Tektronix 1L20, Инструкция по эксплуатации

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Operating Instructions— Type 1L20
control is changed. This will be especially evident when
using the .2 M H z/C M and .5 M H z/C M setting. Since
the same dispersion/cm displays are available at 200 kHz/
CM and 500 kHz/CM, but with a considerable increase in
accuracy, it is recommended that the kH z/C M settings be
used when precise measurements are needed and when the
decreased IF CENTER FREQ control range ( + and — 2.5
MHz versus + and — 25 MHz) is acceptable.
The front-panel D1SP CAL adjustment (in conjunction
with the 1 MHz CAL MARKERS OUT signal) can be reset to
improve the dispersion accuracy for specific setting of the
IF CENTER FREQ control. This is accomplished as follows:
1. Set the front-panel controls for the desired signal dis­
play.
2. Connect the 1 MHz CAL MARKERS OUT signal to the
RF INPUT connector and set the DISP CAL adjustment to
give the correct marker/division display.
3. Reconnect the external signal to the RF INPUT con­
nector and make the desired measurements.
4. Be sure to recalibrate the DISP CAL adjustment when
finished with step 3; center the IF CENTER FREQ controls,
set the DISPERSION/CM to 500 kHz, connect the 1 MHz
MARKERS to the RF INPUT connector, and reset the DISP
CAL adjustment for 1 marker/2 divisions.
Obtaining Optimum Resolution
The resolution of a Spectrum Analyzer is the measure of
the instrument's ability to separate individual signals. The
resolution is a function of the IF bandwidth, sweep fre­
quency rate, and dispersion. At very slow sweep rates,
the effective resolution of the analyzer is determined by
the —6dB bandwidth of its IF circuits and w ill closely
resemble the IF response curve.
The effective resolution at a specific sweep rate and dis­
persion is given by:
R
B
1 + 0.195
2]V l
where.
R = Resolution
B = Bandwidth in hertz
D = Dispersion in hertz
T = Sweep time in seconds
The sensitivity of the instrument is also dependent on the
same factors. The sensitivity to be expected can be calcu­
lated mathematically as:
S S0 1 + 0.195
u
D
TB2
2
v4
where:
S = Sensitivity
S0 = Sensitivity at very slow sweep speeds
and zero dispersion
D = Dispersion in hertz
T == Sweep time in seconds
B = Bandwidth in hertz
Usually, the resolution of the Spectrum Analyzer w ill be
near optimum at a given setting of the DISPERSION con­
trols when the DISPERSION and COUPLED RESOLUTION
controls are coupled together (both switches set to the same
position), although the RESOLUTION control can be turned
separately if desired by pulling out on the knurled section
of the knob.
The time base of the oscilloscope should be set for the
fastest sweep rate a t which no distortion or amplitude loss
is noticed in the display. If the Spectrum Analyzer is being
used in combination with a Type 549 Storage Oscilloscope,
the slow-sweep display may be easier to analyze if the
oscilloscope is operated in the Storage Mode.
Sensitivity of the Spectrum Analyzer to pulse signals is
a function of the bandwidth of the instrument as stated
above. However, if the bandwidth is too large, the minima
of the spectrum are no longer zero. To adjust the RESOLU­
TION for a pulse signal, set the oscilloscope sweep rate for
a pulse repetition frequency of about 40 lines in the prin­
cipal lobe of the spectrum. Then, adjust the RESOLUTION
control for well-defined lobe zeros without ringing (see Fig.
2-6). This setting corresponds to a bandwidth-pulse width
product of 0.1 or less.
Fig. 2 -6. Display of the frequency spectrum of a pulsed cw signal.
Relative Amplitude Measurements
The relative amplitudes o f two signals can be measured
as follows:
1. Center the IF CENTER FREQ controls.
2. Tune the smallest-amplitude signal to the center of
the screen, using the RF CENTER FREQ control.
3. W ith all of the IF ATTEN switches OFF, adjust the
GAIN control for exactly 4 centimeters of vertical deflection
of the smallest signal.
4. Tune the larger-amplitude signal to the center of the
screen.
5. Switch in as many IF ATTEN switches as are required
to make the larger-amplitude signal exactly 4 centimeters
high.
6. Add the settings of the IF ATTEN switches that were
switched in. The total is the relative amplitude difference,
in dB, between the two signals.
2 -7