Evaluation Kit User Manual
EV9000
2001 Consumer Microcircuits Limited
26
UM9000/2
3.3.2.2 Trim TxA and RxB Paths
Table 15 details the procedure for trimming TxA and RxB signal paths. Note that if +5VM is
changed then this procedure will have to be repeated. This is because the bias points remain
static compared to ½ of +5VM. Also, for best performance, the pk-pk voltage of the signal at the
RXFB pin should be set to 20% of +5VM. Care must be exercised in setting the dc levels
accurately, as the variable resistors R15, R16, R17, R30 and R46 all swing b5V and -5V
in less than one turn.
Trim TxA and RxB Procedure
Purpose
1.
Press the HALT switch
Terminate the previous test.
2.
Click on the
Default
button.
Set on screen configuration to default values.
3.
In the channel A Mode register, click
TXRXN to mark it as checked.
Change channel A screen configuration to
transmit mode.
4.
Enter data values into the channel A data
buffer. For the CMX909B, enter $CC.
For the CMX969, FX919B and FX929B,
enter $F5.
These values will produce full excursions of
the channel A transmit output for use as a
reference signal.
5.
Click on the channel A Data Buffer
Write
button.
Transfer this configuration change to modem
A. Modem A is now prepared to transmit.
6.
Connect TXOUTA to RXINB.
Use channel A Tx as a reference signal
source for channel B Rx.
7.
Make sure the selected Test is 2 and
click on the
GO
button.
This will repeatedly cause channel A to
transmit a signal which drives channel B’s
receive path.
8.
Probe TXOUTA and Test Point 6.
Observe channel B Rx signal relative to V
BIAS
.
9.
Adjust R30, centring TXOUTA around
V
BIAS
as observed at Test Point 6.
Adjust TXOUTA bias to a good nominal
operating point.
10 Move probe to pin B of jumper E8.
Observe channel B buffered and the inverted
signal level.
11 Adjust R38 centring the pin B, jumper E8,
signal around V
BIAS
as observed at Test
Point 6.
Adjust the dc bias of the Rx B signal to be
equal to modem B’s V
BIAS
.
12 Adjust R34 to achieve a signal level of 1V
pk-pk at pin B, E8.
Adjust RXINB gain to a proper nominal
amplitude.
13 Move probe to Test Point 5.
Observe Rx signal directly at the modem
receive feedback node. Modem Rx signal
levels are specified for this node.
14 Adjust R43 to achieve a signal level of
1V pk-pk at Test Point 5.
Adjust final RXINA gain to the proper
amplitude.
15 Disconnect TXOUTA from RXINB and
connect TXOUTA to NSB.
Connect channel B Rx noise injection path in
preparation for its gain adjustment.
16 Adjust R35 to obtain a 1.0 Vpk-pk signal
at Test Point 5.
Adjust channel B input noise gain to inject
100% noise signal level.
17 Press the HALT switch.
Terminate the test.
Table 15: Trim TxA and RxB Paths