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Model 5340A
Theory of Operation
4-175. A10 VCO NO. 2 ASSEMBLY, 05340-60008
4-176. When the counter is in the search phase, the VCO (Figure 8-16) provides a n output which
varies from -100 to 185 MHz. When phase lock occurs, the frequency of the VCO is held constant
and used by other circuitry. There are two outputs, one to the sampler driver A2A2, and the other
to the mixer A l l . Buffer U1B drives matching transformer T1 and receives a VCO inhibit signal
from control board A21.
4-177. U1A is a digital device that outputs EECL levels but
feedback through low pass filter LlO, C14, and R21. The VCO consists of Q5, C10, C12, L11,
CR4, and CR5. CR4 and CR5 are voltage variable capacitors used to tune the VCO in response
to voltage variations a t the collector of Q5. The oscillator consists of Q5 and the resonant circuit
formed by CR4, which is capacitiire, and L11-CR5, which appears inductive. C10 and C12 form a
capacitive divider feedback path. The oscillator output is amplified and buffered by Q6.
4-178. R17 and R20 are current limiting resistors and L7,
the two circuits. Input decoupling is accomplished with a wideband ferrite choke Z1. This pre-
vents the VCO signal from leaking back to the input leads and interfering with other circuits. Z1
has the property of looking resistive over a wide band of frequencies and appears
The impedance of Z1 is constant over the frequency range of the oscillator.
4-179. Q3 is a constant current source for Q2. CR1, R8, and R25 set a constant bias voltage for
Q3 to determine the value of the constant current. The emitter of Q2 is a low impedance point
and the resistor networks Rl-R6 and R2-R7 are summed
at AlO(7 and 8) change the current through Q2. This varies the collector voltage of Q2 which in
turn causes a voltage change
VCO frequency. Q2 has a nonlinear collector load to compensate for the nonlinear tuning char-
acteristic of the VCO. The linearizing circuit consists of CR2, R5, and R9. As the collector
voltage of Q2 changes, additional load resistors are switched in by CR2 to change the gain of the
stage. Q4 is a n emitter follower to buffer the linearizer.
the current through Q1 decreases to drop less voltage across CR1 and R8. This tends to reduce
the current in Q3 thereby increasing the voltage on the collector of Q2 to compensate for the
decrease on the +15V supply. Any increase of voltage on the +15V supply is similarly com-
pensated. The circuit is decoupled from noise on the -15V supply by constant current source Q3.
4-181. L5 and C7 constitute a lowpass filter to reduce noise generated by input amplifier Ql-Q3.
response. Filters L1, L2, C1, C5, C6, L6, C9, C11, L9, and C16 prevent noise from being dis-
tributed by the power supply lines.
A l l MIXER ASSEMBLY, 05340-60009
4-182.
4-183. The mixer (Figure 8-17) receives the two VCO signals and provides a difference fre-
quency output to Bandpass Filter/Phase Detector A8. Mixer U2 is a differential amplifier with a
constant current source. The signal from VCO No.
current source. The signal from VCO No. 2 connects to one side of the differential amplifier (U2
pin 12). The other input to the differential amplifier connects to ground (U2 pin
is the substrate which connects to -5 volts. The output of U2 is a 20 kHz (nominal) square wave
output. U1 provides a gain of 22 with low response to the harmonics and supplies a 20 kHz sine
wave a t approximately I V p-p output. The power supply voltages (+15V and -5V) are decoupled
from the power supplies by L1, L2, and C1, C2, and C5 through C9. The
to bandpass filter phase detector A8.
NO.
4-184. A12 VCO
1 ASSEMBLY, 05340-60008
4-185. Figure 8-18 shows' the schematic diagram for A12. The operation of A12 is identical to
that of A10. One additional output
counting.
4-42
at the emitter of Q4 and the cathodes of CR4 and CR5 to change the
is attenuated by R1, C4, R3 and drives the
1
at pin 5 is used to route the VCO No.
it
is biased in its active region by dc
a n d C13 are decoupling filters for
as a
at the emitter of Q2. Voltage changes
Pin 1 of U2
20
1 output to A22 for
DC short.
,
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