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.
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.