Fluke 6080A Service Manual page 179

RF LEVEL CIRCUIT DESCRIPTION
The circuits on the A8 Output PCA, the A10 Premodulator PCA, and the A11
Modulation Control PCA are interrelated and are described here as a unit. The
Premodulator PCA receives a 480 to 1056-MHz RF signal from the Sum Loop VCO,
A9. The Premodulator PCA uses divide-by-two circuits and switches to develop a 15 to
1056-MHz RF signal. This signal is applied to the Output PCA. The Output PCA
contains the level/AM modulator, generates a detected voltage for the leveling loop,
develops the 0.01 to 14.999999-MHz HET BAND signal, contains the pulse modulator
circuits, and provides the final amplification of the 0.01 to 1056 MHz output signal.
The Modulation Control PCA distributes DC power and control signals to the Output
PCA, the Premodulator PCA, the FM PCA, and the Sum Loop PCA. It also controls
and distributes internal and external modulation signals for AM, FM, øM, and Pulse
modulation.
The Output Assembly provides a 0.01 to 1056-MHz RF signal to the A20
Attenuator/RPP Assembly. The Attenuator/RPP provides 0 to 138-dB of attenu-
ation in 6-dB steps and provides protection for the output circuits.
RF Path
The RF path begins with the 480 to 1056-MHz signal at J5 on the Premodulator PCA.
This signal comes from the Sum Loop VCO PCA. A double-pole, double-throw
switch (CR1, CR2, CR3, CR4) sends this signal directly to amplifier U4 or through the
divide-by-two circuit, U2, and then to U4. The switch is controlled by the logic signal
MIDH. Both paths use frequency-shaping networks to flatten the frequency response.
The input frequency to U4 is then 240 to 1056 Mhz. The signal is further amplified by
U5. The output of U5 is first low-pass filtered at 1100 MHz and then filtered again by
the switched filter that is controlled by logic signals HAOCTH and MIDH. This filter
removes harmonics with low-pass filters switched at 350 MHz, 512 MHz, and 730
MHz. The particular filter in place depends on the logic state of HAOCTH and MIDH
which control CR9-19.
The 240 to 1056-MHz signal goes to the double-pole, double-throw switch (CR22,
CR23, CR31, CR32) which is controlled by the logic signal GT256H. This switch
directs the signal directly to the amplifier, Q6, or to the divider chain beginning with
U11.
The U11 output is split. One output provides 128 to 256 MHz to a switched filter for
reducing harmonics, and the other output provides an input signal to the third divider,
U58. The 128 to 256-MHz output passes through a low-pass filter (selected by logic
signal GT180H) for frequencies below 180 MHz. Above that, frequency filtering is
provided by the 260-MHz LPF between U10 and CR31. When a frequency in this band
is selected, logic signal 1HAFH2 is high, which turns on CR71. The network between
CR25,27, and CR71 provides level adjustment of the signal. U58 has two outputs. One
output provides 64 to 128 MHz to an output filter consisting of T2 and the LPF
following it. The second output provides these same frequencies to the fourth divider,
U8. U58 is activated by D1HAFL being high to provide DC bias and D2HAFH low to
activate the divider. U8 provides an output of 32 to 64 MHz. U8 and its output are
selected by logic signals D3HAF and 3HAFH. The fifth divider, U9, generates 15 to 32
MHz. U9 and its output are selected by logic signals D4HAFL and 4HAFH. Filtering
for harmonic suppression, when the frequency is in the 15 to 64-MHz range, is done on
the output board following the modulator. The appropriate band is selected by diodes
CR71, CR28, CR29, or CR30 and is amplified by U10. When a frequency from 15 to
256 MHz is selected, it passes to the amplifier Q6, through CR31. Q6 provides 6 dB of
gain and buffering for the 15 to 1056-MHz signal that is the input to the Output PCA.
TROUBLESHOOTING AND REPAIR
6D-3.
6D-4.
RF LEVEL/AM
6D-3