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RSA3408A I/Q Capture

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Joined: July 18th, 2011, 12:56 am

RSA3408A I/Q Capture

Post by andyclayton » July 18th, 2011, 1:13 am


I'd like to capture and store I/Q data onto a memory stick using an RSA3408A RTSA.

More specifically I want to downconvert a "chunk" of the RF spectrum and store it as an I/Q datafile using the "center frequency" and "span" controls to define the RF spectrum I which to analyise.

My attempts so far has led, (I believe), to the instrument attempting to demodulate signals and not simply down convertering them into a final IF bandwidth defined by the span. So if I wanted to analyise a 100MHz CW signal in the center of a 1MHz span I'm hoping to see I/Q signals with a frequency of 500kHz. Instead I see a very low frequency, almost DC component in the IQ data.

I do not need real time data so i don't have option /05 installed - just need the data to go onto a USB stick

Thanks in advance for your help.


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Re: RSA3408A I/Q Capture

Post by w2aew » August 12th, 2011, 1:01 pm

The IQ signals that are captured and stored in the RSA3408 are base-banded to the Center Frequency that you have set on the unit. Thus, if the center frequency is set to 100MHz, and the input signal is 100MHz, then the IQ signals will indeed be ideally DC values - because the magnitudes of the I and Q 100MHz sinusoidal components of the signal are constant. Remember, the I and Q are the magnitudes of the cosine and sine wave components at the center frequency that would be used to reconstruct the signal - such as:

I * cos(2 * pi * 100MHz * t) + Q * sin(2 * pi * 100MHz * t).

The captured and stored I and Q signals are just these magnitudes vs. time, not the complete quadrature products.

In order for you to see 500kHz I and Q signals, you would have to set an offset of 500kHz to the center frequency of the analyzer. For example, for your 100MHz input signal, set the center frequency of the 3408 to 100.5MHz or 99.5MHz, and you'll see 500kHz sinusoidal I and Q signals.

I hope this is clear.

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