To: | [email protected] |
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Subject: | Re: [rsgb_lf_group] Re:: Fwd: LF: Frequency References GPSDOs etc. (was EbNaut etc...) |
From: | Andy Talbot <[email protected]> |
Date: | Wed, 9 Dec 2015 12:15:25 +0000 |
Cc: | [email protected] |
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In-reply-to: | <[email protected]> |
References: | <CAA8k23RzSeso2RqJDk4AC_BDoT8Y0wDW==z6BwFhtHMk+eOedg@mail.gmail.com> <[email protected]> |
Reply-to: | [email protected] |
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What is needed is a design for a homebrew 1 PPS PHASE LOCKED source. Both the Brooks Shera and VE2ZAZ designs are frequency locked loops - although it could be argued that frequency and phase are the same things, and it is the order of the PLL that is different. But whatever... they give noticeable errors short to medoium term Commercially quite low cost GPSDOs, like one I have from [what used to be] Connor Winfield give results that when averaged over a few minutes are quite accpetable in terms of absolute accuracy, and are in the sub-PPB region over shorter periods. The paper on my web site, URL given in a previous post, shows the CWin GPSDO short term stability. Those low end units use what would appear to be phase locking, and I see no reason why a simple edge triggered phase detector at 1PPS, using high speed logic couldn't be made to work. Problems ion the past have centred round phase detectors being not very useful when asked to deal with difference in tens of ns, but these days, one made using VHF logic or a gate array ought to be able to manage. I don't think an XOR phase detector is feasible, the filtering to get rid of the 2Hz residue, even at very long time constants, would be too much. Getting a real long time constant could be a bit of a headache without going into DSP, but good quality non electrolytics are around for several uF, and if we're not worried by size, several in || to say 50uFm with 20M resistors around CMOS opamps ought to be good enough - that's up to 20 minutes time constant and in the realm of needing an OCXO as the 10MHz source. Construction would need care to avaoid leakage though. My MSF reference way-back-when used CR values of this sort of value, and it did mostly work. Phase changes of RF due to proximity effects around he ferrite rod were its major downfall. And all the rest is just basic division On 9 December 2015 at 02:54, [email protected] [rsgb_lf_group] <[email protected]> wrote:
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