Check Vco - Tait TM9100 Service Manual

Check that the correct receive and transmit frequencies are synthesized.
Task 28 —

Check VCO

The receive frequency is that of the VCO output
shown in
at the
1.
2.
3.
4.
5.
6.
208 Frequency Synthesizer Fault Finding
Figure 9.12. The transmit frequency is that of the output
port.
TX
Enter the CCTM command 335 1 to set the transmit-receive switch
on (transmit mode).
Using a frequency counter, proceed as follows to observe the transmit
frequency at the port before and after grounding the junction
TX
between C541 and R547 (see
While holding the probe from the counter on the
of tweezers to momentarily ground the junction. The frequency
should change to:
TX port: maximum VCO frequency (see
The loop filter will hold its output steady at 13.3 V. This should result
in a frequency equal to the maximum given in
If the maximum frequency measured in
Step 4. If it is incorrect, go to
detected, go to Task 31.
Enter the CCTM command 335 0 to set the transmit-receive switch
off (receive mode).
Proceed as follows to observe the receive frequency at the
before and after applying 3 V DC to the junction of C541 and R547;
there is a convenient 3 V level at R544 (see
While holding the probe on the
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:
RX port: minimum VCO frequency (see
The loop filter will hold its output steady at about 0V. This should
result in a frequency equal to the minimum given in
If the minimum frequency measured in
Task 29. If it is incorrect, go to
go to Task 31.
1 at the
SYN RX LO
Figure 9.12):
port, use a pair
TX
Table 9.4)
Table 9.4.
Step 2 is correct, go to
Task 30, but if no frequency at all is
Figure 9.9):
port, use the tweezers to
RX
Table 9.4)
Table
Step 5 is correct, go to
Task 30. If no frequency is detected,
TM9100 Service Manual
© Tait Electronics Limited August 2005
port
RX
SYN TX LO
port
RX
9.4.