3 Spectrum Analyzer Mode
3.2 Swept SA Measurement
RBW3dB = the 3 dB RBW, which is not normally the annotated RBW for EMC
measurements, which are based on the 6 dB RBW.
deltaA = the amplitude at the marker, in dBm, minus the amplitude of the bucket
one to the left of the marker, in dBm. Which reminds me...we need to protect
against having the marker be the first or last bucket--we can't zoom in these cases.
If the marker is at a peak, as intended, deltaA must be positive. But the algorithm
may be run when deltaA is not positive. In this case, we must "intercept" the
computation to prevent a mathematics failure wherein the argument of the square
root is negative. In this case, we should set fLowerCandidate to CenterOfBucket –
0.5*BucketWidth. This is equivalent to replacing deltaA with max(0,deltaA).
(3dB) is a constant, which reminds us that the units of deltaA are dB.
Step 2. Compute the frequency from the upper pair. Compute
fUpperCandidate = fCenterOfBucket + 0.5*BucketWidth - RBW3dB*sqrt(deltaA/
(3dB))
All the terms are the same except:
deltaA = the amplitude at the marker, in dBm, minus the amplitude of the bucket
one to the right of the marker, in dBm.
As in step 1:
If the marker is at a peak, as intended, deltaA must be positive. But the algorithm
may be run when deltaA is not positive. In this case, we must "intercept" the
computation to prevent a mathematics failure wherein the argument of the square
root is negative. In this case, we should set fUpperCandidate to fCenterOfBucket +
0.5*BucketWidth. This is equivalent to replacing deltaA with max(0,deltaA)
Step 3. Compare fUpperCandidate with fLowerCandidate. If the absolute value of
the difference is less than 0.1*RBW3dB, and deltaA (either one) is positive, then we
conclude that the signal is CW and worth fine-tuning. Set the tuning to the mean
value, which is (fUpperCandidate + fLowerCandidate)/2. Otherwise, use fMarker for
the zero-span acquisition.
Measure at Marker Config
Opens a dialog that lets you configure the Measure at Marker function.
The two most important settings are the detectors and the dwelltime associated
with each. Any of the instrument’s detectors (up to three) can be used as the
Measure at Marker detectors, or any of the three can be turned off. The dwell time
for each detector is also settable.
When performing a Meas at Marker, the dwell time settings that you select will
depend on the characteristics of the emission you are measuring. The default dwell
time (200 ms) should work well for typical EUT emissions, but sometimes you will
366 Spectrum Analyzer Mode User's &Programmer's Reference
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