On Wed, 3 Nov 2021 at 22:15, Daniel Estévez <dan...@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. Thanks, -- Phil. phil...@philpem.me.uk (preferred) phil...@gmail.com (alternate) http://www.philpem.me.uk/
