Fluke 6080A - SUM LOOP (A12) CIRCUIT DESCRIPTION 6 C-29.; RF Section 6 C-30.; Audio Section 6 C-31.

TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
SUM LOOP (A12) CIRCUIT DESCRIPTION 6C-29.
The A12 Sum Loop PCA generates the fundamental frequency band, 480-1056 MHz,
by combining signal frequencies from the FM, the Sub-Synthesizer, and the Coarse
Loop PCAs. The sum loop was designed for spurious signal generation of less than
-100 dBc, and for low phase noise contribution. The relation between sum loop output
frequency and the input frequencies follows.
For f(sum) < 760 MHz:
f(sum) = f(coarse) - f(FM) - f(sub-synth)
For f(sum) >= 760 MHz:
f(sum) = f(coarse) + f(FM) + f(sub-synth)
The A12 Sum Loop PCA includes a phase locked loop circuit that steers the frequency
of the A9 Sum Loop VCO PCA to the correct value, according to the above relations.
The loop includes an RF section with two stages of heterodyne frequency down
conversion, a phase detector circuit, and an audio section containing the loop
amplifier, acquisition circuits, and VCO coarse steering circuits. These sections will be
discussed in the following paragraphs.
The output signal from the A9 Sum Loop VCO PCA is coupled into the Output
Section, where the signal is further processed to generate the instrument output signal.
RF Section 6C-30.
The RF section contains the RF amplifiers, mixers, and filters required to process the
Sum Loop VCO signal through three successive stages of frequency down-conversion.
The first stage subtracts the coarse loop frequency and the Sum Loop VCO frequency
to generate a first IF signal, referred to as the IF1 signal. The second stage subtracts the
FM PCA signal from the IF1 signal to generate a second IF signal, referred to as the
IF2 signal. The third stage includes a mixer phase detector that compares the IF2
signal to the sub-synthesizer signal and generates the audio frequency phase lock signal
that is further processed in the audio section. Frequency ranges for these signals are
given in Table 6C-8. Note that when FM is on, the programmed deviation will appear
in the Sum Loop VCO signal, the IF1 signal, and the FM PCA signal.
Table 6C-8. Sum Loop Frequencies
6C-34
SIGNAL
Sum Loop VCO
Coarse Loop
IF1
FM PCA
IF2
Sub-Synthesizer
FREQUENCY RANGE (MHz)
480 - 1056
576 - 960
88 - 96
80
8 - 16
8 - 16