Tektronix 1L40, Instruction Manual

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Operating Instructions— Type 1L40
Fig. 2 -7 . 1 MHz M arkers O utput signal (phase lock reference)
a p p lie d to the RF INPUT connector to check dispersion calibration .
GENERAL OPERATING INFORMATION
RF Center Frequency Tuning
The dial and the analyzer are tuned through the frequency
range of each scale by the RF CENTER FREQ control. The
dial frequency calibration is accurate to within zb (2 MHz - f
1% of the dial reading) when the FINE RF CENTER FREQ
and the IF CENTER FREQ controls are centered. As the
dial knob is rotated clockwise, the dial tape increases in
frequency and true signals (see spurious responses) travel
across the screen from left to right.
The RF CENTER FREQ control is supplemented by a FINE
RF CENTER FREQ control that provides a fine tuning adjust­
ment, through a limited frequency range, on either side of
the dial frequency. This provides a fine tuning adjustment
to establish phase lock operation.
MIXER PEAKING Control
The front-panel MIXER PEAKING control provides an ad­
justment to improve the over-all sensitivity of the Spectrum
Analyzer. Its action is broad; therefore it can usually be
set to an optimum setting and left unless there is a large
change in the RF center signal frequency, or a change in
RF dial scale.
Phase Lock Operation
The 1st local oscillator can be phase locked to either
an internal 1 MHz reference oscillator or an external fre­
quency source when it is applied to the EXT REF FREQ IN
connector. Locking the local oscillator to a stable fre­
quency, such as the internal 1 MHz crystal controlled
oscillator, reduces the local oscillator incidental frequency
modulation and frequency drift. This allows narrow disper­
sion and high resolution settings for signal analysis.
The frequency range for an external reference frequency
is 1 MHz to 5 MHz, and amplitude limitations are 1 to 5
volts peak to peak. The external signal fo r phase lock
operation is applied to the phase lock circuit when the
INT REF FREQ control is turned ccw to the OFF OR EXT
REF FREQ IN position.
The LOCK CHECK pushbutton applies the output of the
phase lock am plifier to the vertical display system. The
output of the phase lock am plifier contains the following:
(1) Beat frequency signals between the local oscillator and
the reference frequency when the oscillator frequency is
very close to a lock with the reference frequency. (2) A DC
reference
level of the output amplifier. This DC level
changes as the FINE RF CENTER FREQ control is rotated and
shifts the local oscillator frequency a slight amount. It also
affects the vertical position of the display baseline. Thus,
by depressing the LOCK CHECK button and slowly turning
the FINE RF CENTER FREQ control, the operator w ill o bserve
the
baseline of the display shift until a lock mode is
reached. The baseline w ill then remain stationary over a
portion of the control range as the circuit holds the local
oscillator locked to the reference frequency. Turning the
control further causes the circuit to lose its lock and the
baseline jumps from the locked position.
Beat frequency signals are usually displayed just before
a T ock point is reached. See Fig. 2-6B. However, through
part of the Type 1L40 frequency range, the phase lock
operation may be very positive and the local oscillator
w ill jump from one lock mode to another without displaying
the beat signals or the smooth shift of the display baseline
between lock points.
When the DC operating level of the phase lock amplifier
reaches either extreme (top or bottom of the graticule area),
the operation of the am plifier becomes non-linear and com­
pression of the beat signals w ill be noted. Phase lock
operation becomes difficult to achieve. The displayed DC
level therefore aids in setting a phase lock condition within
the linear operating range of the phase lock amplifier.
Part of the input signal is coupled through and displayed
when the LOCK CHECK button is pushed. This permits the
operator to re-establish a particular lock point that may be
lost because of oscillator d rift or other reasons. The opera­
tor adjusts the FINE RF CENTER FREQ control while observ­
ing the display until the signal is again at a particular lock
point (the point where the baseline or the signal position locks)
is set.
The local oscillator fundamental frequency locks in 1 MHz
steps, (from one lock mode to the next) when the internal
1 MHz reference frequency is used for phase lock operation.
This produces gaps as much as 5 MHz in the upper fre­
quency scale, where the upper harmonic of the local oscilla­
tor is used. Continuous tuning through these gaps is
provided by the INT REF FREQ control. Rotating the control
through its range pulls the crystal controlled reference fre­
quency approximately 1 kHz. This is sufficient to shift the
local oscillator frequency through these gaps and maintain
phase lock operation.
Phase lock operation is established as follows:
1. Tune the desired signal to the center of the display with
the RF CENTER FREQ control.
2. Depress the LOCK CHECK button and adjust the FINE
RF CENTER FREQ control for a lock indication within the
center (4 cm) of the graticule. If the lock indication or beat
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