I'm talking about reasonable expectations. Why have a 100 Hz resolution on a device that will drift 10 times that amount? You will never be able to keep it on frequency. It's as useful as the 1/10th cent piece when everything costs an even amount of cents (except fuel).
Now, if you used an external reference to keep the oscillator on frequency, that might work. That would also make a 100 Hz step useful. But, the dongles don't have provisions for external oscillators.
The objective is not to stay on some arbitrary frequency, like [
insert some number here], but to stay centered on whatever signal you're trying to receive.
OK. Saw the video now. I see it's adjusting the ppm error, but I don't see on the video where it is showing how much.
From the video description:
Dynamic frequency correction (in Hz) is displayed in spectrum window titles.
When the PPM value is set 10 below the correct value, the tuning eventually warps by 8 kHz to get back on center. Dongles drift slowly, so TRUNK88 corrects slowly.
Still, warm-up drift is something other than we have discussed above, and I have said many times that warm-up drift is significant until the unit reaches operating temp. I just let it warm up. After that point I've never seen any drift that would affect decoding.
That generally works for me, but for some users, it doesn't. With a laptop or netbook, I could see where a dongle plugged in near the CPU fan exhaust holes could be affected when the CPU load goes ballistic (hello SDR#...)
Some users have reported that wrapping the entire dongle exterior in foil greatly reduces drift. Not something I'd want to do or have had need to do.
In fact, what Trunk88 is doing you could do yourself with the ppm adjustment. And again, even the TCXO dongle will behave the same way.
Not really. TRUNK88 adjusts in 200 Hz steps, while at 800 MHz, a change of one PPM is over 800 Hz. Adjusting PPM will not center the demodulated audio - it will have a noticeable DC shift.
Also of note is that you are not putting the dongle on-frequency - you are setting it to the same error as that single transmitter.
Which is what you need to do to have your bandpass filter centered on the signal. It's critical when dealing with closely packed NXDN4800 signals (got lots of them here)
If you use the dongle on another transmitter, the adjustment may have thrown it farther off than it was when you started.
When monitoring a single system, like a Moto TRS or a NEXEDGE TRS, I assume that all of the site transmitters are running off of the same reference oscillator, so not an issue. Also, nothing says that a fine tuning system can't maintain a per-channel correction factor to handle off-frequency transmitters. And I've rarely seen such transmitters, so not really much of a concern.
Still, interesting that Trunk88 can do what it's doing, and useful in place of manual ppm adjustment. Is it tuning in 100 Hz steps or 1 kHz steps? (or something else)
If you watch the spectrum window titles, you'll see it's 200 Hz steps. You can see the effects of each step on the CC audio's DC bias.
And yes, if a plugin is developed that does the same thing, it will be useful provided it can handle going from a - 1 kHz error TX to a + 1kHz error TX, and won't try to tune adjacent frequency signals.
+/-1 kHz doesn't sound like enough to cause problems. You'd start only 2 kHz off from the target signal. Any adjacent signal would be considerably farther away, so the centering algorithm will move towards the correct signal.
Still, if it can only tune in 1 ppm increments, you will still be off on average half a ppm, or +/- 250 Hz at 500 MHz.
No need to touch PPM. Just adjust the tuning to get on center. PPM is so ghetto.
