OP25 tuning errors

[maredzki]

Why not fire up GQRX and tune to a weather channel and setup your PPM with it? Let it run for about 20 mins. and see where your PPM is after the warm up. I'll run my PPM setup that way to get a value.

Can I contact you via PM directly if I have any questions about the usage of Gqrx to make sure I have it tuned properly?

 

[KA1RBI]

Why not fire up GQRX and tune to a weather channel and setup your PPM with it? Let it run for about 20 mins. and see where your PPM is after the warm up. I'll run my PPM setup that way to get a value.

Unfortunately, this is not sufficient. At 800 MHz a PPM error of 1 is 800 Hz. The slicer can only tolerate an error < 600 Hz (approx.; in FDM P25 symbols differ by a deviation of 1,200 Hz). So, corrections must be more precise than 1 part per million...

Also, separately, the weather channels are in the 162 MHz band where PPM errors are only about 1/5 as severe as they would be in the 800 band...

73

Max

 

[maredzki]

Unfortunately, this is not sufficient. At 800 MHz a PPM error of 1 is 800 Hz. The slicer can only tolerate an error < 600 Hz (approx.; in FDM P25 symbols differ by a deviation of 1,200 Hz). So, corrections must be more precise than 1 part per million...

Also, separately, the weather channels are in the 162 MHz band where PPM errors are only about 1/5 as severe as they would be in the 800 band...

73

Max

Ok, should I just wait for the -X switch to be added to the syntax this weekend?

 

[boatbod]

Ok, should I just wait for the -X switch to be added to the syntax this weekend?

It's really not that hard to tweak the tuning manually. Simply open the 'mixer' plot (key #5) and look at the signal hump that should be centered on the tuning line. Adjust the -q parameter (by +/-1 each time) to move the hump into the center.

It should look similar to this:

 

[boatbod]

Code is pushed to the boatbod repo.
See op25 frequency tracking

 

[maredzki]

Cool, pulled it, but wasnt able to rebuild, so I pulled it and just installed it as a fresh copy.

It works perfectly now even though I do get a few tuning errors and a few PCM underruns.

 

[boatbod]

Cool, pulled it, but wasnt able to rebuild, so I pulled it and just installed it as a fresh copy.

It works perfectly now even though I do get a few tuning errors and a few PCM underruns.

You may see a tuning errors initially as op25 establishes a lock on the signal and sorts out the needed adjustments but after that it should remain fairly stable. Depending on your hardware there may be intermittent small adjustments in fine tuning (which you might notice on the terminal screen) but it really shouldn't issue any subsequent +/-1200/2400hz tuning errors. Control Channel timeouts and PCM underruns are an unfortunate fact of life, especially if the signal strength is a little on the marginal side.

 

[maredzki]

It's really not that hard to tweak the tuning manually. Simply open the 'mixer' plot (key #5) and look at the signal hump that should be centered on the tuning line. Adjust the -q parameter (by +/-1 each time) to move the hump into the center.

It should look similar to this:
View attachment 81490

After this hint, I was able to quickly fine tune the offset and in my case it was -500.

 

[logan437]

WOW - that really works great ! Thank you soooo much.

 

[boatbod]

After this hint, I was able to quickly fine tune the offset and in my case it was -500.

500 is a little on the high side for a fine tuning adjustment. It might be worth adjusting ppm up or down by 1 and then re-tweaking fine tune.

 

[maredzki]

500 is a little on the high side for a fine tuning adjustment. It might be worth adjusting ppm up or down by 1 and then re-tweaking fine tune.

ppm being -q value?

 

[boatbod]

ppm being -q value?

Yes that it correct.
General theory is to make the gross adjustments by varying ppm (-q) then finish off with fine-tune (-d). Typically you shouldn't need more than about +/-350 of fine tune.

 

[maredzki]

Yes that it correct.
General theory is to make the gross adjustments by varying ppm (-q) then finish off with fine-tune (-d). Typically you shouldn't need more than about +/-350 of fine tune.

Ended up with -q -1 -d 300. I think this could be the optimal setting.