What is a quadrature signal and how to process this signal with Sagax SRM-3000 software defined radio receiver? Find out it the post! For additional content visit our resources page or subscribe to our newsletter here!
What is a quadrature signal?
A quadrature signal is a BaseBand (BB) representation of a modulated signal. It contains all of the signal information except for the carrier frequency itself. Therefore we can call it the complex envelope or the In-phase and Quadrature-phase (IQ) signal. http://www.dspguru.com/dsp/tutorials/quadrature-signals In this tutorial, Rick Lyons, the author of the best-selling DSP books Understanding Digital Signal Processing and Streamlining Digital Signal Processing: A Tricks of the Trade Guidebook, clears the fog around this difficult subject by providing the clearest, most intuitive explanation yet of quadrature signals and their importance in Digital Signal Processing (DSP).
Processing quadrature signal
Signal processing software
There are lots of signal processing software packages to handle complex signals – the fundamental representation of radio signals. As a result we’ve selected the well-known SpectrumLab, by Wolf, DL4YHF, to show the quadrature signals originating from the SRM-3000 SDR platform.
Setting up SpecLab for IQ mode
SDRs with baseband output via audio provide the I and Q sample streams in the L and R channels respectively of the stereo audio stream. To set up a spectrum calculation from the complex samples, one has to turn on the CFFT function.
Fortunately, SpecLab could handle IQ signals from different SDR platforms. We just need to select the appropriate setup from the menu.
Connecting to the SRM-3000 SDR software
The SRM-3000 is one of the software package of our SDR receiver platform. It has different demodulators including an IQ demodulator, which allows further processing of the quadrature signal itself. We can select IQ demodulator from the drop down list.
The In-phase (I) samples will go to the left channel (L) and the Quadrature-phase (Q) samples will go to the right (R) channel of the audio stream. You can connect signal processing applications to a Virtual Audio Connection (VAC). In this case, we can direct the IQ signal from the receiver software to the signal processing software.
Sampling rate and spectrum bandwidth
We selected different audio output rates in the SRM-3000 SDR receiver software. The audio settings are located on the demodulator panel. Either the audio High-Pass Filter (HPF) or the audio Low-Pass Filter (LPF) can be selected, which are applied on the demodulated signal before it gets routed to the audio out device.
In our example, we’ve selected the maximum 48 kHz sampling rate. We sampled both the I and Q with 48 kHz with this setup, which is equivalent to a 96 kHz total sampling rate. That is why the Nyquist bandwidth of the baseband quadrature signal is 48 kHz, and +/-24 kHz of bandwidth is shown on the (complex) spectrum display.
Quadrature signal on the displays
The receiver IF spectrum display shows the incoming signal. In this example, the SRM-3000 SDR receiver operates the in +/-50 kHz display mode. We tuned CH1 to the 180 MHz carrier frequency. The incoming signal is about ~11 kHz higher at around 180.010 kHz.
On the baseband spectrum display of the Spectrum Lab you can see the same spectrum however, the bandwidth is only +/-24 kHz corresponding to the 48 kHz sampling rate of the IQ demodulated signal. The IF filter, before the demodulator, was set to 20 kHz bandwidth. This is indicated on the IF spectrum display as well with the two yellow lines. This is the part of the IF signal that will be filtered and fed into the demodulator. The baseband spectrum display shows that the noise level is higher within the +/-20 kHz band, which passed through the IF filter.
We can use quadrature signals to represent the complex, modulated signals in the baseband, where we removed the carrier frequency from the signal, but all other information is still there. You can run different types of the demodulation on the IQ baseband signal, including multi-level, multi-phase modulation types. The SRM-3000 SDR receiver can provide the IF filtered IQ signals for further processing. We’ve introduced an example how to connect the SRM SDR receiver to an external signal processing software, Spectrum Lab, with a virtual audio cable.
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