TROUBLESHOOTING AND REPAIR
FREQUENCY SYNTHESIS
signal at TP2 is next applied to DAC U109, which is programmed by SUMCOMP bits
0-7. These 8 bits encode a number proportional to Sum Loop VCO 1 / Kv. Note that Kv
is the slope of the frequency vs. tuning voltage function. Thus, DAC U109 scales the
signal to account for VCO tuning voltage sensitivity variations with RF frequency.
Gain adjustment for DAC U109 is provided by R116. The DAC output at U108 pin 6 is
next applied to a switched R-C network including R105-108 and related components,
that is programmed by FM range switching bits SUMVCO4-6 depending on FM
deviation range. This network scales the signal to the appropriate level. The output of
the network, at TP3, the VCO steering port, is the desired AC cancellation signal.
Noise contribution at the VCO steering port is reduced by C105, which is switched to
ground by Q101 when the cancellation circuit is not active.
SUM LOOP TROUBLESHOOTING 6C-32.
Since the primary function of the sum loop is to combine various signal frequencies
into the desired fundamental band frequency, sum loop problems will generally cause
frequency errors at the UUT output. A first step in troubleshooting is to check for sum
loop fault status codes 244 and 245. The implications and suggested troubleshooting
sequence in response to these codes are described below. Reading and understanding
the detailed circuit descriptions for the sum loop and Sum Loop VCO assemblies
(paragraphs 6C-28 through 6C-31 and 6C-39) is highly recommended prior to
troubleshooting.
Status code 244 indicates that the sum loop is not properly phase locked, and is
triggered by the free running loop acquisition oscillator. This fault condition can be
caused by either a problem with the input signals to the sum loop, or by a problem in
the A12 Sum Loop PCA or the A9 Sum Loop VCO PCA. A faulty input signal from
either the Coarse Loop, the Sub-Synthesizer, or the FM assembly could result in sum
loop unlock.
First, check for status codes that indicate faulty operation of the Sub-Synthesizer
PCA, the Coarse Loop PCA, and the FM PCA. Repair any indicated assemblies and
check whether Status Code 244 still appears. If it does, check that the following three
input signals have the correct frequency and level. The FM signal is measured using a
500-ohm probe with the spectrum analyzer, while the coarse loop and sub-synthesizer
cables are detached from the Sum Loop PCA and are connected to the spectrum
analyzer directly. Note that for any UUT frequency, the expected coarse loop and
sub-synthesizer frequencies can be displayed by entering SPCL 946 and SPCL 947,
respectively.
SIGNAL DESCRIPTION
Coarse Loop
FM
Sub-Synthesizer
TEST LOCATION
Cable W14
TP14
Cable W13
FREQUENCY
Use SPCL 946
80 Mhz
See equations below
Or, use SPCL 947
LEVEL
+7 dBm
-13 dBm
+3 dBm
For f(sum) < 760 Mhz, f in Mhz:
f(sub-synth) = 2*(f(coarse) -80 -f(sum))
For f(sum) >= 760 Mhz, f in Mhz:
f(sub-synth) = 2*(f(sum) -80 -f(coarse))
6C-38
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