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Replaying a signal


We have an RTL dongle and an HackRF One, both connected at the same time to the computer.

HackRF One w/ fw 2017.02.1,

1 Mhz - 7.250 Ghz
recommanded sample rates: 2M/s - 20M/s

RTL dongle (RTL2832U // RTL2838UHIDIR) w/ Rafael Micro R820T/2,

24 - 1766 MHz (Can be improved to ~13 – 1864 MHz with experimental drivers)
recommanded sample rates: 1.8M/s - 2.56M/s

Capturing some signal

With Gnu Radio, the payload looks as such just to identify the signal,

osmocom source _1 (RTL dongle) -- sample rate 2.56M/s
--> QT GUI Frequency Sink freq=434e6 RFgain=0 IFgain=16 BBgain=16 bandwidth=samp_rate

middle click > max hold (shows up as yellow)

In case nothing is seen with HackRF, some further attempts could be realized by improving the IF and BB gains (and in the last resort, the RF gain) for receiving. Since we did not see anything with HackRF, while we could indeed identify the signal with the RTL dongle, we simply used the latter.

Then for capturing into a file, we are adding,

--> file sink (path /var/tmp/

Checking the resulting file size (depends on the amount of samples),

ls -lhF /var/tmp/car*.iq

gives, with a sample rate of 2.56M/s,

-rw-rw-r-- 1 elge elge 424M Jan 30 17:15 /var/tmp/
-rw-rw-r-- 1 elge elge  79M Jan 30 17:17 /var/tmp/

Analyzing the captured signal

The Gnu Radio payload looks as simple as follows,

File Operators/File Source: captured-IQ-file with sample rate 2.56M/s
--> Misc/Throttle w/ 2.56 million signals per second (even though there is a sample rate at the file source, the signals would just be pushed as fast as the CPU is able to)
--> QT GUI Frequency Sink (or WX QUI FFT Sink)
--> QT GUI Time Sink (or WX GUI Scope Sink) to show the waves amplitude

The Inspectrum tool is also helpful to review the captured data:

Replaying the signal

With GNU Radio, the payload to emulate a replay is as simple as follows,

File source: captured file
--> Math Operators/Multiply Const: e.g. 6
--> Osmocom Sink
    freq=same as original capture (the offset is restored)
    RF gain=0
    IF gain=max (47)

Note. BB gain does not apply as for RX.

Then checking the resulting signal with the RTL dongle as TX, identifying the relevant bandwidth,

osmocom source _1 (RTL dongle) -- sample rate 2.56M/s
--> Frequency Sink (sample rate can be higher, it is ok)

and croping the TX payload to that range.

Base band filter


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