To: | [email protected] |
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Subject: | Re: LF: Coherent BPSK on LF using EbNaut |
From: | Andy Talbot <[email protected]> |
Date: | Sat, 26 Sep 2015 15:41:03 +0100 |
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Yessss...
Hadn't appreciated that was what it was doing until I read the description in more detail. I suppose that if soundcards are going to be used as the interface some sort of locking will have to be used. G3PLX used his clicklock technique for phase coherency, where the 1 PPS is bled into the receive path. You go one further and mix in the NMEA stream audio as well for time stamping. A neat solution to making use of what is really unsuitable hardware. Soundcards really aren't precision measurement tools ! It may be time to revisit the LF receiver in that case. Something like a direct I/Q downconversion from LF to baseband, low pass filtering then I/Q sampling into an A/D at something like 1kHz (or even lower), sending the resulting samples to a PC using a basic USB (or even a COM ) port. Then for backwards compatibility adding in the GPS timestamp information in the same way . Getting a certain sense of Deja-vue here. Back when LF was new, and soundcards were rarely used for radio and GPS receivers rare, I did start to build a custom LF Rx doing just this technique and managed to receive coherent carriers from such as MSF and the Swiss one and see how their phase shifted with the ionosphere. For freqeuncy reference I used my caesium standard. But it never went anywhere long term on 73 & 137kHz because a) PC technology moved faster and everyone else was adopting soundcard ideas and b) The PIC processors I was using were still new than and had pretty basic functionality making everything hard work and c) no one seemed bothered since QRSS has just become widely popular and they all used that mode. N ow, nearly 20 years later, the direct conversion LF receiver (Softrock type perhaps) and a new generation of processor chips make the job a lot easier. DDS or Fract-N synth LO, Quadrature mixer, simple (just analogue perhaps) low pass filter, dual A/D , PIC, PC interface Andy G4JNT On 26 September 2015 at 14:17, Paul Nicholson <[email protected]> wrote:
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