Specifications

Chapter 11 93
Demodulating AM and FM Signals
Measuring the Modulation Rate of an AM Signal
Demodulating AM and FM Signals
NOTE If the trigger level is set too high or too low when video trigger mode is
activated, the sweep stops. You need to adjust the trigger level up or
down with the front-panel knob until the sweep begins again.
Step 8. Measure the AM rate using delta markers:
Press
Peak Search, Marker, Delta, Peak Search, Next Pk Right or Next Pk
Left
.
Use markers and delta markers to measure the AM rate. Place the
marker on a peak and then use a delta marker to measure the time
difference between the peaks (this is the AM rate of the signal)
NOTE Make sure the delta markers above are placed on adjacent peaks. See
Figure 11-1. The frequency or the AM rate is 1 divided by the time
between adjacent peaks:
AM Rate = 1/1.0 ms = 1 kHz
The spectrum analyzer can also make this rate calculation by changing
the marker readout to inverse time.
Press Marker, More, Readout, Inverse Time.
In this example we calculated the time between time-domain peaks at
the frequency of the peak of the AM signal. To make this measurement
more accurately, set the center frequency to a point on the AM signal
where the slope of the AM signal’s skirt is steep and then set the
analyzer in zero span. Set the analyzer sweep time to about 2 times the
modulation rate and increase the number of sweep points.
Another way to calculate the modulation rate would be to view the
signal in the frequency domain and measure the delta frequency
between the peak of the carrier and the first sideband.
Figure 11-1 Measuring Time Parameters