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RTL Dongle - how to fix PPM drift?

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#1
OK, this is driving me nuts. How do I keep this RTL dongle from drifting around? On my feed I have to keep adjusting the PPM value in order to decode the control channel and digital voice accurately. It wants to be anywhere from 68 to 70 ppm, depending on, well, who knows? The alignment of the planets? The position of the moon? The time of day? I have no idea. I set it, it works for anywhere from an hour to a day or so, and then it gets out of whack again and I have to keep changing it. As a result, either the feed sounds like crap at times or worse, there's no sound at all for hours on end while I'm at work or asleep. VERY frustrating.

I'm ready to fix this. Do I need a new dongle, or can I do something to this one to fix it? It's the NooElec dongle - not sure what the model number is but I can post that later today. Just wondering if anyone else has run into this and if so what worked to fix it.

Thanks guys (and gals),
-AZ
 

jfr

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#2
I think the drift might be temperature induced. My settings change from cold start vs warm dongle (hehehe)

-JR
 

rbm

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#4
It takes mine as long as 40 minutes or so to 'stabilize' from a cold start.

After that, they may drift one or two ppm at the most.

You can alleviate some of the effect on your decoding by 'slightly' widening your 'Radio' filter bandwidth.

Rich

 

rbm

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#6
I have nine dongles and to keep from having to change the ppm etc every time I use one or another of them, I've just created multiple copies of the SDRSharp folder. ((And corresponding shortcuts.)

If you look at my desktop in the image, you'll see shortcuts for each instance of SDRSharp across the top.

I've numbered and labeled each dongle with the proper ppm for it.
They run from around 14 ppm to around 90 ppm.

At times, I've run six of them at one time.
So, for example, I can 'watch' around 12 MHz of SatCom spectrum at the same time.
Or MilAir or whatever my interest is at the time.

I also have one folder/config setup for digital modes.
That already has VB-Audio CABLE all set up and ready to go for DSD+, MMSSTV, WXtoImg, etc. so I don't have to play around to get decoding going.

Each SDRSharp folder contains around 200 KB or so.

Rich

 

KC1UA

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#7
Someone was/is selling the modified dongles, but as I recall they were "pricey" by comparison to the standard R820T's.

I will definitely own an airspy when they become available but I have a feeling it's going to be several months before that happens.

Most of mine stay on target pretty much once I let them settle. I run two with UniTrunker all the time at work (nice show and tell item too) and they sound fine, although it's a 3600 baud 99% analog system so there's more tolerance.
 
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#8
If the dongle has an issue where it can't stabilize after 40 or so minutes, I'd say it's fair to get an RMA replacement from nooelec.
I have 2 nooelec dongles that stabilize after 7 minutes, they start at 61 and drift towards 57 where they stay for as long as they're powered.
 
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#9
I also have two sticks purchased from NooElec, one is labeled "NooElec" on the body, the other isn't, but I've verified that internally they are exactly the same (they both have the diodes on the antenna for ESD protection which the NooElec versions all do unlike some of the really cheap RTL sticks that don't). One has a 56 ppm drift after about 5 minutes and the other is 49 ppm; once they reach those points of drift they don't typically change UNLESS - and that's the big point here - I stop actively monitoring actual content meaning I may tune to a dead spot "on the dial" and the sticks aren't actively creating content. If I do that then the stick cools down some since it's not working nearly as hard.

Sounds weird but it's true: tune to a known persistently active frequency (NOAA weather is perfect for this, or a nice clear ATIS signal from an airport perhaps) and let it sit for 15-20 minutes, check the temp of the stick with your fingers (or maybe you're one of those people with a bunch of geek gadgets and you have one of those infrared laser thermometers or something to measure surface temps from a distance, who knows) - the stick will be somewhat warm because it's been "busy" pumping out actual data the whole time.

Now tune off to dead air space with no signal (squelched so there's no static either) and give it 5 minutes or so - the stick will cool down and the ppm shifts as expected.

But I do agree with what tylerwatt12 just said: if the stick just never stabilizes at all either a) it's defective to a point or b) you're in some environment where the physical temp of said environment fluctuates so much it's affecting the stick itself (airflow, fans, vents, air conditioning - in January no less) - or something else could be going on.
Yes, there is a TCXO (Temperature Controller Oscillator) mod for these RTL sticks available but it's tricky to get it done if you're not really expert at soldering/desoldering, especially with solid state circuits which are known to be temperature sensitive (no pun intended, I mean heating one up with a soldering iron too long could damage/destroy it entirely). There's even a guy (in Japan, iirc) selling the pre-modded RTL sticks for like $50+ (and shipping charges too) so, while it would be awesome if every RTL stick had a TCXO already on it, I have no issues with the 49/56 ppm shifts because I keep the sticks busy more often than not.
 
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#11
~$91 USD... good lord, I thought it was available from some other distributor for around $60-ish, geez. I suppose some folks would buy it, however, based on their needs. So far once I figured out the ppm for these sticks, it's never been an issue since for me.
 

KC1UA

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#12
~$91 USD... good lord, I thought it was available from some other distributor for around $60-ish, geez. I suppose some folks would buy it, however, based on their needs. So far once I figured out the ppm for these sticks, it's never been an issue since for me.
I think we've been spoiled by the el cheapo price of these devices, but I agree, that's $$ better spent on something a little more robust like Airspy.

All of the dongles I have seem to settle fine after no more than 5 minutes, and at most I've only had to adjust them by 2 PPM thereafter, usually if I switch bands.
 
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#13
Re rbm's post a couple up, having 6 running at one time might be good thing today giving off a little extra heat in the house. Currently 4 outside with -14 wind chill.
 

rbm

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#14
Re rbm's post a couple up, having 6 running at one time might be good thing today giving off a little extra heat in the house. Currently 4 outside with -14 wind chill.
It got down to -15 here last night.

I don't even turn the heat on in my den.

I run seven computers 24x7 and more than 40 scanners and that keeps it around 66 degrees in here on the even the coldest days. ;)

Rich
 

rbm

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#16
I hereby declare you King of the Scanning Universe!! ;-)
You're too late.

My wife has wanted to 'crown' me more than once for all the scanners. ;)

Besides the scanners in my den, I have one in the kitchen, and one in the living room.

She tells me that she doesn't like them, but .....................
She's always saying ................. "Did you hear what just happened on the scanner?"
Sometimes I think she's listening more than I am.

Rich
 
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#17
It got down to -15 here last night.

I don't even turn the heat on in my den.

I run seven computers 24x7 and more than 40 scanners and that keeps it around 66 degrees in here on the even the coldest days. ;)

Rich
Damn!!! You da man :)

Hate to be paying your power bill :)
 
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#18
An update:

After trying just about everything I could think of, I found a solution that (so far, knock on wood) seems to be working. But it's very, very weird. I found that if I set the mode to WFM (yeah, I know - weird, right?) and the filter bandwidth to 15K, it seems to stay stable enough to at least continue to function without me having to constantly adjust the PPM setting. Sounds pretty good, too, surprisingly. I need to optimize DSDplus again to account for the changes and then it should be spot on, but if you give the feed a listen now, it's not too bad. I'd say I'm getting about 90-95% decoding right now, which for this system (LSM simulcast) is huge. I dare say the performance I'm getting out of this little dongle on this system with the settings I'm using in SDR# and DSDPlus will rival my 436 whenever it finally arrives (ugh.. don't get me started on THAT).

Anyway, I thought I would share my findings with anyone else who might be dealing with the same issue.

-AZ
 
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#19
Does anyone know how to write plugins for SDRSharp?

Imaging there was a plug in where any signal centered in the waterfall display (or close), what ever highest point is centered automatically by ppm.

Or something like it.,?

Just saying, it sounds not that hard. And would solve a lot of problems and not having to buy the new rtl stickTCXO controlled.
 
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