Recommended dongle

[acasadoalonso]

I wonder if you can recommend a good dongle for this project, I am using the Realtek Semiconductor Corp. RTL2832U DVB-T
with mixed results, it seems OK for FM, but not so much for AM in the Airband.
Thanks in advance.
AC/.

[szpajder]

On Thu, May 21, 2015 at 12:15 PM, acasadoalonso [email protected]
wrote:

I wonder if you can recommend a good dongle for this project, I am using
the Realtek Semiconductor Corp. RTL2832U DVB-T
with mixed results, it seems OK for FM, but not so much for AM in the
Airband.
Thanks in advance.

I used these:

http://www.dx.com/p/rtl2832u-r820t-mini-dvb-t-dab-fm-usb-digital-tv-dongle-black-170541#.VV2wwvCK3E0

and these:

https://www.cosycave.co.uk/product.php?id_product=320

They both work equally well.

In fact, they are both using the same chipset as yours, so don't expect a
huge difference. If your airband reception is bad, it might rather be due
to poor location or antenna.

Also be sure that you set the proper value of ppm correction in config.txt.
Airband channels are quite narrow, so if your dongle has a large offset,
you can easily miss the channel you want to receive.

Tomasz Lemiech

[acasadoalonso]

Tomasz,
I may buy one of the R820T2 … looks a little bit better than the R820T
What are the values for gain and frequency offset that you use in your config.txt file ???
Thanks in advance for your help.
AC/.

On 21 May 2015, at 12:35, szpajder [email protected] wrote:

On Thu, May 21, 2015 at 12:15 PM, acasadoalonso [email protected]
wrote:

I wonder if you can recommend a good dongle for this project, I am using
the Realtek Semiconductor Corp. RTL2832U DVB-T
with mixed results, it seems OK for FM, but not so much for AM in the
Airband.
Thanks in advance.

I used these:

http://www.dx.com/p/rtl2832u-r820t-mini-dvb-t-dab-fm-usb-digital-tv-dongle-black-170541#.VV2wwvCK3E0

and these:

https://www.cosycave.co.uk/product.php?id_product=320

They both work equally well.

In fact, they are both using the same chipset as yours, so don't expect a
huge difference. If your airband reception is bad, it might rather be due
to poor location or antenna.

Also be sure that you set the proper value of ppm correction in config.txt.
Airband channels are quite narrow, so if your dongle has a large offset,
you can easily miss the channel you want to receive.

Tomasz Lemiech
—
Reply to this email directly or view it on GitHub #13 (comment).

[szpajder]

On Thu, May 21, 2015 at 3:03 PM, acasadoalonso [email protected]
wrote:

Tomasz,
I may buy one of the R820T2 … looks a little bit better than the R820T
What are the values for gain and frequency offset that you use in your
config.txt file ???

I usually set gain to somewhere between 28 and 34 dB (that means 280-340 in
config.txt). There is no universal rule for offset, because every dongle is
different - it's a matter of manufacturing dispersion. I own five dongles
and the only common thing to them is that they all tune a bit too high, so
I need to set negative offset to counteract this. The values are between
4000 and 8200, depending on which dongle I use. That's why you should not
get these numbers from the Internet, but rather measure them for your own
hardware. You can do this with SDRSharp or GQRX:

1. connect the dongle and antenna, launch the program
2. set the exact frequency of the channel you want to receive (for example,
   if your ATC freq is 131.1 MHz, then the numbers at
   the top of the screen should show exactly 131100000). Start decoding.
3. allow the dongle to warm up at least 15 minutes (the frequency offset
   changes with temperature)
4. go to receiver configuration screen and adjust gain - go up from the
   lowest possible values and observe the spectrum graph. Increase gain as
   long as the noise floor level does not increase significantly. Stop when it
   starts to rise - this is your optimal gain value.
5. when there is some chatter on the channel, you will probably notice that
   the spike on the spectrum is not exactly centered on the line of your
   currently tuned frequency (use the Zoom slider in SDRSharp to see it
   better). Go to dongle configuration screen and adjust PPM correction until
   the spike is exactly centered. Multiply the PPM setting by 1000 and change
   it to negative if your spike was to the right of the center line before
   correction. This is your correction setting. Put this into config.txt, note
   it down somewhere or apply a sticker to the dongle, so that you don't need
   to repeat all this again.

Tomasz Lemiech

[hamav8tor]

The difference between the R820T and the R820T2 in actual measurements shows some gain difference at higher frequencies and a little better frequency stability and spurious performance. More details here: http://www.rtl-sdr.com/rtl-sdr-tests-r820t-vs-r820t2-stability-tests-for-radio-astronomy/

[TobbyTravel]

You should write the PPM value to the eeprom of the dongle.
That way if you have multiple dongles connected at once and don't have to keep track of it.
just run the following.
rtl_eeprom -s PPM+30

You will see the PPM value for each unit with
rtl_test

[tc0nn]

"3. allow the dongle to warm up at least 15 minutes (the frequency offset changes with temperature)"

So sticking this in my attic that swings from 80 to 110 each day would be a bad idea?

[microtony]

Assuming you set it to the average (95 ppm) then the max error would be 125 MHz x 0.000015 = 1.875kHz. This program's bandwidth is set to 5 kHz and because it's AM so most amplitude changes can still be captured if the peak is not outside the band.