Fluke 6080A Service Manual page 149

The 40-MHz voltage-controlled crystal oscillator (VCXO) consists of the 40-MHz
third overtone crystal (Y601), a grounded base stage (Q606 and associated com-
ponents), a low Q tuned circuit to ensure the crystal operates at the third overtone
(L601, C604-5, R602), and varactors (CR603-5). This oscillator can be tuned
approximately ± 1 kHz. The feedback is from the collector to the emitter. The output,
from the collector tuned circuit is lightly coupled (C607) to a buffer amplifier (U601).
The output of the buffer is connected to the 80-MHz section (R606, R617, C610) and
another pair of grounded base stages (Q607-8). The outputs of these buffers are
low-pass filtered (C616-17, L604 and C622-23, L605). One output is converted to ECL
(U602), and the other output drives the main loop phase detector buffer amplifier
(Q205).
One output of U602 provides the 40-MHz reference for the Sub-Synthesizer via J16.
The other output is divided by 2 (U506A) and buffered by U513. The 20-MHz square
wave output is band-pass filtered (R36, L510, C545) before being sent to the
modulation oscillator (J8). A second divide-by-2 (U506B) divides the 20 MHz to 10
MHz. This ECL signal is converted to TTL (Q504-05) and buffered (U507C). Two
inverters in parallel (U507A,B) drive a low pass-filter (C518-19, C550, L501) whose
output is a 10-MHz sine wave. This is the reference out for the instrument (J7).
When in internal reference, the 10-MHz signal from U507C is routed through a
multiplexer (U504A) to the R input of the phase frequency detector (U503B, U511D).
When in external reference (10 MHz), divider U505 is disabled and the signal from
U507C is routed through a multiplexer (U504C), logic (U511 A,B) to the R input of the
phase frequency detector. When Special Function 761 is enabled, the 10-MHz signal is
divided down to either 1, 2, or 5 MHz (U505), depending on how the switches are set
(S502). The output of the divider is also routed to the phase frequency detector.
The phase frequency detector (U503, U511D) compares the phase/frequency of the
divided down 40-MHz VCXO (V input) with either the 10-MHz TCXO or an external
reference. The phase detector normally operates at 10 MHz except when an external
reference of 1 ,2, or 5 MHz is used. In these cases, it operates at the reference frequency.
The output of this phase detector operates in the differential voltage mode. If the
frequency/phase of the V input (U503B-11) is greater than the R input (U503A-3),
there will be a net positive voltage from U503A-6 to U503B-8. The loop amplifier/
integrator (U508 and associated components), which drives a lead-lag network (R532,
R530, C546, Q502), integrates the phase detector voltage and causes the voltage on the
control line to the varactors (CR603-5) to drop until the two frequencies match and the
loop is locked. Consequently, if the frequency/phase of the R input is greater than the
V input, there will be a net negative voltage, and the control voltage will rise until the
loop is locked. A small phase offset is set (R512) to keep the phase detector in its linear
region. The bandwidth of this reference loop is changed by switching in various
lead-lag networks (R505, R510, R530, R532, C530, C547, C549, C546). When in
internal TCXO or external 10 or 5 MHz, Q506 and Q502 are on and the loop
bandwidth is approximately 30 Hz. When the external reference is 1 or 2 MHz, Q506 is
on and Q502 is off. By turning Q502 off, the gain is increased 5 times, but since the
division factor is 5 times greater, the overall loop gain remains approximately
constant. The loop control voltage (U508-6) is sensed by two comparators (U509A,B).
If the voltage is below approximately 1.2V and above approximately 11V or there are
pulses, this indicates the loop is unlocked. A one-shot (U512) converts these pulses into
a logic signal. This is combined with the low and high voltage detector to produce
REFUNLKL, which is sent to the controller.
The 40 MHz from the buffer (U601) is amplified (Q609) and doubled to 80 MHz in a
full wave rectifier (CR601, CR602, T601). The 80 MHz is filtered (L611, L612, C643,
TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
6C-19