168 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 17 —
Check Receive VCO
Check that the correct receive frequency is synthesized. This is the
frequency of the receive VCO output
SYN RX LO1 at the RX port shown in
Figure 11.9.
1. Enter the CCTM command 335 0 to set the transmit-receive switch
off (receive mode).
2. Using a frequency counter, proceed as follows to observe the receive
frequency at the
RX port before and after grounding the junction
between C541 and R547 (see Figure 11.9):
While holding the probe from the counter on the
RX port, use a pair
of tweezers to momentarily
ground the junction. The frequency
should change to:
The loop filter will hold its output steady at 13.3 V. This should result
in a frequency equal to the maximum given in Table 11.2.
3. If the receive frequency measured in Step 2 is correct, go to Step 4.
If it is incorrect, go to Task 19
, but if no frequency is detected, go to
Task 20
.
4. Proceed as follows to observe the receive frequency at the
RX port
before and after applying 3 V DC to the junction of C541 and R547;
there is a convenient 3 V level at R544 (see Figure 11.9):
While holding the probe on the
RX port, use the tweezers to
momentarily
apply 3 V DC to the junction; do not touch the board
with your hand, and do not
allow the tweezers to touch any cans
when you remove them. The frequency should change to:
The loop filter will hold its output steady at about 0V. This should
result in a frequency equal to the minimum given in Table 11.2.
5. If the receive frequency measured in Step 4 is correct, go to Task 18
.
If it is incorrect, go to Task 19
. If no frequency is detected, go to
Task 20
.
RX port: maximum receive frequency (see Table 11.2)
RX port: minimum receive frequency (see Table 11.2)
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