Dual SDR Channel Lock to Control Channel Frequency

[taylor...@gmail.com]

Been searching everywhere and cannot seem to find an answer. I am not sure if this is an option or could be an option. I had 2 SDR's running in SDRTrunk. I am trying to Monitor a single P25 Phase 2 System ( https://www.radioreference.com/db/sid/8677). The main control frequency is 771.11875, with backup control channels of 770.15625, 770.40625 and 770.86875. There are an additional 6 channels for voice and data 769.65625, 769.90625, 771.61875. 773.30625, 773.59375 and 774.34375. Now i know one SDR would not cover the entire frequency range so I have 2 trying to cover it at this time. My problem is I have no way of locking down one SDR to stay within the control channels frequency range,, and whenever a call comes in to the upper ranges like 773.59375 and 774.34375 the SDR monitoring the Control channel jumps to that frequency and then looses the Control frequency until the call is done. I was hoping there was a way to set one SDR to cover the lower end frequencies including the control channel and have the 2nd one cover calls within the upper range.

[Ben Saladino]

It depends on your SDR. A single AirSpy R2 will cover it, or two RTL-SDRs set to 2.5Mhz width will cover it. In any case, you can set up 2 channels for each SDR to cover the lower and upper frequencies of the range you want to set them to. These two channels are in addition to the the channel that is set up to cover the trunked system you're trying to monitor. It looks like the screen shots below.

I hope that helps.

Ben

[taylor...@gmail.com]

Ok i have tried this and it still seems as tho the SDR device that is monitoring the control frequency follows the voice channel to the high end and we loose the control frequency for a few seconds till the end of the voice call

I will try and get a few screenshots of how i have it configured and upload them here.

[taylor...@gmail.com]

Here are the screenshots of what i currently have setup.

[Eric F.]

I have similar behavior. I monitor two P25 systems, one at a time, using RTL blog dongle. Each has frequencies too far apart for the dongle to tune them all, so be it. One stays locked on the control channel. The other however does not. When a call occurs too far below the control channel I get the 'fade' indicator and the call is missed. But when the call is too far above the control it drops the control channel and tunes to the call. The it will rotate through frequencies looking again for the control channel. Both systems are using the same settings, frequencies of course and aliases are different. I thought maybe it was related to the selection of a preferred tuner or not but I cannot alter the behavior by altering this setting. Perhaps it is related to the 'distance' of the voice channel.

System 1, which behaves as expected
control channel 773.33125

highest freq. 774.83125 for a difference of 1.5 which the dongle handles

lowest freq. 769.90625 for a difference of 3.425 which the dongle cannot reach. 

Of course this shifts around as needed to tune as much as possible. On this system using 2.56 bandwidth it will skip 3 frequencies and at 2.88 which doesn't perform as well due to noise and images but it usually acceptable it will only skip the lowest frequency. But it does not drop the control channel.

System 2, does strange things. This system rotates control channels and is much more difficult to troubleshoot.
control channel presently 855.3075

lowest is 851.275, difference of 3.765, this and the next lowest will be skipped, 'fade' is indicated

highest is 859.4125, difference of 4.375, this and the next highest would normally be skipped but instead the control channel drops. If I try to start a couple conventional channels at 858 it will also drop the control channel and include the conventional in scanning.

[Eric F.]

Testing further, this is in Het mode. In Polyphase mode I receive the "no tuner available" message. In Heterodyne mode it will accept and attempt to tune whatever I tell it to.
If for example I'm monitoring a P25 system and play anything else:
If out of range less than whatever the tuner is actively doing it simply shows a gray line of idle and it continues on normally with a frequency it just cannot tune. It will at times show the unreachable frequency as a control channel or do some other odd things. 

If out of range by 3.59 MHz above the tuned frequency it goes bananas, dropping the control channel, displaying any of the programmed frequencies, sometimes showing others as control. Sure, I'm asking the program to do what it can't but it handles it badly. This poses a problem if the system you're trying to monitor does the same as does my previous "System 2".

It does not matter whether or not any items have a preferred tuner selected.

[taylor...@gmail.com]

I am getting the exact same issues. I find that what ever SDR is capturing the Control Channel when it attempts to tune a Voice Channel that leaves the Control Channel outside of the SDR's range instead of just tuning the other tuner that is free to the voice channel, the control channel SDR follows it instead and then I drop the control channel till the voice call is complete. 

It would be nice if we could designate a Tuner that specifically stays with the control channel and have the other one do all the voice channels, or have a a way of specifying the range of each tuner so it does not jump around .

[Ben Saladino]

Are you sure that both tuners are staying enabled? Maybe there's a USB issue where even though you start with both tuners enabled, one of them locks up or goes to sleep after a while.  Also, are the serial numbers different for each RTL-SDR? If they are both set to serial #1, SDR may not be able to tell them apart.

[taylor...@gmail.com]

The Serial Numbers are different, and from what i can see looking through the looks im not seeing any disconnects from the either SDR. My First sdr has 3 Locked Channels (Lower and Upper Range, and the Control) and the 2nd has 2 Locked Channels (Lower and Upper Range). The 2nd one never changes, I never see any activity on it. The first one will show all the activity, for the most part it works fine but when there is heavy radio traffic it will start to go out of range of the control channel and move around the spectrum.

[DaveNF2G]

I don't lock channels on my SDRs and I have never seen this happen.