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For older receivers with a fixed (crystal?) audio filter for CW, what is the usual center frequency of the audio bandpass of the filter? Or cut-off frequency for low-pass CW filters?

For newer receivers (and external audio DSP boxes) with configurable narrow audio filters for CW, what is the usual default or factory configured center frequency for the narrowest audio filter?

Do experienced operators more commonly increase or decrease the narrow CW filter factory frequency setting?

But I’m especially interested in old radios with fixed frequency (crystal or ???) audio CW filters.

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  • $\begingroup$ Some of the answers and comments in your January 2013 question partially answer your question about what tone do experienced operators prefer; however, I think preferences vary. $\endgroup$
    – Mike Waters
    Commented Nov 24, 2019 at 20:46
  • $\begingroup$ If you are trying for the narrowest filter, consider CCW. I hope this is not more than you want to know. •Coherent CW arrl.org/files/file/Technology/tis/info/pdf/7509026.pdf $\endgroup$
    – Optionparty
    Commented Nov 25, 2019 at 22:39
  • $\begingroup$ Not asking about the width, just the audio filter center frequency. $\endgroup$
    – hotpaw2
    Commented Nov 25, 2019 at 22:59

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I have seen default centre frequencies of between 600Hz and 800Hz depending on the manufacturer. These have always been configurable though, sometimes using a knob, and sometimes with a menu setting.

Not everyone's hearing is the same, and so this should always be configurable. But if I were designing such a system, I would use a default of 700Hz, and allow it to be configurable between 200-2000Hz or so.

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600 to 800Hz range is most common, as Scott Earle answered. I also use 700 Hz and don't see a need to change that frequency.

Many operators find a higher pitch is better for working faster codes, while a lower pitch is easier on the ear for rag chewing. Even with that factored in, 500 to 900 Hz is probably the common range.

So, I don't really see a need to make variable center frequency BPF, but if you don't know which frequency works best for you, you might want to consider variable center frequency. If implementing in analog filter, a switched capacitor filter would be the easiest way to do that, as the center frequency will scale with the clock frequency.

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