Is it normal for an RTL-SDR stick to suddenly need a different tuning offset?

[Papagei]

Hello all!

tl,dr: My circa-2017 RTL-SDR stick has been running well under OP25 since fall 2019 with a +400 Hz offset. A couple of weeks ago, the +400 Hz offset stopped working very well; I changed it to +1200 Hz and it works better. Is it normal for the "best" offset to suddenly change like that?

I bought an RTL-SDR stick around 2017. I played around with it when I first got it, listening to various things. I didn't have any P25 software for it, so I eventually put it back on the shelf.

A couple of years later, I got it down again, and set about getting OP25 working. At first I didn't understand how setting a tuning offset would help; when I started OP25 with a 0 offset, I would hear traffic, but after a while it would die down. After a few weeks, I figured out how to use the plots in OP25, in particular plot 5, "mixer balance". I got the best results (numbers closest to 0) on that plot with a +400 Hz offset, and if I set that as soon as I started OP25, I could let it run continuously and not suffer a loss of traffic. I let it run that way, more or less 24/7, from fall 2019 onwards.

A couple of weeks ago, I had to restart OP25 on my PC. I did so, applied the normal +400 Hz offset, but wasn't getting much traffic. I eventually pulled up plot 5 again and noticed I had rather higher numbers for "mixer balance" than I previously had, like 80-150 or so. I played with the tuning and an offset of +1200 Hz got the mixer balance back down to 0-30 or so, and resulted in a steady flow of traffic.

The thermal environment for the stick hasn't changed much. I have a USB hub with about a 2 foot cable plugged into the back of my PC, and the stick plugged into that hub. It's sitting on the desk behind my monitor. I run the furnace in the winter and the A/C in the summer, and the stick has been through a couple of seasons now, without needing a major tuning change.

As far as I remember, I haven't dropped the stick, or dropped something on the stick, or otherwise given it a mechanical shock. There wasn't a thunderstorm or power outage or something that would tend to feed it a voltage spike around that time. It just suddenly wanted a different tuning offset one day.

Is that normal? Or has something odd happened to my stick?

Thanks!

Papagei

 

[kb8wna]

I've run -10kc offset all the time.

 

[boatbod]

To the OP:
- by "offset" do you mean the fine tune adjustment (-d) or the tuning offset adjustment (-o)?

Have you recently changed/upgraded the librtlsdr driver?
Is there any change the physical radio (tower) infrastructure might have ben upgraded/changed?
If nothing has changed, perhaps the rtl device is beginning to fail?

 

[Papagei]

- by "offset" do you mean the fine tune adjustment (-d) or the tuning offset adjustment (-o)?

By "offset" I mean the one that you can adjust with the , . < > keys (-/+ 100 Hz, -/+ 1200 Hz respectively ) in the curses interface provided by rx.py. These keys cause the third line from the bottom of the screen to change from something like "851.325000(0)" to "851.325000(1200)".

I don't think I have changed librtlsdr for several months. I checked just now and I am running librtlsdr.so.0.5.3, dated 2016-10-08.

There is a low, but nonzero, chance the towers have changed. I only listen to two towers on the same P25 system (MARRS, in the Kansas City area), and as far as I know, their tower configuration has been the same for at least a few years now. I had heard earlier in 2021 that they were preparing to move to Phase II, with narrower channels, but I don't know if they've actually made that move yet or not.

It's possible that the tuning offset change coincided with either or both towers changing their control channel. When I first set up OP25, I only put in one control channel for each tower. That worked well, until one of them changed control channels. After that, I put in all four control channels for each tower in the "Control Channel List" column in trunk.tsv . As far as I know, the control channel changes aren't logged, so I can't go back and tell if one of them changed at around the time I had to change the tuning offset.

If nothing has changed, perhaps the rtl device is beginning to fail?

That's why I was asking... I didn't know if the offset change was a prelude to failure.

It's always run somewhat warm to the touch; I can stand to put my fingers on it, but it's noticeably hotter than (say) your average USB flash drive.

With the new offset, it seems to work as it always has. The audio sounds the same as before and isn't choppy/intermittent or distorted. OP25 doesn't randomly crash or otherwise report trouble with the hardware.

Thanks!

 

[wgbecks]

I think you likely have it nailed down to ageing of the SDR, or perhaps a component is beginning to fail. However, these devices are rather voltage sensitive (frequency wise) and it's not uncommon for USB extension cables, jacks and hub devices to contribute to voltage fluctuations over time. I have found that even a somewhat loose plug/jack connection will increase I2R losses that may account for additional voltage drop across the connection.

I have had decent luck stabilizing USB operating voltages by cleaning and tightening contact pressure across the junction of the SDR plug and USB jack, by avoiding use of USB extension cables, and by avoiding external USB hubs if at all possible. It's not at all uncommon to move any given RTL SDR device from one computer or Raspberry Pi to another and end up with different frequency corrections with all else remaining identical.

One last thought is the operating temperature of your SDR. Has anything change in the environment of the SDR from previous use to your most current use and subsequent need to re-compensate the device?

 

[jonwienke]

Get a SDR with a TCXO, and you avoid most such issues.

 

[boatbod]

By "offset" I mean the one that you can adjust with the , . < > keys (-/+ 100 Hz, -/+ 1200 Hz respectively ) in the curses interface provided by rx.py. These keys cause the third line from the bottom of the screen to change from something like "851.325000(0)" to "851.325000(1200)".

If the fine tune offset has to change and you haven't altered the tuned frequency it sounds like the RTL hardware is no longer tuning the same as it was. Could be temperature, but these things run hot, so I'm inclined to agree with @wgbecks that it's age related. That said, I've got dongles approaching 5 years of 24x7 use that are as good now as the day they were purchased. Hard to say... keep tweaking the tuning until it no longer works then buy a new one!

 

[Unitrunker2]

1200 hertz offset corresponds to a warp value of 1.2 ppm. Try adding 1 or 1.2 to your current warp settings.
The previous 400 hertz offset suggests your warp value was off by about 0.5 ppm.

 

[awasser1]

have you done a git pull in awhile? and maybe -X (autotune). that works great for me at least.

 

[Papagei]

Thanks for the responses! I am replying to several people at once.

I think you likely have it nailed down to ageing of the SDR, or perhaps a component is beginning to fail. However, these devices are rather voltage sensitive (frequency wise) and it's not uncommon for USB extension cables, jacks and hub devices to contribute to voltage fluctuations over time.

I run the USB hub just because the antenna cable isn't quite long enough to allow the antenna to sit in my window and still plug the RTL-SDR directly into the PC. In the past, I did try plugging the RTL-SDR directly into the back of the PC, and moving the antenna closer to the PC, but I felt like the received signal strength went down somewhat when I moved the antenna out of the window.

I have absolutely everything I need in the way of F connectors and 75-ohm coax, and even a few BNCs and some 50-ohm coax floating around. My habits have not yet acquired any extra cables with SMA connectors. If I get one of those, I'll try plugging the RTL-SDR directly into the PC, and putting an extension on the antenna cable.

One last thought is the operating temperature of your SDR.

Nothing has really changed; it's indoors in my office, sitting behind the deck of my PC. I did start running the air conditioning about two weeks ago, but that wasn't when the tuning change was required.

Get a SDR with a TCXO, and you avoid most such issues.

I think my radio has one. It says "DVB-T + DAB + FM + SDR" and "RTL2832U R820T2 TCXO" on the front. The QA sticker is marked 2015-12.

Hard to say... keep tweaking the tuning until it no longer works then buy a new one!

I like this answer the best. Mostly what I didn't want to hear is "oh yeah, if you have to change the tuning offset, then in three weeks it'll die completely", and it doesn't sound like that's the case.

1200 hertz offset corresponds to a warp value of 1.2 ppm. Try adding 1 or 1.2 to your current warp settings.

I don't think I have a warp setting. I don't even think I have an impulse setting! My copy of OP25 is from September 2018, so that may be why.

have you done a git pull in awhile? and maybe -X (autotune).

I haven't done a git pull lately. I have hacked on my copy of OP25 somewhat, but I was a bad developer and didn't use any source code control. If I pull a new version, I'll have to spend some time re-implementing my changes on the new version. My copy is from September 2018 and doesn't seem to support the -X option.

Papagei

 

[boatbod]

I haven't done a git pull lately. I have hacked on my copy of OP25 somewhat, but I was a bad developer and didn't use any source code control. If I pull a new version, I'll have to spend some time re-implementing my changes on the new version. My copy is from September 2018 and doesn't seem to support the -X option.

Oh good lord that's old!
What were the changes you made? We've implemented a lot of new features since then.

 

[Papagei]

What were the changes you made?

I patched trunking.py to add the ability to associate a name with each RID, similar to how you could already associate a name with each talkgroup. It uses a new TSV file to list each RID and its associated name.

I patched terminal.py to use desktop notifications (under X), to have a text log file, and to log the RIDs and names to that log file.

I sent these patches to your email in early July, 2019, but I don't know if you got them or not.

Papagei