erence 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 de-
pressing the LOCK CHECK button and slowly turning the
FINE RF CENTER FREQ control (Fig. 2-15), the operator will
observe the baseline of the display shift until a lock mode is
reached. The baseline will then remain stationary over a
portion of the control range as the circuit holds the local
oscillator locked to the reference frequency. Turning the con-
trol 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
lock point is reached. See Fig. 2-16. However, through part
of the frequency range, the phase lock operation may be
very positive and the local oscillator will 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 amplifier becomes non-linear and com-
pression of the beat signals will be noted. Phase lock opera-
tion becomes difficult to achieve. The displayed DC level
thus 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 drift 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).
The local oscillator fundamental frequency locks in 1 MHz
steps, (from one lock mode to the next) then the internal
1 MHz reference frequency is used for phase lock operation.
This produces gaps of 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 frequency ap-
proximately 1 kHz. This is sufficient to shift the local oscil-
lator frequency through these gaps and maintain phase lock
operation.
I
Phase lock operation is established as follows:
1. Tune the desired signal to the center of the display with
the RF CENTER FREQUENCY control.
2. Depress the LOCK CHECK button and adjust the FINE RF
CENTER FREQ control for a lock indication within the center
(4 div) of the graticule. if the lock indication or beat signal
is outside the linear operating range of the amplifier (base-
line of display at the top or bottom of the graticule), center
the display with the FINE RF CENTER FREQ control, then
adjust the RF CENTER FREQUENCY control to shift the signal
towards a beat mode. Adjust the FINE RF CENTER FREQ con-
trol while observing the desired signal for phase lock opera-
tion, then release the LOCK CHECK button.
2-15
