Tektronix 1L30 Instruction Manual page 19

Fig. 2 -11. Measuring pulse repetition time.
the bandwidth capabilities of the analyzer.
can now be set to examine the modulation pattern.
Timing information such as pulse repetition rate may be
obtained by triggering the sweep on the signal source
(Internal mode) and switching the Time/Cm control to a
calibrated sweep time that will permit time measurement
between the modulation pulses. See Fig. 2-11.
Triggering ihe Sweep
For most applications the oscilloscope triggering is set
for free run operation, however, there are applications, for
example; at 0 dispersion, or when slaving the Type 1L30
to a recorder, that it may be desirable or necessary to trig­
ger the display.
The display may be. triggered internally by setting the
oscilloscope Source switch to the Int position and adjusting
the triggering controls to trigger on the display. The oscil­
loscope requires approxim ately 2 millimeters of signal am­
plitude to trigger satisfactorily.
fore, to adjust the FINE RF or IF CENTER FREQ control to
shift the sweep start away from a spectrum null point. See
Fig. 2-12.
If th e 'signal is time related to the power supply line fre­
quency, it is best to trigger the oscilloscope on the Line
frequency.
In applications that are single shot events, the triggering
can be set for single sweep operation and the trace trig­
gered before the event by some external source.
Recorder Out
Signals on the display may be recorded by plugging a
phone plug into the TO RECORDER output jack. A linear out­
put is provided when the VERTICAL DISPLAY switch is in
the LOG and LIN positions.
Sweep time
It may be necessary, there­
W ith the DISPLAY switch in
Fig. 2-12. To trigger the analyzer from the display requires 0.2 cm
of signal. Tune the spectrum null point away from the sweep start­
ing point, with the RF CENTER- FREQ control.
the SQ LAW position, the output to the RECORDER con­
nector is square law.
SPECTRUM ANALYZER DISPLAYS
The Spectrum Analyzer display is a plot of signal am­
plitude as a function of frequency. W ith this type of dis­
play, individual frequency components within the signal
are displayed and readily analyzed. This section describes
some of these basic spectrum displays and basic applica­
tions for the Type 1L30.
Spectra of Amplitude Modulation
When a single frequency (CW) signal is amplitude-modu-
.lated by a signal frequency,, two additional frequencies will
be generated; the carrier plus the two sidebands.
Fig. 2-13. The amplitude of either sideband with respect
to the carrier voltage is V2 the percentage of modulation.
The frequency difference between the carrier and either
sideband is equal to the modulating frequency.
Figure 2-14 illustrates how the spectrum is generated
when a fundamental carrier frequency F is modulated by
two frequencies Fx and F2.
The sideband spectrum, of a multiple frequency ampli­
tude-modulated signal spectrum, is determined by the mod­
ulating frequencies. To resolve this complex spectrum, the
analyzer resolution bandwidth must be less than the low­
est modulating frequency, or the bandwidth must be less
than the difference between any two modulating frequencies,
whichever is the smaller.
In wideband amplitude-modulation applications, such as
television picture information, the spectrum analyzer may
be used to measure both the sideband energy distribution
and the modulation bandwidth.
See
2 -13
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