Spectrum Analyzer Menus
6–32
2715 User Manual
400.0MHz
(AUTO SWEEP)
ATTN 12DB
28.8DBMV
VF WIDE
100MHz/
10 DB/
5MHz RBW (AUTO)
CALIBRATOR
The B,C MINUS A feature of the Display menu enables you to subtract a sweep
stored in register A from an active sweep in registers B or C. You can use it to
flatten a noise spectrum, get rid of unwanted signals, easily detect signal
changes, and so forth.
Observe the current sweep, especially the zero-frequency and calibration signal
peaks. In your instrument the noise floor may rise slightly with increasing
frequency.
1. Perform a 24 sweep MAX/MIN ensemble average and store the result in A.
The average indicator is displayed only when the 2715 is in the non-CATV
mode.
2. Wait until the ensemble average is complete.
3. Ensure item 3 of the Display menu reads B,C MINUS A OFFSET TO
CENTER. (If it does not, select item 3 once.)
4. Press [DSPL] [2] to activate the B,C MINUS A mode.
5. Turn on the B register and turn off A and D.
The display now consists of a much-reduced and totally flat noise floor and some
intermittent peaks similar to Figure 6–5. The zero-frequency and calibration
signal peaks have almost disappeared. You have subtracted the average max/min
spectrum stored in the A register from the active sweep in the B register (you can
also use the C register), and are displaying the result that consists only of the
sweep-by-sweep variations. The vertical center of the screen represents zero
amplitude difference between the waveforms.
Waveform subtraction can be used as a sensitive detector of signal changes.
Suppose you were trying to measure a weak signal, which you could turn on and
off, in the presence of interfering noise and signals. You could turn the signal off,
compile the ensemble average and subtract it as above. Then, when you turn the
weak signal back on, it would show up because it was not part of the stored
average.
Subtracting Stored
Signals
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