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AF Spectrum display

Hi John,

I have recently been trying to make more use of the AF Spectrum display, primarily to help identify the characteristics of HF radars, such as their Pulse Repetition Frequency, and if CTCSS tones are present on NBFM signals.

I normally use the KiWi in conjunction with Soundcard Oscilloscope to look at the amplitude and frequency domain of the demodulated audio.


My plan was to try and use the KiWi AF Spectrum display and cursor to see if I could pick out the dominant AF components from the demodulated AM and FM signals.

However (Doh) I now realise that the AF spectrum display isn't actually showing the demodulated output, but it is a zoomed in version of the RF spectrum FFT.

JUst a thought but maybe the AF spec button should be renamed to RX or IF ? spectrum or something similar ?

Regards,

Martin

Comments

  • It does show the audio spectrum (AF) of the rx channel, not the RF spectrum of the waterfall channel. But it's the pre-demod data from the FFT-based passband filter (since you get that FFT data for free essentially).

  • One big problem with the Spec AF display is that it really needs its own min/max controls. Right now there is just a heuristic to try and derive the values from the waterfall (hence Spec RF) min/max.

    But it doesn't work across all situations (e.g. when tuned to work well for a band full of signals the settings are too low for a quiet band and the noise floor drops off the bottom of the scale). This is something I'm thinking about putting on a second page of the Audio tab.

  • Hi John,

    OK maybe we are cross purposes ?

    It doesn't seem to show the demodulated audio, for example if you use Synchronous AM but slightly off tune, the carrier shows up offset from the AF spectrum centre line. This would not be the case if it was the actual demodulated audio, plus it would just be the audio spectrum from 0Hz to the upper bandwidth limit of the filter.

    Maybe it would work better as a separate extension ?

    Regards,

    Martin

  • For example this is what I see when I use the KiWi AF spectrum to look at a radar signal.


    And this is with Sound Card Oscilloscope looking at the AM demodulated output.

    Frequency domain

    Time domain


    And using NBFM to determine that it is swept and not just pulsed.

    Frequency domain


    Time Domain


    One of the weird HF Trading data signals using SSB demod


    Frequency domain, not easy to pick out the separate data channels this time


    Time domain

    You can at least pick out the base line data rate

  • It doesn't seem to show the demodulated audio

    That's why I said pre-demod. The FFT-based passband filtering happens before any of the demodulators.

  • Ak OK, I misunderstood that point, it makes sense now.

    No wonder I got confused about what I was seeing. I don't think it will allow me to examine what I hoped I could, but never mind I'll just continue to use the Oscilloscope software.

    Regards,

    Martin

  • Am I missing something here? If "Spec AF" is pre-demod, what is the point of it? What does it show that "Spec RF" doesn't already show, suitably zoomed?

    Regards,

    Richard G4DYA

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