Tektronix 1L20, Инструкция по эксплуатации, Страница: 15 / 117

Operating Instructions— Type 1L20
5. The equivalent pulse width of the modulation signal
can now be determined by measuring the frequency width
(see Fig. 2-8) of either the main lobe (Afmain) or a side lobe
(Afside) as directed in Frequency Difference Measurements
on page 2-8 and calculating for pulse width:
Afmain A f side
where:
t = pulse width in microseconds
Afmain = frequency w idth of main lobe in
MHz
A fSide = frequency width of side lobe in
MHz
Repetition Rate Measurements of Pulsed Signals
The spectrum analyzer can also be used to measure the
repetition frequency of the pulsed signal. Note in Fig. 2-8
the frequency distribution for the pulsed signal is a series
o f vertical lines. Each line represents one sampling (inter­
ception) of a pulse during the sweep interval. Thus, at zero
dispersion, the line spacing will correspond directly to the
sweep time.
The pulse rate can therefore be determined directly from
the number of lines per division when the sweep speed is
known. The procedure to be followed is given below:
1. Set the RF CENTER FREQ and the IF CENTER FREQ con­
trols so that the signal appears at the center of the screen.
2. Set the DISPERSION switch to 0 and set the DISPER­
SION RANGE switch to kHz/CM. The oscilloscope is now
operating as a time-base oscilloscope, and each line on the
- - - -
- j
I —
UUL.
b
screen is equivalent to one pulse. (It might be helpful if the
RESOLUTION control is uncoupled and reset for better resolu­
tion.)
3. Set the sweep controls of the oscilloscope for + IN T
triggering and set the Stability and Trig Level controls for
a stable trace.
4. Set the Time/CM switch of the oscilloscope to display
several pulses of the applied signal (see Fig. 2-9). Be sure
the Variable Tim e/CM control o f the oscilloscope is in the
Calibrated position.
5. Count the number of lines (pulses) in one or more d ivi­
sions and use the oscilloscope sweep rate to compute the time
between each pulse. In the example of Fig. 2-9, the pulse
repetition rate is:
(1 cm) 50 fis/cm) _ 50 /is _ 2Q kHz
'I pulse pulse
Use of Expanded Sweep
The expanded (magnified) sweep can be used to advan­
tage in certain spectrum analyzer measurements. It is often
desired to examine a very small portion o f the display in
more detail. Theoretically, such an examination could be
performed by changing the DISPERSION— COUPLED RESOLU­
TION control to give the desired display. A faster and easier
method may be to set that part of the display to be examined
to the center of the screen and expand the sweep.
Fig. 2-10 shows an expanded display of the pulsed RF
signal that was shown in Fig. 2-8. The spectrum is being
examined in detail in the vicinity of one of the nulls.
Fig. 2-9 . Pulse repetition rate measurement: 50 /tS /C M sweep.
Fig. 2 -1 0 . X 5 magnification applied to the display shown in
Fig. 2-9.