However, I wonder if people have a real use for a 10 ppb reference. I could see it being handy for checking calibration, but that is about it. Most of the world seems to be built on +/- 1% for most specs, and maybe +/-50ppm for frequency (about that of a cheap XTAL)
What are you going to do for an antenna?
Any RF engineer will frown on this who needs (at least occasionally calibrated) accurate reference? E.g. it's very typical to down mix. Also it's not uncommon (especially for hobbyist use) to down mix narrow band signals. From this on it's probably easy to figure. Sure for many digital stuff it does not matter.
For trialing this kind of things either a good/affordable reference is required or measuring frequency with some precision. So maybe a 2ppm signal does not sound too bad. At 2.2GHz (lowest value of the VCO of the ADF4351) this is around 4 KHz. Once not so far in the past I had the idea of using the ADF4351 as signal source. There are several examples/cook books on the net about this. There are however some trade offs, as outside of the VCO range it outputs a divided down, thus square like signal. Also it cannot go below 34.375 MHz by its own.
So let's mix (as with this a huge range can be covered). I've actually made this exercise. To throw in even more part numbers beyond the ADF4351 I've used a mini circuits SIM-73L+ (what I like/so great about this mixer that it can mix with such low level LO that the ADF's output can be directly used without anything like balun, amplifier etc.) + ADF4355 (apart from register structure it's very similar to ADF4351 so for trialing the same cookbook can be used).
So let's down mix to e.g. 12 MHz. Oh wait, the 4 KHz drift is still there but it's not 2ppm anymore...
You mean the GPS antenna I guess. Some cheapo stuff but may I could also build it from wires (basically nothing) if I'm really pushed.
I see a lot of buzz words and part numbers being thrown around but I'm not sure I see any real application information to connect any of this stuff together to make a working system. I wish you luck.
Connecting the RF stuff I've used SMA male to male cables so far and for the digital part some Dupont wire. Really nothing else.
True that I did not factor these in in the bill of materials as I have some of these lying around (although SMA stuff may actually be not so cheap). So connecting stuff together is mostly SW, not HW (where except for the averaging there are tons of SW examples on the net, e.g. for the ADF4351 I found at least 3, I've found DDS code as well for many DDSs), however I'd still like to avoid that due to limitation of the building components SW connecting cannot work even in theory, hence this thread.
Note: I've started out this thread as precise frequency measurement but as you can see thing are interchangeable and so wondered to GPSDO now. One of them is needed but having one, the other can be created with relative ease.
So what do you think?
You seem to describe something that resembles this:
http://www.knology.net/~gmarcus/VFO/GPS_Sig_Gen_v1_1.pdf
Exactly. Apart from using the more powerful ESP32 that does not need the extra circuitry for dividing down the signal (so less components, really just wiring; and also conversion can be faster) it pretty much covers what I've tried to describe.
As the AD9850 seems to have more bits so tuning can be more precise.
So to me it's not obvious at first checking what does actually limit the precision only to 0.3Hz @ 10 MHz. Anyhow, it's pretty much in the expected range.
So after all the idea existed quite some time already, just people are oriented towards the more specialized PLL+ special loop filter direction (that unfortunately cannot be built from off the shelf generic boards).
Counter:
1. I would not use 1PPS directly for "gating" the input frequency and counting the pulses (the traditional old school counter)
even it is feasible with the 1sec 1PPS (mind the one-shot jitter there)
But it would not be one shot, several measurements averaged so jitter averaged out. Look the conversion figure and the description of the linked material.
Well, it was a good discussion, thanks especially for DimitriP for the link.