SDR Dongle frequency offset

[bharvey2]

I have several of the $20 820T dongles That I've purchased over the last 18 months from NooElec in an attempt to put together a working P25 trunk-tracking setup for the Alameda County, CA EBRCS System. I can successfully monitor and decode one audio frequency with SDR# and DSD or monitor a control channel with Unitrunker but I can't seem to get two dongles to work together successfully. I'm inclined to believe that it may be an issue with the frequency offsets that need to be determined for each of the dongles.

Using any one of the dongles, I find a known frequency (e.g. a NOAA weather frequency), calculate the difference between the known frequency and displayed frequency in SDR# and enter the offset in the "Shift" entry under the Radio tab. I restart SDR# and everything seems to be spot on. However, when I move up into the T band or 700MHZ band, the offset isn't accurate anymore.

Is the offset linear throughout the range of the dongles or am I doing something wrong? The offset seems to be off by as much as 30% as I move up in frequency with any dongle that I use.

 

[RonnieUSA]

See if this helps You to get it going.

RTL-SDR Tutorial: Following Trunked Radio with Unitrunker - rtl-sdr.com

 

[rbm]

SHIFT is for use with a converter.
So ............... set it back to 00000 and uncheck the box.

Frequency Correction (ppm) is for correcting oscillator variations.
You should be using the 'Frequency Correction ppm)' field in the configuration popup.

It is used to correct the frequency variation of a particular dongle. (They all vary)

Typically, once it's set it doesn't vary much from day to day. But you must do it after the dongle has been powered on for a while.
Then, every time you start it up from cold, it will take a while to settle back down.

There are several ways to find that correction. (after about a 30 minute warm up).

The easiest way is to use the 'pilot' of an ATSC TV broadcast.

That is supposed to be 'exactly' 310 kHz within the TV signal. (ie. Channel 14 is at 470.310 MHz)
Using the highest frequency local channels works the best.

You can see a list of them here. Just check against a few local TV signals because they DO vary slightly.

North American television frequencies - Wikipedia, the free encyclopedia

Rich

Here's an example using that method. If you zoom in on the pilot signal, you can see how much difference just 1 ppm makes.
(you can click through to the full resolution image to see the details)

 

[bharvey2]

RBM,

Thanks for that tidbit of information on the configuration page. I definitely messed that one up.

I was aware that the dongles need to run for a while before determining the offset. I usually run the waterfall for 30 minutes or more and make sure the dongle is warm to the touch before figuring the offset. It still isn't clear to me whether that offset is linear throughout the range of the dongle. Anyone have any feedback on that? I've tested three different dongles and I haven't found one that holds the same offset. Since I've never seen mention of this problem, it makes me think I'm not figuring it out properly.

 

[RonnieUSA]

RBM,

Thanks for that tidbit of information on the configuration page. I definitely messed that one up.

I was aware that the dongles need to run for a while before determining the offset. I usually run the waterfall for 30 minutes or more and make sure the dongle is warm to the touch before figuring the offset. It still isn't clear to me whether that offset is linear throughout the range of the dongle. Anyone have any feedback on that? I've tested three different dongles and I haven't found one that holds the same offset. Since I've never seen mention of this problem, it makes me think I'm not figuring it out properly.

Forget the offset, it's the PPM You want to adjust.
Once set it should be stable a point or 2 either way.

 

[allandunn]

I agree it should be ppm, but I only see a Shift on 1131.

 

[allandunn]

I meant 1331. Don't see a ppm shift. I may have in earlier releases.

 

[RonnieUSA]

I meant 1331. Don't see a ppm shift. I may have in earlier releases.

Look at the top of the tool bar, you will see a Cog, click on that, and that is where You adjust the PPM.

 

[rbm]

I meant 1331. Don't see a ppm shift. I may have in earlier releases.

Red marks the spot!

Rich

 

[marksmith]

RBM,

I've tested three different dongles and I haven't found one that holds the same offset. Since I've never seen mention of this problem, it makes me think I'm not figuring it out properly.

I've tested three of them too. All were different but not a lot different from each other. None were very close to zero.

BCD536HP/HP2/996XT/PSR800/396XT/996T

 

[Voyager]

All will be different, as the crystals are not that high of a tolerance. Mine are generally grouped around the 20s or the 60s with a few oddballs around 40-ish. That's why there IS an adjustment. If they were all the same, there would be no need for it.

 

[bharvey2]

All,

Just wanted to say thanks for the help. Messing with the Offset rather than the PPM correction was my undoing. Once I established the proper PPM correction (and put it in the right place), I could decode Alameda County EBRCS.

 

[Voyager]

Make sure you (re)set the shift back to 0.