I design , manufacturer , and sell such SDRs (XIlinx MPSoC based)  with such said clock distribution..., and they're not on VME buses....but you'd need to write your own FPGA firmware, unless it was an interesting weekend project for me... there's a few things you can'tt easily do that come to mind. But it depends on your sample rate, frequency of interest.... Distribution and minimum jitter,  and fastest risetimes you can manage, means the clock around the board, while LVPECL, runs usually at 4x the ADC rate (400-1000 MHz) to keep edges nice and fast. Problem comes that the dividers in the ADCs, while they should be deterministic in their state, when all out of reset at the same time,  they're not perfect, so there needs to be out - of -reset knowledge of something... Actually JESD204 sync input ADCs has somewhat fixed some of those problems. So, you'd need to distribute a clock at the sample rate , not a multiple, which might have some impact on the SNR for very high frequency applications (aperture uncertainty etc etc etc) -glen On 27/03/2025 02:50, Jim Lux via time-nuts wrote: > > > > Yes, that scheme looks like it would work.  Of course it needs switches on the input to switch to a sufficiently loud noise source (the RTL-SDR isn't particularly low noise - at 20 MHz it's about 22 dB NF). > My idea of a tone burst works similarly. Instead of correlating among the channels for noise, you correlate with a known pulse. > > The more i look at cheap SDRs (or even expensive ones) the whole idea of consistent coherent sampling isn't done very well - you can feed an external reference in, but there's inevitably a PLL synthesizing a LO that has uncertain initial state. Almost nobody sells SDRs that have a common sampling clock AND LO distribution. (well, they do, they're real expensive and plug into VME type chassis). > > The fancy X series USRPs (which are quite popular) don't have LO distribution between boxes (although they do have ADC clock sync among boxes) > > > > On Wed, 26 Mar 2025 10:14:22 +1100, Geoff Van der Wagen via time-nuts<time-nuts@lists.febo.com> wrote: > > Hi Jim, > > Take a look at how it's been done on RTL-SDRs: > > https://github.com/tejeez/rtl_coherent > > Cheers > > Geoff > > > On 26/2/25 06:40, Jim Lux via time-nuts wrote: >> >> >> I have an interesting problem which someone has probably solved. >> I've got a bunch of distributed SDR receivers (something like a SDRPlay unit) which can record a swath of RF. >> What I'd like to do is have a way to transmit an "in-band" signal that can be used in post processing to precisely synchronize the data streams. Ideally to, say, 1 ns. >> >> Conceptually, this could be something like taking a 1pps from a GPS receiver (with whatever uncertainty it has), or a GPSDO,  and generating a synchronized tone burst that's in band. So the question is, what off the shelf box, or combination of inexpensive boxes, could generate such a signal. >> >> Once that signal is in the sampled data stream, it's easy to do a matched filter/correlator and find out where it is. >> >> There's plenty of GPSDOs with, say, 10 MHz outputs, but they're continuous, and I don't know if the "zero crossing" of the 10 MHz is synchronized with the 1pps (or, at least, consistently timed). >> >> Or is this something that a modified PICDIV might do?  (with not too much work to modify) >> >> The key here is "don't develop new widget" - an off the shelf piece of lab gear might work (I was thinking about a Keysight function generator 33622 might generate a tone burst triggered with sufficient stability, but I have to go check - when last I checked, the 33622 takes an external frequency reference, but it drives a FLL, not a PLL) >> >> >> >> >> _______________________________________________ >> time-nuts mailing list --time-nuts@lists.febo.com >> To unsubscribe send an email totime-nuts-leave@lists.febo.com > _______________________________________________ > time-nuts mailing list --time-nuts@lists.febo.com > To unsubscribe send an email totime-nuts-leave@lists.febo.com > > > > > _______________________________________________ > time-nuts mailing list --time-nuts@lists.febo.com > To unsubscribe send an email totime-nuts-leave@lists.febo.com