Hi Said, A few questions regarding Delay line vs. Software correction:
Before asking the question let me check if I understood you correctly. Noting that you have a resolution of 6.66ns, I presume you are running at a 150MHz clock speed. In other words you delay the 1PPS by some integer number of clock cycles (e.g. n times 6.66ns). This is done by rounding the sawtooth error provided by the GPS (typically something between -128ns and +128ns for the M12+T) to the nearest 6.66ns. Now the random sawtooth jitter of the 1PPS signal and the slight drift on your 150MHz clock adds out of phase (most of the time), or put otherwise - it is sometimes less, and sometimes more - which averages to a nice final resolution of 3.33ns. Now, the question: I'm primarily interested in relative time stability between two or more identical GPSDO devices. If I use a delay line it will add an absolute offset (+- a few ns) plus some jitter (the Maxim/Dallas datasheets doesn't specify the added jitter) to each GPSDO. The absolute offset doesn't really worry me since I'm only interested in relative stability (Maybe I'm to calibrate the "+- few ns" out first using the method you suggested e.g. cable delay function). It is the jitter that worries me. If I try correct for a 2ns sawtooth error with the delay adding jitter of that same order it really defies the purpose. On the other hand, if I delay it in software (which I had in mind in the first place) I have to go for a high frequency clock oscillator. Nothing wrong with that, I guess you could go as high as your processor can handle. However, the EMI generated (radiated EM, ground bounce, power supply noise etc.) by such a high speed clock worries me. For digital stuff it is not as bad I guess, but the DACs, VRefs and FS will suffer due to this. My gut tells me that if the EMI is going to be a problem I would rather go for the delay line option with a much lower clock speed say 64MHz (capture resolution of 15.625ns). However, no I'm back where I started since now I make a new sawtooth error of 15.625ns! So, the only option is to go for a higher clock speed. What are your experiences with the resulting EMI? Is it a problem? How do you combat it? Regards, Stephan Sandenbergh --------------------------------------------------------------------- Said wrote: Message: 10 Date: Tue, 27 Jun 2006 21:38:02 EDT From: [EMAIL PROTECTED] Subject: Re: [time-nuts] Some results of PRS10 and Trimble Resolution To: [EMAIL PROTECTED], [email protected] Message-ID: <[EMAIL PROTECTED]> Content-Type: text/plain; charset="US-ASCII" Hello Tom, We approached the sawtooth correction on the software side: we sample the 1PPS with 6.66ns resolution in our new Fury GPSDO and apply software adjusted sawtooth correction (post-capture), and this yields an easily visible, very significant reduction in the 1PPS capture noise with the Motorola M12+ receivers. You can really see the difference second-to-second when the correction is turned on/off. This yields an average sampling quantization noise of +-3.33ns. The physical sawtooth on the 1PPS signal is actually helpfull in this setup as you mention since it dithers the LSB quantization noise, and thus actually improves the quantization resolution over time (with simple averaging low pass filtering of the captured data). This trick is called dithering in audio etc. Using delay lines may be tricky and expensive, they usually are temperature sensitive, and only yield good results if the capture of the 1PPS is done in fast enough (<10ns capture resolution). But if your capture is fast enough anyways, there is no need to use a delay line since the correction can be done in software. Delay lines also have another disadvantage: the 1PPS correction from the GPS can be positive or negative in time, so in theory you would need a negative time delay (Einstein would be happy :). So for the delay line to work correctly, you have to set the 0ns delay tap equivalent to the most negative pulse, all other pulses will incur more than 0ns delay. You are now effectively delaying the average 1PPS by 1/2 the spread of the pulses. Thus the output of the delay line is always late (by about 30ns for commercial receivers) on average for standard GPS receivers. This error will have to be subtracted later. One trick when using Motorola GPS's is to set the cable delay to an additional 30ns to make the GPS receiver itself compensate for this delay by issuing the 1PPS output 30ns "too fast". An interesting effect of sampling at +-3.33ns is that the GPS errors such as multipath, atmospheric, and GPS crystal temperature related issues become clearly visible with this kind of resolution... Bye, Said _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
