Re: [Hackrf-dev] Precise synthesizer tuning
I revisited this problem yesterday and have it *almost* completely resolved. By running the synthesizer programming formulas in parallel, I am able to determine the actual tuned frequency as a function of a requested frequency and adjust my software local oscillator to compensate. The synthesizers in *both* the MAX2837 and the RFFC5072 have some weird behaviors that made it difficult to figure out what was really going on. (My signal generator stops at 2100 MHz so I was unable to test with the MAX2837 alone, which requires a signal between 2150-2750 MHz.) Digging into the firmware source and the synthesizer data sheets, I found that the tuning step for the MAX2837 is: 30 MHz / 2^20 = 28.610229492... Hz However, the synthesizer occasionally settles one step away from the specified one. Reprogramming the same frequency seems to usually fix it. Could this be a timing problem in how the firmware writes the registers? The RFFC5072 step size depends on the frequency range, but is some small multiple (or fraction) of 50 MHz / 2^24 = 2.980232239... Hz But the RFFC5072 synthesizer always seems to settle one *half* step above the desired frequency. This seems very strange, but maybe there's something I don't understand about fractional-N synthesizers. (So far my tests have been limited to frequencies where the step is 100 MHz/2^24 Hz). In the source file firmware/common/max2837.c is the following mysterious comment: /* ASSUME 40MHz PLL. Ratio = F*(4/3)/40,000,000 = F/30,000,000 */ This implies the internal synthesizer reference is 30 MHz despite 40 MHz being applied to the part by the Si5351C. This appears correct, but I can't find any mention of this 4/3 (or 3/4) factor anywhere in the MAX2837 documentation or register settings. Where does it come from? Also in the firmware, when the RFFC5071 is used, an approximate frequency that's an integer multiple of 1 MHz is passed to rffc5071_config_synth_int in rffc5071.c. That function returns the actual frequency, which is then used to adjust the tuning frequency passed to the max2837. However, rffc5071_config_synth_int returns an integer, so this also introduces a slight error. (It should probably return a double). I'd like to put in a plea to the designers of SDR front end APIs: please have your "tune" function return its best estimate of the radio's actual tuned frequency, taking into account synthesizer step sizes and TCXO offset estimates. It's perfectly OK for a radio to not give me the exact frequency I ask for *PROVIDED* I know the actual frequency, because I can then easily compensate for it in my software local oscillator. Thanks! Phil Karn PS. One of the reasons I'm interested in precise frequency knowledge is that I'm comparing local ATSC pilot carriers to my GPSDO. ATSC pilots have attracted some interest as poor-man's frequency references, but my observation is that only *some* TV stations actually use high stability oscillators to control them. The others can be a few hertz off (not counting those still operating with transition-era frequency offsets.) ___ HackRF-dev mailing list HackRF-dev@greatscottgadgets.com https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
Re: [Hackrf-dev] Precise synthesizer tuning
Phil, Unfortunately the answer is rather complicated because there are two frequency synthesizers involved, not just one. The code that handles tuning in the firmware is set_freq() here: https://github.com/mossmann/hackrf/blob/master/firmware/common/tuning.c#L57 To determine the exact frequency you would have to look at that function and the functions it calls to set the frequency of the two mixer/synthesizer parts (and read the data sheets of those parts). It's a bit simpler if you use set_freq_explicit (which is invoked when using certain command line options with hackrf_transfer) because it lacks the fancy automatic IF determination that tries to avoid spurs that can be caused by various harmonics. I think it would be a good idea to provide an easy way to retrieve this information from a running HackRF, so I've added an enhancement to the issues list: https://github.com/mossmann/hackrf/issues/524 One way to accomplish part of that functionality today would be to use the hackrf_debug command to retrieve register values from the max2837 and rffc5072, but you'd still have to read the data sheets to know how to compute the frequencies from the register values. Mike On Sat, Sep 22, 2018 at 07:06:32PM -0700, Phil Karn wrote: > > I would like to determine the *exact* frequency of the HackRF when set > to a given frequency. Like nearly all modern tuners they use > "fractional-N" synthesis. They can only tune to frequency steps that are > often not a simple fraction of 1 Hz, and this step size often varies > with operating band. > > I'd like to have either of two things, ideally both: > > A formula that gives the exact frequency to which the HackRF tunes when > set to frequency 'f'. > > A frequency where the HackRF synthesizer is exactly correct (assuming an > accurate timing reference) and the minimum frequency step that will keep > the synthesizer exactly on frequency throughout the HackRF's coverage > range. (This step size may vary with band as various dividers are > changed, but using the largest step size across the full coverage is the > easiest way.) > > If I know the actual frequency of the synthesizer I can easily correct > for any offset in the software oscillator I use to downconvert the low > digital IF to DC. > > I could probably figure this out by digging into the firmware and the > synthesizer data sheets, but I wanted to first see if someone could save > me the trouble. > > Thanks! > > Phil Karn, KA9Q > ___ > HackRF-dev mailing list > HackRF-dev@greatscottgadgets.com > https://pairlist9.pair.net/mailman/listinfo/hackrf-dev > ___ HackRF-dev mailing list HackRF-dev@greatscottgadgets.com https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
[Hackrf-dev] Precise synthesizer tuning
I would like to determine the *exact* frequency of the HackRF when set to a given frequency. Like nearly all modern tuners they use "fractional-N" synthesis. They can only tune to frequency steps that are often not a simple fraction of 1 Hz, and this step size often varies with operating band. I'd like to have either of two things, ideally both: A formula that gives the exact frequency to which the HackRF tunes when set to frequency 'f'. A frequency where the HackRF synthesizer is exactly correct (assuming an accurate timing reference) and the minimum frequency step that will keep the synthesizer exactly on frequency throughout the HackRF's coverage range. (This step size may vary with band as various dividers are changed, but using the largest step size across the full coverage is the easiest way.) If I know the actual frequency of the synthesizer I can easily correct for any offset in the software oscillator I use to downconvert the low digital IF to DC. I could probably figure this out by digging into the firmware and the synthesizer data sheets, but I wanted to first see if someone could save me the trouble. Thanks! Phil Karn, KA9Q ___ HackRF-dev mailing list HackRF-dev@greatscottgadgets.com https://pairlist9.pair.net/mailman/listinfo/hackrf-dev
