Custom IQ Demodulation (IFO & IQA)

There are a number of methods for capturing and exporting the IQ data for custom analysis of these types of communication signals from Siglent analyzers. The first method is an analog method for IF signal capture called IFO or IF Output. This option is on the SSA5000A series. The analog IF signal is output from a BNC on the instrument rear panel. The IFO can be used is swept mode when set to zero span. The Input/Output menu enables the IF output. The IF signal appears on a 120 MHz center frequency with approximately 40 MHz of bandwidth.

This enables higher speed digitization of signals because the carrier has been reduced to a range where it can be easily captured by an oscilloscope or digitizer. Here is an FFT of an IQ signal from the IF output shown on an oscilloscope:

The second method is to use the analyzer to digitize the IQ data itself and then store that for further analysis. This option is called IQA for IQ Analysis. This is available on the SSA5000A and SHA800A series handhelds. Here is a comparison table for the memory depth, bandwidth, and sample rate they can provide:

It is also possible to extract IQ data from the SDS7000A using the SigVSA tools. The IQA option also includes an additional visualization. This view makes it possible to see the changes in I and Q over time separately like so:

This data can then be moved to the computer using “:TRAC:DATA?” queries resulting in:

The raw IQ data can also be saved as a txt file and then moved later for offline analysis. On the SSA5000A, this can be done with the command:

:SAVE:RAW:IQ 1000000,”raw_iq.txt”

This data can be analyzed using the same programming methodology of the following demodulation example with these commands above when operating in IQA mode. Enter IQA mode using “:INST:SEL IQA”.

Review the next section for more details on programming tips for Python and VISA.