leif at sm5bsz.com
Mon Feb 2 02:46:57 UTC 2015
The rtl-sdr library from Osmocom uses R820T with an
IF filter bandwidth of 5 MHz. This means that the
peak amplitude of all the signals within this bandwidth
has to be below the point of saturation for tha ADC.
To accomplish that on a crowded FM band it is necessary
to turn down the gain which means that weak stations
can not be heard because of the degraded NF.
It is possible to do much better. The IF filter which
actually is a low pass filter and a high pass filter
can be set for a bandwidth of 300 kHz. Dynamic range
increases by something like 30 dB for the second next
channel 400 kHz away. It is also possible to get some
more improvement by changing the gain distribution.
Sampling should remain at 2.4 MHz because the stop band
attenuation is not very high and if sampling at 300
kHz one would get aliases from signals outside the filter
if they are strong.
Here is a video showing the difference between the Osmocom
library and a modified library:
Amusing that the cheap dongle has lower phase noise than
the low noise synthetized generator from Hewlet Packard.
An oldfashioned vacuumtube generator is needed to find
the performance of the dongle...
The modified library can be obtained like this:
git clone https://github.com/dl8aau/librtlsdr
A dll for Windows is available here:
Linrad-04.04 will soon become available here:
More information about the osmocom-sdr