Re: [Ngspice-users] Instantaneous frequency

[Daniel M. <pr...@gm...>]

You can compute the complex analytic signal, which turns a real-valued
signal into a complex-valued signal, and then take the derivative of the
phase with respect to time to get the instantaneous frequency.  The complex
analytic signal can be obtained by taking the FFT of the signal, setting
the negative frequencies to zero and doubling the positive frequencies, and
taking the inverse FFT.

For my guitar pedal design, I use zero crossings, the zero crossings being
determined by a comparator with hysteresis.  The frequency is determined by
measuring the time during zero crossings and using these crossings to
compute the 1-bit autocorrelation of the signal, choosing the lowest
harmonic of the autocorrelation.  Here's the code I wrote to do it:

https://github.com/profdc9/GuitarPico/blob/main/Code/guitarpico/gpico/src/pitch.c

It is very fast because it only needs to keep track of the time between
edges and computes the autocorrelation based on those.  It needs to be very
fast because most of the time needs to be spent on the actual guitar
effects.

On Fri, Feb 6, 2026 at 2:34 PM Patrick Kurth <p....@po...> wrote:

> Hi list,
>
> Is there a way of measuring the instantaneous frequency of a signal
> over time? Something like the TRIG measurement, but as a waveform, as
> if you would call the TRIG measurement repeatedly and concatenated all
> results into a waveform. More precisely I am looking for a crossing
> function.
> Is there a simple way to achieve this?
>
> Kind regards,
> Patrick
>
>
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