rtl_fm and r820t tuner

Miguel A. Vallejo ea4eoz at gmail.com
Tue Nov 12 21:02:30 UTC 2013


*>>>  Are you sure?*

Yes.

For the skeptics:

~$ rtl_fm -f 89000000 -R -s 1000000 -g 0 - | baudline -stdin -samplerate
1000000 -quadrature -flipcomplex -channels 2 -format le16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000013

Using device 0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Found Rafael Micro R820T tuner
Oversampling input by: 2x.
Oversampling output by: 1x.
Buffer size: 4.10ms
Tuned to 89500000 Hz.
Sampling at 2000000 Hz.
Output at 1000000 Hz.
Exact sample rate is: 2000000.052982 Hz
Tuner gain set to 0.00 dB.

Result: https://dl.dropboxusercontent.com/u/102802145/baudline0dB.jpg



~$ rtl_fm -f 89000000 -R -s 1000000 -g 30 - | baudline -stdin -samplerate
1000000 -quadrature -flipcomplex -channels 2 -format le16
Found 1 device(s):
  0:  Realtek, RTL2838UHIDIR, SN: 00000013

Using device 0: ezcap USB 2.0 DVB-T/DAB/FM dongle
Found Rafael Micro R820T tuner
Oversampling input by: 2x.
Oversampling output by: 1x.
Buffer size: 4.10ms
Tuned to 89500000 Hz.
Sampling at 2000000 Hz.
Output at 1000000 Hz.
Exact sample rate is: 2000000.052982 Hz
Tuner gain set to 29.70 dB.

Result: https://dl.dropboxusercontent.com/u/102802145/baudline30dB.jpg


GQRX, LNA gain = 0dB

Result: https://dl.dropboxusercontent.com/u/102802145/gqrx0dB.jpg



GQRX, LNA gain=30dB

Result: https://dl.dropboxusercontent.com/u/102802145/gqrx30dB.jpg



It's clear rtl_fm does not set any gain to the r820t tuner
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