Fluke 6080A - Alternate Reference Frequency Selection 6 C-25.; COARSE LOOP VCO (A5) CIRCUIT DESCRIPTION 6 C-26.

TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
Alternate Reference Frequency Selection 6C-25.
REMARKS:
The unit is configured at the factory for 5-MHz external reference when in alternate
reference frequency mode, SPCL 761. These are the switch settings for 1- or 2- MHz
external reference.
PROCEDURE:
On the synthesizer module bottom cover, remove the metal hole plug labeled A2S502.
Set the switches as follows:
REFERENCE
5 MHz
2 MHz
1 MHz
1
on
off
off
2
on
off
on
3
off
on
off
4
off
on
on
5
on
off
off
6
off
off
on
BAND
1
2
3
FREQUENCY RANGE (MHz)
576 - 704
712 - 824
832 - 968
CSVCO0H
0
0
1
CSVCO1H
0
1
1
COARSE LOOP VCO (A5) CIRCUIT DESCRIPTION 6C-26.
The A5 Coarse Loop VCO PCA is controlled by the A2 Coarse Loop PCA and
produces a signal that is further processed in the A12 Sum Loop PCA. This assembly
includes three varactor-tuned oscillator circuits that cover the frequency range 576 to
960 MHz, programmed by binary control signals CSVCO0H and CSVCO1H, as
follows:
The three oscillator circuits are of similar design but have different element values and
printed transmission line lengths to cover the three bands. In the following discussion,
reference designators for the band 1 oscillator are specified. Corresponding elements
for the other oscillators are obvious from the schematic.
Each oscillator uses a common-base transistor (Q3) configured for negative resistance
at the emitter. The emitter is coupled to a resonator that consists of a printed
transmission line in series with varactor diodes (CR5, CR6) and low loss porcelain
capacitors (C5, C6). Two tuning voltage lines connect to the varactor cathodes and
anodes via RF chokes L6 and L3, respectively. The cathode lines connect to the VCO
steering port, J6. The anode lines connect to the VCO phase lock port, J5. These ports
are used by the A2 Coarse Loop PCA to control the operating frequency. The voltage
across the varactors, measured between J6 and J5, varies approximately linearly with
frequency in each band, from about 2V to 20V.
The +13 dBm nominal signal at the oscillator transistor collector is applied to an 8-dB
attenuator that provides isolation (R13-R15), and then to a low-pass filter that
attenuates harmonics to less than -20 dBc (C41, C42, and printed lines). PIN diode
CR9 has low RF resistance and passes the oscillator signal when the oscillator is on and
goes to a high impedance when the oscillator is off.
6C-31