On the PXA and many of the VSA, in IQ analyzer mode, the IQ BW is set as 0.8 times the sample rate. What is the logic for this 0.8?
Hello Leyo, Great question - there is not much documentation on this subject, yet there is a very detailed, intentional method to this.
When viewing this modulated, digital signal, there are several steps made to make the signal clearer to view, with the removal of spurs and harmonics. In this case, a window function is applied to a low pass filter. You will see something like the following image:
The roll off component of the filter (the circles area) is not able to be removed but contains unwanted data. You want your signal to fall within the 80% of this image, which is why the .8 represents a ratio of what you wanted to view, meaning .2 represents that unwanted, circled area. That 80 covers the 0 point of this graph, to the span/bandwidth section. The section between span/bandwidth to sample rate is unwanted. If this graph is cut off too early, you may not have an accurate view of the signal, as things that may not necessarily be there, will look like they are there. This equation can be represented in other ways besides the one you have provided (.8 * samplerate).When using the something like the M9391, you can see it represented like this: 1.25* bandwidth = sample rate, where 1/.8 gives you the 1.25. When using VSA software, you can see it represented like this:1.28 * bandwidth = sample rate.
When you need the real sample, and not just the sample rate, simply double the value (i.e 1.28 * 2 = 2.56).
Hope this helps!
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