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Re: Demodulating slow phase-modulated data

From: Philip Pemberton
Subject: Re: Demodulating slow phase-modulated data
Date: Fri, 5 Nov 2021 20:21:18 +0000

On Wed, 3 Nov 2021 at 22:15, Daniel Estévez <daniel@destevez.net> wrote:
Downconverting to baseband and low-pass filtering seems a good start.
What to do next depends on the specifics of the modulation. I didn't
understand what you mean by

the modulating wave is sinusoidal, either
37.5Hz or 50Hz depending on the part of the signal being sent

Do you perhaps mean that the carrier is amplitude or phase modulated
with an FSK signal that uses tones of 37.5 Hz and 50 Hz to encoded the bits?

Not quite. The carrier is phase-modulated in an analog form, not digital. Perhaps it might make more sense if I explained how the original receiver decodes it.

The receiver downconverts the incoming signal to about 20kHz, then measures its phase against a 20MHz TCXO. The measured phase value is then sent to the CPU, where they are stored (1kHz sample rate). Once a full 'slot' (part of the transmission) is received, the whole captured block is filtered digitally and matched against a template using cross-correlation.

A "Burst" is a 1.68-second long transmission which starts with a Gold Code Bit and a Clock.
A "Loop" is 64 "Bursts" which forms a complete transmission with a given Clock value.

There are two separate signal blocks modulated onto the carrier:

* Gold Code Bit -- encodes one bit of the 64-bit sync code per transmission burst. 1.5 cycles of 37.5Hz for a 1, or one cycle of 50Hz for a 0. The filter for the GCB is a two-peak notch filter which allows 37.5Hz and 50Hz through.
* Clock -- encodes two bits of a 16-bit time-of-day value, repeated eight times in every 64-burst loop. Filtered with a 50Hz zero-phase-offset filter. The data is encoded by the starting phase at the start of the block: 0, 90, 180, or 270 degrees.

I want to recover the analog modulating signal -- or in other words, the series of phase values the original receiver would have recorded.

Once I have that modulating signal, I'm hoping to use the same template matching the receiver uses in order to extract the data bits.

I'm not at all sure if that's what you mean, and it doesn't seem a very
conventional modulation scheme, so perhaps it's something else. I don't
know anything about Datatrak.

I've been slowly reverse-engineering it -- possibly the most useful page in regards the LF signal is this one: https://www.philpem.me.uk/datatrak/signal
There's a lot more information on the signal there, far more than what I've put in this email.

I'm eventually hoping to build a signal generator which will generate a navigation signal that's "good enough" for my Locator receivers to tune to it and boot up.


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