GPS locking a TCXO that drives an si5351? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 05, 2016, 09:02:18 pm
content: Hey I am making a touch screen multi channel signal source for RF, with 1 adjustable coherent clock outputs to drive other RF gear, basically it just a 2.8tft and encoder that drive 2 AD DDS chips that can do Square/Triangle/Sine up to 47mhz (AD9834these things arent to much it weird how much people will pay for a sine only DDS in a plastic case that isnt even 50mhz). Im also going to add two AD3451 or MAX2780 wideband PLL/Synths with integrated VCO in order to do my microwave work (it may not be the best signal fidelity but its something). This solution may not be a "replacement" for a bench sigGen that can do SHF/Microwave speed sine waves but i cant afford something like that and when paired with an Airspy SDR running spectrum spy (IF you haven't seen this you can sweep almost 2ghz of bandwidth in like 3 seconds), directional coupler, and either a 4 dollar moded directTV down converter or a better home made one you can have the basic capabilities of a 4ghz Network Analyzer, for under $150 if you vouch for the airspy mini.So the basic architecture of this thing is going to be an 8 core propeller with each DDS/Synth having there own dedicated SPI port, this way every channel is deterministic an coherent and all channel can run at once. Im going to use the si5351 as the master clock, one channel will drive the pair of AD9834s, The Second will drive the VFO's, and the third output will be the clock output.So , first off do I need to drive the Propeller micros clock from the si5351 I would think that any clock error from a regular crystal on the micro wouldn't matter because all the outputs are clocked togather and cant lose sink? It doesnt matter if it does, I can always use a divider to derive the 5mhz for the propeller clock from the 75mhz clock driving the AP9834's. I just dont really want to unless it will make an acuall difference.Now the more complicated question I ordered cheapest e-bay GPS module I could find, its based on aNEO-6M-0-001. I haven't looked to much at the data sheet yet, just briefly browsed. Im a bit worried about two things I read in other places at some point. I remember seeing a down fall of the si5351 was it cant be locked to a GPS, not sure where I read it. This is my first time using a PLL and locking clocks etc. It seems to me that I am just going to build the locking pll and use that to lock the .2PPM TCXO that is on my si5351 board, is there any issues with that?Moving on assuming the GPS will be able to lock the TCXO, there is the issue of weather the GPS is actually a good time source...? After I placed the order for the NEO I went searching the net to figure out how a PLL work and how you lock its time to a TCXO or OCXO or whatever your using. It may be my crappy search skills but I didn't find much, but I did find a pretty to the point good article that was much older maybe 2005ish, about locking a jupiter GPS to a tcxo. In the article the guy said he chose the jupiter because of the 10hz output, and explained that even when the GPS has a good sat lock, the 1PPS pin isn't a good time source and that it is stable when used for timing over very long periods but in short periods there is a lot of jitters between each 1pps. I thought that the NEO6 did have 10hz timing you just had to extract it from the binary protocal and not the NEMA but when running a simple search on the data sheet for 10hz I get no hits. So this article was old and admitted newer gps of the day(~2004) were getting better about there 1pps timing. So whats the deal can you you use the 1pps signal as a good time source?This sounds a bit goofy but I know before GPS that lab time sources would lock to time stations like WWVB, I have a WWVB module I ordered from some UK time keeping hobby site laying around ive never used. Now im in CA and WWVB is in Colorado so I know there is going to be some small propagation issues with the signals timing, but if I were to make a bigass loop stick resonant at 60KHz like the ones on the 50's/60's locked frequency sources would it be worth locking to a TCXO of today? I was thinking of either using it as a fallback if the GPS isnt connected to a sat, or to make a completely different frequency source.Im sure to you guys who know all this stuff these questions are a bit ridiculous, but I havent had to use a PLL until now and I cant afford to go get a nice prebuilt source, if I could I would just add an input in to the signal source to lock it to that. There is just going to be times Im generating quadrature waves and I need them locked down to the radio so everything is coherent. I just thought of somthig, if you output a 10mhz source out from your GPS locked oscillator to equipment your building or even equipment that just needs a different frequency isnt doing division and multiplication on the clock with 74HC's going to introduce the delay between a logic chip input and output throwing the system out of sink, not coherent anymore?

author: Paul Rose
author_rank: Regular Contributor
timestamp: December 05, 2016, 10:45:38 pm
content: Don't know why you couldn't discipline the source feeding the 5351.PPS from GPS units is a pulse from an internal source that is itself not that good ( for this purpose ). The choice of which pulse you get from that source is good, and over long term it averages out very good.I use a GPSDO from Nick Sayer, which is available on Tindie ( as low as USD $159 with TCXO and no enclosure, but OCXO and enclosure are extra cost options ).https://www.tindie.com/products/nsayer/gps-disciplined-xcxo/Nick was very helpful when I had questions.I took me ~3.5 days to stabilize my OCXO variant to 0.5 PPM ( according to unit itself, I have not audited the stability myself).

author: Theboel
author_rank: Frequent Contributor
timestamp: December 06, 2016, 02:50:28 am
content: what about this ?http://www.knology.net/~gmarcus/Si5351/Si5351_VFO_QEX.pdf

author: CJay
author_rank: Super Contributor
timestamp: December 06, 2016, 12:24:02 pm
content: I strongly suggest you have a look at the timenuts mailing list as I am nowhere near expert but, from reading your needs:The U-Blox GPS receivers can have a programmable frequency output, the one on my Neo 7M is configurable from 1 PPS up to somewhere around 15MHz I think. It's got lots of jitter because of the way it stabilises and synthesizes it (drops cycles I believe) so it needs 'cleaning up', using an integer divisor of the main 48MHz clock gives lower jitter (10MHz gives jitter that's visible on a 'scope', 8MHz is *much* better but still not perfect, however, measuring on a decent counter gives only millihertz errors on either).You need a 'timing' class receiver, again the U-Blox receivers can be had in versions designed for the sort of application you envisage, they're actually not expensive, you could buy the 'T' versions integrated into drone GPS modules for ~$15 last time I looked.PPS is not stable over a short period, tends to be lots of jitter but there are ways to use these outputs to discipline a high stability OCXO which will be 'clean' and stable, again the timenuts mail list archive will give you far more information and detail on the methods used than I can.I think that only one edge of the PPS will be 'accurate', you need to know which for your application.Depending how far down the time rabbit hole you want to go, you can achieve ridiculously accurate frequency generation but you need to quantify the level of accuracy you want before you start or you could waste a *lot* of time and money.Locking to WWVB is possible but I believe it's accuracy is dependent on propagation and distance, they can induce errors which are more or less significant depending on where you've set your decimal point accuracy requirement.Hans Summers has a site qrplabs, which offers an ovenised Si5351, it's better than good enough for WSPR, might that be useful?I seem to remember there's also a GPS disciplined version but ICBW.