182 Frequency Synthesizer Fault Finding TB7100 Service Manual
© Tait Electronics Limited October 2005
Task 28 —
Check VCO
Check that the correct receive and transmit frequencies are synthesized.
The receive frequency is that of the VCO output
SYN RX LO1 at the RX port
shown in Figure 11.12. The transmit frequency is that of the output
SYN TX
LO at the TX port.
1. Enter the CCTM command 335 1 to set the transmit-receive switch
on (transmit mode).
2. Using a frequency counter, proceed as follows to observe the transmit
frequency at the
TX port before and after grounding the junction
between C541 and R547 (see Figure 11.12):
While holding the probe from the counter on the
TX 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 11.4.
3. If the maximum frequency measured in Step 2 is correct, go to
Step 4. If it is incorrect, go to Task 30
, but if no frequency at all is
detected, go to Task 31
.
4. Enter the CCTM command 335 0 to set the transmit-receive switch
off (receive mode).
5. 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 11.4.
6. If the minimum frequency measured in Step 5 is correct, go to
Task 29
. If it is incorrect, go to Task 30. If no frequency is detected,
go to Task 31
.
TX port: maximum VCO frequency (see Table 11.4)
RX port: minimum VCO frequency (see Table 11.4)
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