CHECK switch to the vertical deflection system. At zero beat,
Sweeper Circuit
the output signal amplitude snaps to a minimum and the cir-
This circuit (see Fig. 3-5)
provides a swept frequency,
cuit locks the local oscillator to the reference frequency by
centered at 275 MHz, to the
wide band mixer amplifier. The
feeding a corrective voltage to a Varactor diode in the
swept frequency amplitude
is constant and the dispersion
oscillator circuit.
can be varied from about O
Hz to 100 MHz.
FINE RF CENTER FREQ control R1182 tunes the local oscil-
A positive-going sawtooth voltage from the sweep genera-
lator over a limited range by changing the DC output level
tor circuit is applied through pin AE to the emitter of Q200.
of Q1180. When the circuit is in a lock condition, any small
Q200 is configured as a long-tail amplifier. It converts the
shift of the FINE RF CENTER FREQ control is counteracted
sawtooth voltage input signal to a linear current ramp, which
by the phase lock circuit. If the control is further moved
is applied through the DISPERSION attenuator to one side
the circuit will lose the lock and the oscillator will jump
of comparator amplifier Q220-Q230.
to a different frequency lock point. This jump in frequency
is easily seen as a shift in signal position at dispersion set-
The output DC level of the current ramp from Q200 is
tings of 500kHz/div or less,
set by Sweep Center adjustment R203. Dispersion CAL adjust-
ment R208, shunts the dispersion attenuator. It calibrates
An isolation switch is used to minimize the loading affect
the dispersion for the 10 MHz/div position of the DISPERSION
of the lower band (A & B) phase detectors on the band C
selector by adjusting the output amplitude of the current
phase detector. The junction at diodes D1170 and D1174
ramp from Q200. The remaining positions the selector are
is returned, through a filter network and the Band selector
then within instrument specifications.
switch SW70, to +150V. The diodes are forward biased
Two
dispersion ranges (MHz/DIV and kHz/DIV) are pro-
when the Band selector switch is in the A or B position. When
vided
by the DISPERSION RANGE selector R210, which
the switch is changed to the C position, the diode switch is
selects
a different range of resistance values for each position
open.
of the
DISPERSOIN selector.
Band C Bal and Bands A & B Bal adjustments correct any
Sweep Comparator. The sweep comparator containing
imbalance between the phase detectors so the DC output
level of the amplifier remains balanced as the Band selector
Q220 and Q230 compares the current ramp from the dis-
persion attenuator against a current ramp applied to the
is switched between bands. The FINE RF CENTER FREQ
base of Q230. The signal applied to the base of Q230 is the
control must be centered for this adjustment.
3-5
