Newbie IQ questions

[emonk]

I still have no hardware to play with but I've been reading lots of VST documentation and RF discussions while parsing the coding examples in the Instrument Design Libraries.  I have not yet found the document that answers these nagging questions: 

How do I interpret a streamed array of IQ data to obtain actual quantitative values like center frequency and peak power?  The example programs I've looked at draw spectrums.  While that's nice, I'd like to look at numbers.

If I'm analyzing RF pulses, how can I ascertain typical pulse characteristics like duration, rise time and repetition rate?

Is IQ data the only data that is allowed in the FIFOs?  How would I dump some DIO input data on there for the host to deal with?

Again, I'm new at this.  Be gentle.

I'm also transitioning from LV 8.2 to 14 and grappling with the coolness of that too.

Thanks.

[Andy_Hinde]

• How do I interpret a streamed array of IQ data to obtain actual quantitative values like center frequency and peak power?

IQ data is time-domain data, to get frequency information you would need to compute an FFT/power spectrum. IQ data gives you I and Q vs time, or you can convert from Cartesian to polar coordinates and have magnitude and phase vs time. You could look at the first derivitive of phase vs time to get a frequency value if there is one tone present, but if you have multiple tones or a modulating signal, this will just give you instantaneous frequency, and an FFT/power spectrum would be more useful.

For peak power, convert to polar mag/phase vs time, and find the array index with the max magnitude.

• If I'm analyzing RF pulses, how can I ascertain typical pulse characteristics like duration, rise time and repetition rate?

Convert your IQ data to polar mag/phase vs time. Your mag vs time data can then be analyzed for the properties above. There are LabVIEW functions which will do these measurements for you. Keep in mind that your IQ sample rate sets the input bandwidth and can potentially have an effect on rise time. If you are measuring rise times, I recommend using as large an IQ sample rate as you can, so your input bandwidth (0.8*IQ Rate) is as wide as possible.

• Is IQ data the only data that is allowed in the FIFOs?  How would I dump some DIO input data on there for the host to deal with?

There should be Instrument Design Library (IDL) examples that show how to use the VST DIO lines. I would start there.

Regards,

Andy Hinde

Senior Systems Engineer

RF and Communications

National Instruments