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.
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.
GNURadio Companion Basics Course:
Please send your comments, questions and suggestions to:
#1. – “RTL-SDR for VHF Air Band AM on GNU Radio”
#2 – “AIS GMSK Modulator – Scicos Simulation”