2.
The pulse width is equal to the reciprocal of the main lobe
A visual means is provided to tune the transmitting
frequency width, or the reciprocal of the side lobe frequency
system and obtain most of the output power within the fre-
width. See Fig. 2-27.
quency range of the receiver bandwidth.
3. The frequency difference between the first two minima
Repetition Rate: The pulse repetition rate is measured
of any spectrum is a measure of the modulating pulse width.
when the spectrum analyzer is switched to zero dispersion
See Fig. 2-27.
and the analyzer becomes a fixed tuned receiver. The sweep
is then triggered on the signal and becomes a time domain
4. A spectrum without deep minima points adjacent to
display. The procedure is as follows:
the main Iobe indicates the presence of frequency modula-
tion. See Fig. 2-22.
1. Tune the signal to the display center with the RF CEN-
TER FREQUENCY and the IF CENTER FREQ controls.
5. If the spectrum has two peaks, the oscillator is operating
in two modes or it is pulled in frequency by some external
2. Change the DISPERSION RANGE switch to kHz, then
factor, such as mismatched transmission lines or fluctuating
decrease the DISPERSION to 0. Uncouple the RESOLUTION
voltages (providing the resolution of the analyzer is sufficient).
control and turn to the fully clockwise position. The analyzer
is now a fixed frequency device with no dispersion.
Measurements of Pulse Modulated RF Signals
3. Set the Trigger SOURCE switch to INT, the SLOPE switch
to + position, then adjust the LEVEL control for a stable
Pulse Width: Since the theoretical pulse width for a square
display. The IF CENTER FREQ-FINE control may require
pulse is the reciprocal of the spectral side lobe frequency
slight adjustment to displace the spectrum null point from the
width, the main frequency lobe or its side lobes can be used
sweep start. See Fig. 2-14. The Type 491 requires a 0.2
to measure the pulse width of the pulse modulated spectrum.
divisions of signal to trigger internally.
This is accomplished with the Type 491 as follows:
1. Adiust the DISPERSION control and tune the RF CENTER
4. Set the VARIABLE control to the CAL detent then adjust
FREQUENCY control so the main lobe of the spectrum is
the TIME/DIV selector so several pulses of the received signal
displayed in the center of the graticule, and the side lobes
are displayed. See Fig. 2-27C. The number of pulses dis-
are visible on each side.
played is now a function of the sweep rate and the signal
PRF.
2. Adjust the GAIN control and switch in the necessary
IF ATTENUATION dB switches, so the main lobe and its side
5. Measure the number of divisions between 2 or more
lobes are within the graticule height.
pulses on the graticule.
3. Adjust the TIME/DIV selector for optimum spectrum
6. The pulse repetition frequency is the reciprocal of the
definition,
period between pulses.
4. Adjust the RESOLUTION control so the nulls are easily
In the example of Fig. 2-27C, the repetition time is
discernible without excessive loss of sensitivity. Change the
mode selection of the VERTICAL DISPLAY switch to accen-
tuate these minima points. (Usually LOG position.)
5. Calculate the dispersion of either the main lobe or a
The pulse repetition frequency
1000 Hz.
side lobe as directed under measuring frequency difference.
2-24
