Tektronix 491 Maintenance Manual page 37

CHECK switch to the vertical deflection system. At zero beat,
the output signal amplitude snaps to a minimum and the cir-
cuit locks the local oscillator to the reference frequency by
feeding a corrective voltage to a Varactor diode in the
oscillator circuit.
FINE RF CENTER FREQ control R1182 tunes the local oscil-
lator over a limited range by changing the DC output level
of Q1180. When the circuit is in a lock condition, any small
shift of the FINE RF CENTER FREQ control is counteracted
by the phase lock circuit.
the circuit will lose the lock and the oscillator will jump
to a different frequency lock point. This jump in frequency
is easily seen as a shift in signal position at dispersion set-
tings of 500kHz/div or less,
An isolation switch is used to minimize the loading affect
of the lower band (A & B) phase detectors on the band C
phase detector. The junction at diodes D1170 and D1174
is returned, through a filter network and the Band selector
s w i t c h S W 7 0 , t o + 1 5 0 V . T h e d i o d e s a r e f o r w a r d b i a s e d
when the Band selector switch is in the A or B position. When
the switch is changed to the C position, the diode switch is
open.
Band C Bal and Bands A & B Bal adjustments correct any
imbalance between the phase detectors so the DC output
level of the amplifier remains balanced
i s s w i t c h e d b e t w e e n b a n d s . T h e F I N E R F C E N T E R F R E Q
control must be centered for this adjustment.
If the control is further moved
the Band selector
as
Fig. 3-4. Simplified phase detector circuit.
Sweeper Circuit
This circuit (see Fig. 3-5) provides a swept frequency,
centered at 275 MHz, to the wide band mixer amplifier. The
swept frequency amplitude is constant and the dispersion
can be varied from about O Hz to 100 MHz.
A positive-going sawtooth voltage from the sweep genera-
tor circuit is applied through pin AE to the emitter of Q200.
Q200 is configured as a long-tail amplifier. It converts the
sawtooth voltage input signal to a linear current ramp, which
is applied through the DISPERSION attenuator to one side
of comparator amplifier Q220-Q230.
The output DC level of the current ramp from Q200 is
set by Sweep Center adjustment R203. Dispersion CAL adjust-
ment R208, shunts the dispersion attenuator. It calibrates
the dispersion for the 10 MHz/div position of the DISPERSION
selector by adjusting the output amplitude of the current
ramp from Q200. The remaining positions the selector are
then within instrument specifications.
Two dispersion ranges (MHz/DIV and kHz/DIV) are pro-
v i d e d b y t h e D I S P E R S I O N R A N G E s e l e c t o r R 2 1 0 , w h i c h
selects a different range of resistance values for each position
of the DISPERSOIN selector.
Sweep Comparator.
The sweep comparator containing
Q220 and Q230 compares the current ramp from the dis-
persion attenuator against a current ramp applied to the
base of Q230. The signal applied to the base of Q230 is the
3-5