RTL-SDR for RF Signal Capture in GNU Radio


In previous posts I examined using GNU Radio to receive various signals such as VOR, VHF Air Band, SSB, AM, WBFM, AIS and ADS-B (Ref.1). I also examined how these signals could be simulated using Scicos (Ref.2). In many cases for testing purposes, we need to capture a sample of the signal in question to study its properties. This can easily be done using the RTL-SDR with GNURadio.

Signal Capture

Fig.1 RTL-SDR for Signal Capture to an IQ Data File
Fig.2 RTL-SDR for Signal Capture Spectrum Fs=2.4MHz
Fig.3 IQ File Playback to QT GUI Sink
Fig.4 IQ File Playback to QT GUI Sink Fs=200KHz

Figure 1 shows the general principle of Signal Capture. An RTL-SDR Source block is connected to a File Sink and QT GUI Sink. Several variables are present:

Sample Rate: Set to desired receive bandwidth (2.4MHz)
Centre Frequency: Set to desired frequency (WBFM 104.5MHz)
RF Gain: Adjust from 0 – 40dB (30dB)
File Sink: rtlsdr_rf_rx.dat (data file containing IQ samples)

In this example we are sampling a wideband FM broadcast station at 2.4MHz. The spectrum goes from centre frequency -1.2MHz to centre frequency + 1.2MHz = 2.4MHz. So a 10 second sample of complex float 32bit (4bytes) IQ data would contain:
2.4*(10^6)*2*10*4 = 192Mbytes of data.

The capture files are like video files and grow fast! Let’s say we only want to look at the spectrum of the received FM station. Then we can sample at reduced channel bandwidth of 200KHz vs 2.4MHz. This is shown in Figure 4. The spectrum extends -100KHz below the centre frequency and +100KHz above the centre frequency.

Fig.5 YouTube Video RTL-SDR for RF Signal Capture on GNU Radio

GNURadio Companion Basics Course:

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#1. – “RTL-SDR for VHF Air Band AM on GNU Radio”

#2 – “AIS GMSK Modulator – Scicos Simulation”

By Jeremy Clark

Jeremy Clark is a Senior Telecommunications Engineer and Advanced Amateur Radio Operator VE3PKC. He is the author of E-Books on Telecommunications, Navigation & Electronics.