Too Long, Won't Read
Add rtl,bias=1 in the device arguments. Both options are needed.
i like my Low-Noise Amplifier. It helps my dingy little antenna and RTL-SDR dongle pick up signals that otherwise would be very much out of reach. That little amplifier is plugged on the transmission line as close as possible to the antenna, and powered through bias-tee. A bias-tee (or bias-t) is a little circuit designed to add a constant DC voltage to an alternating one, such as radio-frequency signals. Here, it lets me send power from the USB port my dongle is plugged in, through the dongle and along the transmission line all the way to the amplifier, which is then powered by that DC voltage (in my case 5V DC) .
For most situations where i used my SDR
so far, i could use the rtl_biast command line tool to enable
bias-tee prior to running the utility i needed. Other times, there was a
dedicated option for enabling it.
Then, i played with receiving GSM data using Piotr Krysik's
gr-gsm1. Recently, i forked
it to fix build issues on newer
versions of Boost. gr-gsm's scripts use GNURadio,
a flow-based system for building digital signal processing pipelines from basic
building blocks. Scripting it helps provide a clean front-end for users to just,
for example, receive GSM traffic from a nearby antenna, or split a single
recording into multiple capture files depending on the antenna frequency, all
without having to open the GNURadio companion software to tweak a massive
diagram of thousands of blocks.
gr-gsm is supposed to work with basically any radio source that is supported
by OsmoSDR, a SDR software
abstraction layer developed by the OsmoCom group. In order to provide
device-specific arguments, however, we need to fill a free-form text field in
the OsmoSDR Source block. gr-gsm gives you the option to do so, but does not
document anywhere what options are available for what dongles.
After stumbling upon a reddit
post,
i opened the documentation of
gnss-sdr
of all places2. In that specific page lies a table with some information
on how to enable bias-tee for HackRF One, bladeRF and RTL-SDR Blog V3 dongles. i
own a V4 dongle, but the option hasn't changed:
SignalSource.osmosdr_args=rtl,bias=1
So, back to gr-gsm, i just input:
grgsm_livemon -f 921.8M -g 30.0 --serverport 4730 --args="rtl,bias=1"
That last bit, rtl,bias=1, is the part you need to provide as device
arguments. Both rtl and bias=1 need to be present for a RTL-SDR dongle's
bias-tee to turn on.