Tektronix 1L40, Instruction Manual

Page: 19 / 145

Operating Instructions— Type 1L40
signifies that both the sensitivity and resolution of the ana­
lyzer have been degraded by the increased sweep rate.
The loss of the analyzer sensitivity due to sweep rate and
the dispersion can be expressed mathematically as:
where the S/S0 is the ratio of the effective sensitivity to the
analyzer measured sensitivity, at very slow sweep times or
with zero dispersion.
D is the dispersion in hertz
B is the — 3 dB bandwidth of the analyzer in hertz
T is the sweep time in seconds, or — — is the scan­
ning velocity.
These same variables also determine the resolution of
the analyzer. The loss in resolution can be expressed as
fo llo w s :
W here R/R0 is the ratio of the effective resolution of the
analyzer to the analyzer measured resolution bandwidth
at very slow sweep speeds. R0 is somewhat arbitrary and
is taken as the displayed width of the CW signal at the
— 6dB point.
The best resolution for a given dispersion and sweep
Dispersion (in Hz)
time is expressed as:
Sweep Time (in s)
See S p e c tru m
Analyzer definitions.
The resolution of the Type 1L40 Spectrum Analyzer is
optimized for most settings of the DISPERSION selector when
the RESOLUTION control is in the coupled position. Resolu­
tion, however, can be varied from approximately 100 kHz
to less than 1 kHz by uncoupling the RESOLUTION control
and adjusting it as an independent function of the DISPER­
SION selector.
To adequately resolve pulsed spectrum information, the
resolution bandwidth of the analyzer should be on the
order of 1/10 of the side lobe frequency width or the
reciprocal of the pulse width. The RESOLUTION control
is usually set, after the sweep rate has been adjusted, for
optimum main lobe detail. See Fig. 2-10.
Selecting the Sweep Rate
The sweep rate for wide resolution coupled settings is
usually set just above the visual flicker setting; however,
as the DISPERSION is decreased the sweep rate w ill begin
to affect the resolution and sensitivity of the analyzer as
described under Resolution. Therefore, as the DISPERSION
settings are reduced the sweep rate should also be reduced
to maintain sensitivity and resolution.
W ith the DISPERSION control set to 0, the analyzer func­
tions as a fixed tuned receiver. The analyzer then displays
time domain characteristics of the signal modulation within
the bandwidth capabilities of the analyzer. Sweep time
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 (In­
ternal Mode) and switching the oscilloscope Time/Cm
control to a calibrated sweep time that w ill permit time
measurement between the modulation pulses. See Fig. 2-11.
Triggering the Sweep
For most applications the oscilloscope triggering is set
for free run operation; however, there are applications
where it may be desirable or necessary to trigger the
1
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TIM E /C M = 1 m s/C M
_ , . . . Number o f CM measured ^ . . . .
Pulse re p e titio n tim e = ------------------------------ ------------ X T im e /C M
Number o f pulses
— X 1 ms = 2 ms, PRF — ~ ~ or 5 0 0 Hz
3 2
Fig. 2 -1 0 . Frequency spectrum o f a pulse m odulated signal.
®
2-13
Fig. 2 -1 1 . M easuring pulse repetition time.