Setting the PPM

When using SDR Sharp and the RTL dongles,I have noticed that the PPM setting has to be set for scanning different frequency bands,I thought one PPM setting was all that was necessary, but apparently not for my dongles. I was just wondering if anyone else has noticed the same.

I have 2 RTL R820T2 and 2 R820t dongles and I find that when scanning, I have to change the PPM for each band. So if I am scanning the low VHF and go to UHF, I have to up the PPM buy at least 10 to 15 ppm's for either the R820T or the R820T2. This as far as I know only involves the cheaper $20 dongles.

MB

I try to calibrate at the highest frequency possible. Then, all the lower frequencies fall into place.
However, there is some drift for the first 20 minutes or so when the dongles are first turned on.



Here's the method I use:
http://forums.radioreference.com/software-defined-radio/294056-frequency-correction.html#post2213388

Since you're up north, there may be another set of frequencies you can depend on being accurate enough for calibration. (GSM etc.)

Rich

Edit:
Here's an alternative calibration method using GSM as the frequency reference:
http://forums.radioreference.com/so...quency-calibration-reference.html#post2262579

You can download the Kalibrate software at this link:
http://rtl-sdr.sceners.org/?p=193

The procedure I use is here: (however, I haven't used it since I started using the 'ATSC TV' pilot tones)
The link in that old post for Kalibrate is dead. Use the one above.
http://forums.radioreference.com/so...275611-calibrating-sdr-sharp.html#post2050418

+1 on what RBM said. The PPM calibration should not change on the same dongle.

This is how a 'typical' dongle drifts over the first 12 minutes from a cold start.

This specific dongle requires about 47.5 ppm correction. Which can't be selected.
(47 ppm and 48 ppm are off by the same amount on either side of center.)

Total drift is close to 7 kHz.

Using SDRSharp v1430

Rich

rbm said:

This is how a 'typical' dongle drifts over the first 12 minutes from a cold start.

This specific dongle requires about 47.5 ppm correction. Which can't be selected.
(47 ppm and 48 ppm are off by the same amount on either side of center.)

Total drift is close to 7 kHz.

Using SDRSharp v1430

Rich

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Hey Rich good to hear from you again, just got home and fired up the computer and dongle, I will let it warm up and then hunt for the local OTA tv station about 5 blocks from my home. If that doesn't work I will download Kalibrate software, that looks like a pretty handy tool to have.

I had no idea that the drift from a cold start to warm up was so dramatic. I would assume that dongles with the TXO crystals don't have that problem of drift going from a cold start to warm up? If that is so I will have to update to one of them.

Thanks for the information.
MB

The TCXO models obviously are much better in terms of drift but since I leave my RTL sticks plugged in 24/7 it's rarely an issue. I have been considering getting two new ones, those from RTL-SDR.com that are aluminum shielded with SMA-F connectors (and with antennas for $24.95) but that of course would require me to buy new pigtails as well - while they do come with slightly better (and adjustable) antennas they're still crap in the long run because of no ground plane and just cheap low-end construction. Was thinking about buying SMA antennas to attach directly to the RTL sticks themselves and negate any potential cable losses completely, seems like a more practical solution I suppose. I use 12 foot shielded USB extension cables with RF chokes on each end to cut down on noise as well and the newer sticks with the aluminum shielding surely are better in that respect as well along with the slightly more sensitive R820T2 tuners.

I did a similar graphic in the past like the one rbm posted above, was interesting to note from a cold start just how far off that spectrum/waterfall line curved as it warmed up. My sticks are at 49 and 55 ppm but I learned that a few hours after first using them a long time ago. Probably the best ~$20 I think I've ever spent on actual hardware for monitoring but I think it's time for an upgrade soon to the TCXO models.

moonbounce said:

Hey Rich good to hear from you again, just got home and fired up the computer and dongle, I will let it warm up and then hunt for the local OTA tv station about 5 blocks from my home. If that doesn't work I will download Kalibrate software, that looks like a pretty handy tool to have.

I had no idea that the drift from a cold start to warm up was so dramatic. I would assume that dongles with the TXO crystals don't have that problem of drift going from a cold start to warm up? If that is so I will have to update to one of them.

Thanks for the information.
MB

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Hi moonbounce.
With an OTA TV station that close, you have your own, private, calibration standard.

Kalibrate works well, but it takes more time.
I added the local pilot frequencies to frequency manager and I can calibrate any of my dongles in just a minute or two. (after they've warmed up a little.)

I have around a dozen dongles, and they vary from 12 ppm to 85 ppm.
Once calibrated, they pretty much stay at the same place.

I put a label on each with the dongle number and required ppm setting as shown in the image below.
It cuts down on the confusion.

Rich

Nice Droid MAXX (or is that the Razr MAXX) there, had one myself recently and already miss it and I did use it in a similar manner with an RTL stick OTG (on-the-go) with SDR Touch too. Great phone, especially the battery life which makes using an RTL stick in that manner pretty handy.

br0adband said:

The TCXO models obviously are much better in terms of drift but since I leave my RTL sticks plugged in 24/7 it's rarely an issue. I have been considering getting two new ones, those from RTL-SDR.com that are aluminum shielded with SMA-F connectors (and with antennas for $24.95) but that of course would require me to buy new pigtails as well - while they do come with slightly better (and adjustable) antennas they're still crap in the long run because of no ground plane and just cheap low-end construction. Was thinking about buying SMA antennas to attach directly to the RTL sticks themselves and negate any potential cable losses completely, seems like a more practical solution I suppose. I use 12 foot shielded USB extension cables with RF chokes on each end to cut down on noise as well and the newer sticks with the aluminum shielding surely are better in that respect as well along with the slightly more sensitive R820T2 tuners.

I did a similar graphic in the past like the one rbm posted above, was interesting to note from a cold start just how far off that spectrum/waterfall line curved as it warmed up. My sticks are at 49 and 55 ppm but I learned that a few hours after first using them a long time ago. Probably the best ~$20 I think I've ever spent on actual hardware for monitoring but I think it's time for an upgrade soon to the TCXO models.

Click to expand...

Hello BB

I have been searching for an antenna that will attach directly to the dongle as opposed to the cable tether, but have been unable to find one. If you come across such an antenna could you post a link.I think that is a great idea to cut down on cable loss as well.

I was reading a while back about the aluminum cases as well and I heard that the anodized ones ( not sure if they are all anodized or not ) affect performance and not in a good way I don't remember the whole story but it made me kind of leary about them. I also think I will stick to the Pilot frequency as it is in the middle of most of the listening that i do.

Thanks for the input guys.

MB

My dongles (even the pre-TXCO ones) usually warm to stable in less than a minute.

moonbounce said:

Hello BB

I have been searching for an antenna that will attach directly to the dongle as opposed to the cable tether, but have been unable to find one. If you come across such an antenna could you post a link.I think that is a great idea to cut down on cable loss as well.

Click to expand...

If you're using the original RTL sticks which have the MCX connector:



you're more than likely not going to find any antennas that will directly connect to that; in rbm's shot above with the smartphone you can see he's using an MCX to SMA right-angle adapter so that's about the best/closest thing you'll ever be able to manage. What I was meaning was that if I do decide to get those newer RTL sticks (from the link I provided) they have SMA-F connectors which means I'd be able to use any antenna with an SMA-M connector and attach it directly to the stick itself. Most every relevant antenna nowadays has BNC and SMA-M connectors available so that definitely ensures the best possible signal coming into the stick itself.

The newer RTL sticks using the R820T2 tuner (as opposed to the original R820T) are slightly more sensitive, better/lower noise floor (not much but it is a bit lower), and the aluminum/metal (because both types are available) shielding definitely should assist with reducing EMI/noise too. I know some folks complained about the heat buildup inside the first batches of those sticks from RTL-SDR.com so they improved them by adding the heat/thermal pad which alleviates some of the internal heat buildup to the casing itself by transferring heat from the board to the casing. The anodizing itself has no relevance on anything except the visual aspect of the stick itself.

The addition of the TCXO, the improved R820T2 tuner, the aluminum housing, and the SMA connector certainly make them an almost "must buy" if you're serious about using RTL-based hardware for SDR. I've been putting off upgrading my original RTL sticks but I suppose I'll be placing an order here soon for several of the newer models along with some new antennas (all my current ones are BNC) or I may spring for a bunch of BNC to SMA adapters, who knows. A handful of these adapters would certainly save me a ton of cash and allow me to keep using the antennas I've owned for many years now.

br0adband said:

Nice Droid MAXX (or is that the Razr MAXX) there, had one myself recently and already miss it and I did use it in a similar manner with an RTL stick OTG (on-the-go) with SDR Touch too. Great phone, especially the battery life which makes using an RTL stick in that manner pretty handy.

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br0adband, That's a nice portable setup for some specific purposes.

I've used it to isolate sources of RFI for a few friends. (Switching power supplies and LED lighting mostly)

I don't use it as much as I thought I would, but when I do need it, nothing else works as well.

Rich

Thanks again I think I will be downing some shopping on rtl-sdr.com, they have everything, and I need to I'm feeling the need to treet myself .

My apologies if this is getting off topic but I found the link to the article I read about the aluminum case for the RTL dongles. I find it to be an important topic so I thought I would post the link to the talk group on the cases.

https://www.reddit.com/r/RTLSDR/comments/3yifvi/new_model_of_the_rtlsdrcom_dongles_are_available/

For what it's worth, if you have an SDR stick with an MCX connector as shown above, the GPS world has two different 'pigtail' adapter cables which will help you out.

I'll link to them on Amazon, but you can find them in various other online stores too.

MCX to SMA: Robot Check

MCX to BNC: Robot Check

My local GPS dealer has virtually the same MCX to SMA cable for $4.95 (Canadian).

Jay911 said:

For what it's worth, if you have an SDR stick with an MCX connector as shown above, the GPS world has two different 'pigtail' adapter cables which will help you out.

I'll link to them on Amazon, but you can find them in various other online stores too.

MCX to SMA: Robot Check

MCX to BNC: Robot Check

My local GPS dealer has virtually the same MCX to SMA cable for $4.95 (Canadian).

Click to expand...

I bought two MCX to BNC addapters from a dealer in Alibaba for $2.50 shipped to my door. Only problem is you have to wait 3 to 4 weeks.