Tait TP9100 - Page 198

198 Frequency Synthesizer Fault Finding TP9100 Service Manual
© Tait Electronics Limited May 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 9.3. 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 (see Figure 9.16) before and after grounding
the junction between C541 and R547 (see Figure 9.15):
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 9.4.
3. If the maximum frequency measured in Step 2 is correct, go to
Step 4. If it is incorrect, go to Tas k 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 3V DC to the junction of C541 and R547;
there is a convenient 3V level at R544 (see Figure 9.15):
While holding the probe on the
RX port, use the tweezers to momen-
tarily apply 3V 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 9.4.
6. If the minimum frequency measured in Step 5 is correct, go to
Ta sk 2 9
. If it is incorrect, go to Tas k 3 0 . If no frequency is detected,
go to Ta sk 3 1
.
TX port: maximum VCO frequency (see Table 9.4)
RX port: minimum VCO frequency (see Table 9.4)