Tektronix 491, Maintenance Manual

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2.
A visual means is provided to tune the transmitting
system and obtain most of the output power within the fre-
quency range of the receiver bandwidth.
3. The frequency difference between the first two minima
of any spectrum is a measure of the modulating pulse width.
See Fig. 2-27.
4. A spectrum without deep minima points adjacent to
the main Iobe indicates the presence of frequency modula-
tion. See Fig. 2-22.
5. If the spectrum has two peaks, the oscillator is operating
in two modes or it is pulled in frequency by some external
factor, such as mismatched transmission lines or fluctuating
voltages (providing the resolution of the analyzer is sufficient).
Measurements of Pulse Modulated RF Signals
Pulse Width: Since the theoretical pulse width for a square
pulse is the reciprocal of the spectral side lobe frequency
width, the main frequency lobe or its side lobes can be used
to measure the pulse width of the pulse modulated spectrum.
This is accomplished with the Type 491 as follows:
1. Adiust the DISPERSION control and tune the RF CENTER
FREQUENCY control so the main lobe of the spectrum is
displayed in the center of the graticule, and the side lobes
are visible on each side.
2. Adjust the GAIN control and switch in the necessary
IF ATTENUATION dB switches, so the main lobe and its side
lobes are within the graticule height.
3. Adjust the TIME/DIV selector for optimum spectrum
definition,
4. Adjust the RESOLUTION control so the nulls are easily
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
side lobe as directed under measuring frequency difference.
The pulse width is equal to the reciprocal of ½ the main lobe
frequency width, or the reciprocal of the side lobe frequency
width. See Fig. 2-27.
Repetition Rate: The pulse repetition rate is measured
when the spectrum analyzer is switched to zero dispersion
and the analyzer becomes a fixed tuned receiver. The sweep
is then triggered on the signal and becomes a time domain
display. The procedure is as follows:
1. Tune the signal to the display center with the RF CEN-
TER FREQUENCY and the IF CENTER FREQ controls.
2. Change the DISPERSION RANGE switch to kHz, then
decrease the DISPERSION to 0. Uncouple the RESOLUTION
control and turn to the fully clockwise position. The analyzer
is now a fixed frequency device with no dispersion.
3. Set the Trigger SOURCE switch to INT, the SLOPE switch
to + position, then adjust the LEVEL control for a stable
d i s p l a y . T h e I F C E N T E R F R E Q - F I N E c o n t r o l m a y r e q u i r e
slight adjustment to displace the spectrum null point from the
sweep start. See Fig. 2-14. The Type 491 requires a 0.2
divisions of signal to trigger internally.
4. Set the VARIABLE control to the CAL detent then adjust
the TIME/DIV selector so several pulses of the received signal
are displayed. See Fig. 2-27C. The number of pulses dis-
played is now a function of the sweep rate and the signal
PRF.
5. Measure the number of divisions between 2 or more
pulses on the graticule.
6. The pulse repetition frequency is the reciprocal of the
period between pulses.
In the example of Fig. 2-27C, the repetition time is
The pulse repetition frequency 1000 Hz.
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