Specifications

66 Chapter 8
Making Time-Gated Measurements
Gated FFT Measurement (PSA)
Making Time-Gated Measurements
Gated FFT Measurement (PSA)
This procedure utilizes gated FFT to gate the FM signal. For concept
and theory information about gated FFT see “How Time Gating Works”
on page 135.
Step 1. Set the PSA center frequency, span and reference level:
Press
FREQUENCY Channel, Center Freq, 40, MHz.
Press
SPAN X Scale, Span, 500, kHz.
Press
AMPLITUDE Y Scale, Ref Level, −15, dBm.
Step 2. Set the trigger to the external rear trigger input:
Press
Trig, Ext Rear.
Step 3. Select the minimum resolution bandwidth required:
Press
BW/Avg, Res BW (Auto).
The duration of the analysis required is determined by the RBW. Divide
1.83 (always constant) by 4 ms to calculate the minimum RBW. The
pulse width in our case is 4 ms so we need a minimum RBW of 458 Hz.
In this case because the RBW is so narrow let the analyzer choose the
RBW for the current analyzer settings (span). Check that the RBW is
greater than 458 Hz.
With the above PSA settings, the RBW should be 4.7 kHz. Note that the
measurement speed is faster than the gated LO example. Typically
gated FFT is faster than gated LO for spans less than 10 MHz.
Vary the RBW settings and note the signal changes shape as the RBW
transitions from 1 kHz to 300 Hz.
Figure 8-8 Viewing the Gated FFT Measurement Results from the PSA
NOTE If the trigger event needs to be delayed use the Trig Delay function under
the
Trig menu. It is recommended to apply some small amount of trigger
delay to allow time for the pulse modulator to settle.