I am trying to determine what the dwell time is for these instruments. By dwell time I mean the amount of time it samples the signal for each data point. This does not appear to be a configurable value, nor does there seem to be a way to query this value.
For these instruments, is this parameter fixed or variable? If it is fixed how do I find out what it is? If it is variable, is it calcualted by simply dividing the sweep time by the number of data points?
TIA,
-larry
Larry I'm not certain about the 856X series but I suspect it is the same as for the newer analyzers. If so, the way the detector works depends on a number of factors: o how fast one is sweeping. 0 detection mode (peak,sample, neg. peak) 0 resolution bandwidth
As the analyzer's LO traverses the spectrum between measurement bins (each bin of width = span/#measurment points), depending on how the detector is selected, the A/D takes 1-to-many measurements. Maximum sweep rate is limited by the need to let the detected signal settle (the group delay of the RBW filter) and the ability of the LO itself to move quickly and accurately. For the ESA series, multiple measurements may be made within each bin. Peak Detect modes just save the max or min value for the bin. Sample Detect (probably) measures the value at the center(or end, I 'm not sure) of the bin and throws away any other measurment. For some analyzers there is a non-sweeping mode wherein the A/D runs as fast as it can. I think that is a 50 ns measurement time for ESA.
While there may be no dwell time selection, you may be able to influence the time spent by judicious selection of span, RBW and sweep time. But then, the question arises "why do you want to do that?". The SA should already manage things to avoid over-sweeping, trying to take data faster than the if filters can handle, and the detection mode selection should allow you some fleximbility as to whether you're looking at a peak or sampled mode.
Glenn
It is indirectly adjustable, at least on the 4195. The measurement time depends on your resolution bandwidth (Res BW button) setting. Lower bandwidth setting will take longer to make a measurement. You can divide the total time by number of points to come up with an estimate if you use the same bandwidth for all points (non-auto mode). If you use the Auto Res BW, the resolution bandwidth will be different for different frequency bands which can speed up the total measurement time (slow at low freqs, faster at the higher freqs). If you use the video filter, this will add more time to the measurement process. On the 4195, you can manually change the bandwidth in the middle of the sweep if you need to hurry things up or reduce noise in critical areas.
-- Mark
Thanks much for the reply Mark. This is very helpful.
-larry
Thanks very much for the info Glenn, it is very hlepful.
I have to perform some EMI emissions tests that have specific dwell time requirments (>= 0.15 sec), and I will need to prove to my customer that I am meeting these.
Thanks!
-larry
Larry
OK, that helps. In that case you'll want to use the peak detect mode (max) and slow the sweep time down to (.15sec/bin * #bins/sweep). That should get you to .15 seconds within each bin and a measured level corresponding to the peak within that time. To be thorough, you'll probably want to make sure that the RBW is set to be at least as wide as a bin.
Glenn
Glenn, thanks very much for the info!
-larry
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