First would be to use your favorite SDR software suite , tune to a known frequency and calibrate the RTL against that known frequency. Usually the correction factor will be around -50 ppm.
The other way is to use Unitrunker and tune your dongle where the park frequency is on a known, such as your local NOAA weather broadcast; even a control channel frequency will work and adjust the correction factor to where the tuned audio sounds the most clear and that will get you close. To decode P25 however, you need to be pretty spot-on.
Have you been able to decode the control channel data to this point?
Okay. First things first: be aware that the stock antenna is cr@p! You will probably not achieve the desired results with them.
Being that these dongle are super cheap, tuning them to a given frequency does not mean that the dongle is actually tuned there. They all have different correction factors, but most seem to hover in the -50 ppm range. Be aware that this factor will drift as the dongle heats up, so it's best to calibrate after it has been running for 5 minutes or so.
First try to decode a control channel (you gotta walk before you run). Try to post a screen shot of the whole signal receiver window. You may need to check the baseband AGC and/or the auto gain box to get a useable signal. Also deemphasis should be unchecked (or checked - can't remember off the top of my head). Start there and see how it goes.
Also, if you look at the scope, you should see some form of "organized" waveform. This is for a 3600 baud SmartZone system, so if you are listening to a P25 or other system, the waveform will look different, but still organized.
Each dongle IS different as my two are 6ppm and 33ppm so you need to find what your unique number is, after about 30 minutes and then got from there. As for the screenshot, make the following changes to the "Signal" receiver:
- Uncheck Auto Correct
- Uncheck AGC
- Squelch to 0
- Change Audio Output to Soundcard and Digital to Unassigned or blank (DSD decodes the Voice, not the Signal receiver)
- Check Mute (after you establish a signal)
Let us know how it all turned out so we can troubleshoot further.