Tektronix 1L20, Instruction Manual, Page: 9 / 117

Operating Instructions— Type 1L20
16. Change the DISPERSION RANGE switch to kHz/CM
and set the DISPERSION-COUPLED RESOLUTION switch to
10 (inner ring).
17. Note the setting of the FINE FREQ control. Slowly
rotate the control through this setting. Note that the signal
will stay locked in place while the control is rotated through
10 to 15 degrees.
18. Disable the phase-lock action by changing the FINE
FREQ control. Note the increased horizontal d rift and FM
shift. (If necessary, bring the signal back onto the CRT by
adjusting the IF CENTER FREQ controls.)
19. Pull out on the COUPLED RESOLUTION knob and
rotate it toward the clockwise end of its rotation without
changing the DISPERSION setting. Note the broadening of
the displayed signal.
FRONT PANEL ADJUSTMENTS
Any time the Type 1L20 Spectrum Analyzer is moved from
one oscilloscope to another, the front-panel IF CENTER FREQ
CAL and DISP CAL adjustments must be recalibrated to com­
pensate for differences in sawtooth amplitude and CRT sensi­
tivity. These adjustments should also be checked occasionally
during the regular use of the instrument.
1. IF CENTER FREQ CAL Adjustment
a. Set the front panel controls and connect the Spectrum
Analyzer to the oscilloscope as directed in steps 1 through 6
of First Time Operation.
b. Apply an external signal (10-4200 MHz) to the appro­
priate RF INPUT connector.
c. Tune in the signal with the RF CENTER FREQ control
and adjust the G AIN and IF ATTEN controls for a signal
amplitude of about 5 centimeters.
d. Set the RF CENTER FREQ control for no signal shift
as the DISPERSION RANGE control is switched back and
forth between the M H z/C M and kHz/CM positions.
e. Set the IF CENTER FREQ CAL adjustment for no signal
shift as the DISPERSION-COUPLED RESOLUTION control is
rotated.
f. Position the signal to the graticule center with the
oscilloscope HORIZONTAL POSITION control. (On some
oscilloscopes, it may be necessary to readjust R204, the
internal Swp Ctr adjust, located 3 inches behind the analyzer
G A IN control.)
2. DISP CAL Adjustment
a. Set the Type 1L20 front-panel controls as follows:
IF ATTEN
IF CENTER FREQ
DISPERSION RANGE
DISPERSION-COUPLED
RESOLUTION
VERTICAL DISPLAY
All OFF
000 (centered)
kHz/CM
500
SQ LAW
RF INPUTS (switch) BANDS 2-5
PHASE LOCK INT 1 MHz REF FREQ
b. Connect the 1 MHz CAL MARKERS OUT signal to the
BANDS 2-5 RF INPUT connector.
c. Adjust the G A IN control for a marker display about 3
centimeters in height.
d. Use the FINE IF CENTER FREQ control to keep the
markers aligned with the graticule lines while setting the
DISP CAL adjustment for 1 marker/2 centimeters.
Applied Signal Precautions
Signals applied to the RF INPUT connectors should be
connected through a 50-ohm coaxial cable, using a BNC
connector for Band 1 (10 MHz to 275 MHz) signals and a
Type N connector for Bands 2-5 (275 MHz to 4200 MHz).
Unshielded connections will tend to pick up stray unwanted
signals and cause a confusing display.
Reflections caused by mismatches between the signal source
and the Type 1L20 RF INPUT sometimes cause “ dips” in the
signal display on the CRT. When good conversion flatness
is desired and sufficient signal strength is available, a 6dB
50 f2 attenuator pad can be inserted between the source and
the Analyzer to minimize these reflections.
The signal applied to the RF INPUT should not be stronger
than —30 dBm (7 millivolts RMS) for an undistorted display.
(See Fig. 2-2). As a matter o f practice, stay well aw ay from
the point where compression (no increase in height with an
increase in signal strength) is noticed. Back off at least 10 dB
from this point. Otherwise, the Spectrum Analyzer might be
over-driven, resulting in the generation of spurious responses
due to harmonic mixing a t a much higher level than normal.
A t times, this harmonic mixing may be advantageous—
such as during parts o f the Calibration Procedure when it
is desired to calibrate the instrument using harmonics Of the
Time-Mark Generator. Normally, however, such operation
w ill only lead to a confusing display.
A t no time should signals stronger than + 1 5 dBm (1 volt
RMS) be applied to the Spectrum Analyzer. Signals above
this limit are a p t to damage the diode mixers in the RF
INPUT circuits.
Vertical Display
An RF signal is displayed on the screen of the Spectrum
Analyzer as a pip, with the amplitude of the pip represent­
ing the signal strength and the horizontal displacement rep­
resenting the frequency of the signal. The shape of the pip
is a function o f the IF response of the Analyzer. In the Type
1L20, the response can be adjusted by the front panel RESO­
LUTION control in ten steps from 1 kHz (fully counterclock­
wise) to 100 kHz (fully clockwise).
Pulsed signals can be displayed on the screen as a series
of lines, the locus o f which represent the energy distribution
of the signal as a function o f frequency. Frequency domain
representations for a wide variety o f waveforms are to be
found in Design Data for Radio Engineers, ITT Handbook,
Chapter 35.
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