Calibrating RTL-SDR

Main forum to discuss RTL-SDR related topics.
Post Reply
saledinmert
Posts: 1
Joined: Tue Apr 20, 2021 2:26 pm

Calibrating RTL-SDR

Post by saledinmert » Fri Apr 23, 2021 11:31 am

Hello, everyone!
I have some questions! I have been using two different methods in order to calibrate one of my 'old' RTL SDR. It's not V3 with txco. And I got two different results as following:
a) Using SDR sharp : 583 ppm with 20.7gain ( I tuned it to a known frequency of an Airport 120.9 MHz);
calibrare_sdr#_cut.png
calibrare_sdr#_cut.png (618.82 KiB) Viewed 1375 times
b) Using kalibrate-rtl : around 33 ppm using GSM 900, although had the problem with : " Difference of offsets between channels is >1kHz. This likely means that the correct PPM is too far away and you need to provide a rough estimate using the '-e' option. Try tuning against a local FM radio or other known frequency first." and not sure if it's correct.
kal_1.png
kal_1.png (320.91 KiB) Viewed 1375 times
kal_2.png
kal_2.png (303.05 KiB) Viewed 1375 times
The questions I am asking are:
1. Which one is the correct ppm offset? I want to receive signals using rtl_sdr and I wonder which ppm to use on the command;
2. Do I need to calibrate an rtl sdr v3 dongle with 1 ppm TXCO?
Thank you! :)

alanzfq
Posts: 201
Joined: Thu Oct 04, 2018 11:18 am

Re: Calibrating RTL-SDR

Post by alanzfq » Sat Apr 24, 2021 7:39 am

All I have ever done is calibrate on a known frequency.
IF I was certain 120.9 was truly accurate I'd use that.
I am lucky to have 2 accurate ham beacons close enough.
The need for calibration depends on how accurate you need to be. The V3 might be good enough for you but would need calibrating if you wanted absolute (as near as possible) precision.
Alan

Kenn
Posts: 54
Joined: Fri Apr 08, 2016 8:43 pm

Re: Calibrating RTL-SDR

Post by Kenn » Sat Apr 24, 2021 5:41 pm

583ppm seems kinda high, 33ppm more typical. What does it look like on a NOAA/NWS weather radio freqs or an Amateur radio repeater? Also see what another spec analyzer reports.

If all that leaves you in the same place, try both values with command line apps and see which yields more.


-K

qrp
Posts: 71
Joined: Wed May 22, 2019 11:16 pm

Re: Calibrating RTL-SDR

Post by qrp » Wed May 12, 2021 9:33 pm

saledinmert wrote:
Fri Apr 23, 2021 11:31 am
1. Which one is the correct ppm offset? I want to receive signals using rtl_sdr and I wonder which ppm to use on the command;
ppm settings on ExtIO dialog has integer value, this is not enough to get best precision. So I suggest to use software calibration in HDSDR.

You can try frequency standard stations RWM they are located in Russia and working at the following frequencies:
- 4996 kHz, power 5 kW
- 9996 kHz, power 8 kW
- 14996 kHz, power 8 kW

They have specific pattern linked with current time, most of the time it's a beep pattern, but at some period of time it switch to a continuous carrier. Their frequency is synchronized with frequency standard, so it's very precise.

Also you can use carrier of AM broadcast stations for calibration, most of them are calibrated and their frequency is pretty precise, about ±1 Hz, but not all. Some of them have significant random error. Such way is not so precise as using frequency standard stations, but still good enough.
saledinmert wrote:
Fri Apr 23, 2021 11:31 am
2. Do I need to calibrate an rtl sdr v3 dongle with 1 ppm TXCO?
No, RTLSDRv3 precision is good enough. The error is about 2-3 Hz, it doesn't matter and don't worth to spent time on calibration. And even more, since ppm setting in ExtIO dialog accept integer value only, you will be unable to get better result with calibration. But if you use software HDSDR calibration it allows to fix a small frequency error.

Post Reply