter? And how might that be different from an ordinary (narrow) CW bandpass filter, or from tunable audio filters (with fully configurable upper and lower cutoffs)?

Does a peaking filter have anything in common with an audio notch filter?

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    $\begingroup$ would you have a reference where "audio peaking filter" is used? $\endgroup$ – Marcus Müller Aug 2 at 16:15
  • $\begingroup$ According to the Elecraft KX2 owners manual: "In CW mode, holding APF-AN turns on a very narrow filter to improve copy of weak CW signals right at the receiver’s noise floor." $\endgroup$ – aMike Aug 2 at 16:54
  • $\begingroup$ A google search for APF or audio peaking filter returns over 10k results mentioning multiple radio vendors and/or software applications. $\endgroup$ – hotpaw2 Aug 2 at 17:14
  • $\begingroup$ Is this just..."peaking" as opposed to "shelving"? These are two fundamental filter types for audio design. $\endgroup$ – Tyler Stone Aug 2 at 17:45
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    $\begingroup$ Here is a technical note on the Elecraft APF: ftp.elecraft.com/KX3/Mod%20Notes%20Alerts/… $\endgroup$ – K7PEH Aug 2 at 23:10
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A standard CW filter operates on the IF (RF after at least one stage of downmixing, but before demodulation). Elecraft's APF operates on the demodulated audio.

The reason most filtering is done at IF is because making a nice sharp filter becomes easier the higher the filter's center frequency (within reason). In Elecraft's opinion, a filter with the more gradual roll-off implied by a center frequency of only ~1kHz is beneficial to CW copying, when used in addition to the ordinary IF filter.

Since this is a bandpass filter, it's the exact opposite of a notch filter, but other than that, it's the same :)

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