I'm very confused about what frequencies can be set on the Si5351a in terms of what frequency you ask for and what frequency you actually get. I'm not concerned so much about the actual frequency as the accuracy of frequency step sizes which are requested.? I've read the spec sheet, which pretty much numbs my brain after a while. I've read lots of web articles, none of which actually answer the questions I have.? I'm certainly confused by the ability to specify the desired frequency with 0.01Hz resolution in the Arduino libraries. Presumably this is specifications of the Si5351a frequency determining registers rather than any actual ability to generate frequencies in 0.01Hz steps. Most of the arduino sketcchs seem to suggest at least 1Hz accuracy, even though it seems hard to even get exact 1Hz steps. The Si Labs clockbuilder utility for the Si5351 actually has the frequency input in steps of 0.001Hz.
The application is for a low power Q65 beacon on 144MHz. I can handle the Q65 with no problem. I can handle the frequency accuracy and stability with no problem. The problem comes in generating a set of tones with, for example, 6.667Hz spacing.? I have seem people reporting 2m WSPR transmitters, which have tight specs and need 1.4648Hz tone spacing, so 6.667Hz should be possible. For this application and with tone spacing this wide, the frequency step size doesn't have to be accurate to 3 decimal places. In fact anything around 6.6 +/- 0.5Hz would work. I can generate a set of tones which will, in fact, decode correctly, but the tones can be 1 or 2 Hz from where they should be, and that's not going to be a good thing when decoding weak signals. However Q65 tone spacing can be anything from 0.289 to 26.667Hz, depending on the period and submode, so the more accurately the frequency step size can be set, the better. For now I'm just interested in 6.667Hz tone spacing and frequencies set to the nearest 0.1Hz would be fine. Even to the nearest 0.5Hz would probably be OK.
The problem is that, using something like the NT7S Si5351a Arduino library, I can't even get step sizes of 10Hz to me monotonic. They can be anything from 8Hz to 12Hz (though over enough steps they do average out to 10Hz). I have tried a few other arduino sketches and libraries, with similar results. I don't know if this is a fundamental limitation of the Si5351 (I don't think it is) or it's a limitation of the libraries in the way they calculate the values it sends to the Si5351 registers. Or maybe I'm just doing something stupid that I'm not seeing.?
I'm just using one clock output (0) and currently I'm using a 25MHz crystal (mainly because I have an ultra stable 25MHz frequency reference source available). and I'm using an Arduino UNO3. I understand that the exact frequencies which can be generated (and hence exact step size) is related to the crystal frequency, though again I have seen reports (on this group) that WSPR beacons for 2m have been built using a 25MHz xtal and my frequency requirements are quite a bit less critical. I don't have to be on an exact sync frequency either, 144.18 or 144.2 or 144.27 (or pretty much anything else) would be equally good. It would not be channelized operation.
So to generate signals at 144MHz with fairly uniform 6.7Hz steps do I need a different Arduino library (if such a library exists). I now there is a WSPR library, but in it it has the commentt line:
"#define WSPR_FREQ2m? ? ? ? ? ? 14449500000ULL? ?//2m? ? 144.490,000MHz //Not working. No decode in bench test with WSJT-X decoding Software"
Or do I need to write my own code which directly addresses the Si5351 registers (a task I'm not sure I want to try!) or do I need a different reference frequency - or perhaps all of the above?
I could work using the 3rd harmonic of a 48MHz signal with 1/3 the tone spacing if doing things at 48MHz is easier, but then the tone spacing gets 3x smaller, plus it involves even more filtering and amplification, so 144 direct is certainly preferable.
Any help, comments, pointers etc. would be most welcome. As you can probably guess, my knowledge of the inside workings of frequency synthesizers is somewhat limited!
Thanks,
Bob, KA1GT
