EV9900 Datasheet, PDF (23/27 Pages) CML Microcircuits – Evaluation Kit

Evaluation Kit for CMX990
EV9900
6.7 Troubleshooting
The CMX990 is a complex RF and Baseband system. If incorrectly programmed or modified,
results will be at variance from datasheet performance. Please study the datasheet, this manual
and the associated schematics and layout drawings carefully when troubleshooting.
This section provides some suggestions to help users resolve application issues they might
encounter.
6.7.1 Receiver Operation
Error Observed
No packets are received,
however synthesisers are locked
and the receiver otherwise
appears to be operating
correctly.
Receiver sensitivity is 30dB less
than anticipated.
Degraded receiver performance.
Degraded receiver sensitivity
and/or imbalance in adjacent
channel power in the transmitter
(i.e. ACP high is much better
than ACP low or vice versa)
Poor sensitivity on some
channels
or
BER results variable.
Poor co-channel rejection at
negative frequency offsets
Possible Cause
‘INV’ bit incorrectly set.
‘Sign’ bit is incorrectly set.
Incorrect DC offset
correction.
There is a frequency
offset between EV9900
reference oscillator and
the test equipment.
(Note: EV9900 requires
an approximate AFC
value to be programmed
into the DAC1. A value of
0x95 in register 0x0B is a
good starting point.)
The main Tx VCO is not
powered down and can
drift into the Rx band.
The SAW IF filter (F2) is
slightly miss-aligned.
Remedy
Invert current setting of ‘INV’ bit.
Invert current setting on ‘Sign’
bit
Ensure DC offset correction
sequence has been executed.
To check if a frequency error
exists measure the frequency of
the main local oscillator either
using the measuring equipment
in question or compare results
from EV9900 with those from
the measurement equipment.
To overcome a frequency error
either:
a) Use a external reference for
the EV9900
b) Tune to control voltage on
the AFC output (DAC1).
In a normal modem design the
Tx VCO (U18) would be
powered down during Rx mode.
This is not implemented on
EV9900.
To overcome the problem the
Tx VCO should be disabled
during Rx BER testing. This can
be done by removing R67.
Change L4 to 1.8µH.
Note: is necessary to ensure the
centre frequency of the receiver
matches that of the test signal
generators precisely to get
reliable results.
© 2005 CML Microsystems Plc
23
UM9900/1