Envelope-to: dave@picks.force9.co.uk Delivery-date: Thu, 23 Feb 2006 20:56:14 +0000 Received: by ptb-mxcore01.plus.net with spam-scanned (PlusNet MXCore v2.00) id 1FCNVV-0006Iy-Dn for dave@picks.force9.co.uk; Thu, 23 Feb 2006 20:56:14 +0000 Received: from post.thorcom.com ([193.82.116.20]) by ptb-mxcore01.plus.net with esmtp (PlusNet MXCore v2.00) id 1FCNVV-0006I9-5K for dave@picks.force9.co.uk; Thu, 23 Feb 2006 20:56:13 +0000 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1FCNUU-0005VW-R6 for rs_out_1@blacksheep.org; Thu, 23 Feb 2006 20:55:10 +0000 Received: from [193.82.116.32] (helo=relay1.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1FCNUU-0005VN-Bp for rsgb_lf_group@blacksheep.org; Thu, 23 Feb 2006 20:55:10 +0000 Received: from smtp808.mail.ukl.yahoo.com ([217.12.12.198]) by relay1.thorcom.net with smtp (Exim 4.51) id 1FCOwR-0001hZ-CZ for rsgb_lf_group@blacksheep.org; Thu, 23 Feb 2006 22:28:23 +0000 Received: (qmail 61312 invoked from network); 23 Feb 2006 20:54:53 -0000 Received: from unknown (HELO LAPTOP) (peter.martinez@btinternet.com@81.154.77.200 with login) by smtp808.mail.ukl.yahoo.com with SMTP; 23 Feb 2006 20:54:53 -0000 Message-ID: <020a01c638bb$652dfbb0$0300a8c0@LAPTOP> From: "Peter Martinez" To: rsgb_lf_group@blacksheep.org References: <21f.861fa8c.312cfc09@aol.com> Date: Thu, 23 Feb 2006 20:54:53 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 Subject: LF: GPS-locked reception Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-PN-SpamFiltered: by PlusNet MXCore (v2.00) >From Peter G3PLX: Greetings to the group. I was subscribed here some years ago when we first got 136kHz and I was transmitting on the band myself but I have been busy with other projects since. One of these has been an idea for the ultimate in frequency stability, but using an ordinary SSB receiver, a GPS receiver with a 1PPS output, and some soundcard software. Oh, and one more thing is needed - a 47pF capacitor. I contacted a couple of LFers recently, including Brian CT1DRP, VE7TIL, and W1TAG/WD2XES. These stations have been transmitting special signals for me that are locked in frequency to GPS, and these signals have enabled me to confirm that this new idea is working well. The tests also give some idea of how narrow in bandwidth we might be able to go if the propagation will permit. I thought it was time I rejoined the reflector and passed on some of these ideas. I first thought of this trick after I connected the 1PPS signal from my GPS to the antenna of the main receiver, wondering if a narrow spectrum analyser would show this as a comb of harmonics every 1Hz. It sure did, as many people will have found, but I also watched the 'click'on a scope connected to the receiver audio, and noticed that the waveform had a characteristic 'beat' to it. I realised that the beat was the difference frequency between the receiver dial readout and the nearest multiple of 1Hz, and this could be exploited as a drift-cancelling idea. The technique therefore involves connecting the all-important 47pF capacitor from the PPS pulse of a GPS to the LF receiver antenna. The same 1PPS signal connects to the right channel of a stereo soundcard. The receiver audio, with a mix of the distant weak signals and the RF click, connects to the left channel. The software first uses the 1Hz waveform on the right channel to calibrate the soundcard samplerate. The left channel audio is processed through a precision software frequency-shifter, like a very fine RIT control, working over +/-0.5Hz. The RF click in this shifted audio is examined by the clever part of the software and the RIT control is adjusted to zero-beat it. The result is that the frequency-shifted audio has the receiver frequency offset cancelled. The output is as stable as if it had come from a receiver with ALL it's oscillators locked to GPS. This is a simplified description of course. In fact I don't feed the drift-cancelled audio back out again, but process it inside the software, and I think this will be the best way to use this idea for weak-signal LF work. I have done ultra-narrow waterfalls (+/-2mHz), BPSK demodulation, and in particular charts showing phase drift over time to see how well the system is working. Many LF operators have built GPS-locked DDS systems already but this simple technique means we can now GPS-lock the LF reception process, and it's all done in software (except for the 47pF capacitor of course). One thing that this technique is showing is that the propagation medium may be a lot more stable that we thought. I found I can stay within 30 degrees of the LORAN line from Rugby (300km away) on 137.57985440499182885 kHz for days on end. This means there is no lower limit to how narrow we can try to receive extreme QRP over surface-wave paths if both ends are GPS-locked. On the 1500km path from CT1DRP to me (54N 3W) the signal is very stable during the midday period. Around sunset and sunrise the phase changes more rapidly - about 200 degrees/hour. That amounts to a Doppler shift of +/-150uHz at sunrise/sunset. But after the sun rises again, the phase returns to the exact same value as the previous day. I haven't done much yet on easy/west paths but I was able to hold WD2XES within 90 degrees over 2 hours the other night, so QRSS-7200 might be workable over the TA path if we had the patience to do it! There will be no ready-to-fly downloadable software from me. Anyone who wants to have a try at this who has already some experience with soundcard software, is welcome to try it. There's great scope for experiment and new ideas for using this simple idea, perhaps as added front-ends for existing software, or for completely new techniques for ulra-narrowband working. This reflector would be a fine place to discuss these. 73 Peter