Lock status and differial in SAM.

[MisterJ]

I like to use SAM for calibration and yours seems to work well as I can hear the PLL zero beating. Please provide a little window or place it with FFT information showing lock status and differial. Thanks, John.

EDIT: This is similar to SDRuno which is better than all others I have used.

[jdow]

For calibration of what? For front end frequency calibration is this better than setting DSB on WWV and listening for the zero beat? And that is not a very good calibration; but, better than any other HF stations. For VHF and above your best calibration depends on the country you are in. This is usually a fixed frequency pilot tone broadcast as part of the digital signal for GSM or ATSC TV. You'll have to dig on the web for the actual calibration frequency if nobody else posts them here. I don't have them save here. I use GPS for calibration.

For reading off a station's frequency, once the above is accomplished, for AM the DSB trick is good. For FM, it's ugly. You generally must wait for silence and use the FFT with zoom to catch the carrier frequency. Over the decades I've been a ham and professional radio communications design engineer the spectrum scope and the DSB beat note are the two quickest reads. I don't trust PLLs in SDRs. All the source codes I have seen have a math underflow issue that causes them to work but in the wrong mode. So frequency errors can exist. They also lock slower than they should; but, that's hard to notice. A correct loop is fun to watch as it locks on instrumented loops.

{^_^}

[MisterJ]

Thanks, jdow. I like to calibrate at a frequency close to the what I am going to use. Using Leo Bodnar's miniGPS Referenced Clock creating a signal say at 800 MHz for Hydrogen line then calibrate using SAM and watching the differential and lock status, I can get a really good calibration. SDRuno is the best current SDR for this but does not offer lock status. I hope SDR-Radio will consider my Request. Thanks and enjoy, John.

[jdow]

(Error analysis widget stuck in my head keyed in. I can't help it.)

How accurate are you trying to get. In the world of the physics class SDR implementation with (near) infinite precision and resolution and one single oscillator in the front end you should be precisely on target if you pick a frequency your GPS widget can generate (with perfect accuracy of course) then calibrating at as high a frequency as feasible is your best shot. That accuracy carries through the whole SDR setup, within its specific design limits. For example double precision math is not perfect. It is good to a not quite absurd number of N in "parts per N". But it is limited. And that limitation propagates through the math. It is also rather disconnected from the actual values involved, when speaking PPN rather than Hz. And I'd not trust the SAM tuning indicator to better than about 1 Hz unless I had the PLL code to run in a test bench I have built myself.

Moving out of the classroom into reality we face some nasty real world problems.

We got into that SAM PLL. So let's worry that bone a little. Let's consider that a working 2nd order loop will correct a frequency error to zero with a small phase offset. If the signal is shifting in frequency then there will be a frequency error and a lag to consider. But enough of that. Those may be small enough effects to ignore.

Have you considered that most front ends do not step frequencies in 1 Hz steps except by accident? Unless the sampling rate is a power of two times 1 Hz you won't be able to get exactly 1Hz steps. Furthermore most front ends have step sizes that may be significantly larger than 1 Hz. So you get sample rates that are off by an unknown amount on frequency settings that are themselves off by some unreported amount. On top of that the errors depend on complex (as in funkity) math. The actual frequency might involve two variables one dividing into the other with the result multiplied by the reference frequency. Errors will vary all over the place, small but well more than 1Hz in most cases. Unfortunately the front ends do not report back actual calculated sample rate and tuned frequency. So you cannot mathematically back out this error. And even tuning a small distance can leave you considerably off in a different direction. So SDR accuracy is severely constrained for something that is for general purpose uses. Signal levels, frequencies, and such are useful approximations rather than the real thing.

I wonder if any of that discussion above helps you understand some of the freaky results you might be getting in your Hydrogen line tests. I've been interested in precision frequency work since I can remember in my ham and professional careers, both creating precision and very precise errors. (roughly 0.2 PPT steps.) BTW, if you can get the details of the frequency synthesizers involved you may be able to back out the errors I discussed above with post processing. I'd not hold my breath. The only chance of that is the AirSpy R2. I know the frequency synthesizer calculations. And I know the sample rate synthesizer chip. And it is phase lockable to GPS. So in theory - I lack patience these days. {^_-}

It would be nice if Simon reported that value. But, it is not really going to be meaningful at GHz frequencies and the front end tuning accuracies offered by the various front ends.

{^_^}

[Simon G4ELI]

I like to use SAM for calibration and yours seems to work well as I can hear the PLL zero beating. Please provide a little window or place it with FFT information showing lock status and differial. Thanks, John.

EDIT: This is similar to SDRuno which is better than all others I have used.

Hi,

Looking at this it is, in theory possible. Just wondering about coding effort.

[MisterJ]

Thanks, Simon. Hope you will implement my request for Lock Status and LO Differential. Thanks, John.

[MisterJ]

Thanks, jdow. I am not trying to change the world or establish a new frequency standard. I am simply asking to see the SAM reference-to-LO differential and lock status. Thanks and enjoy.