User Guide

PAMS
Technical Documentation
NHD–4
Disassembly & Troubleshooting
Page 34
Original 11/97
AMPS Receiver Troubleshooting
Introduction
Troubleshooting the NHD–4 AMPS receiver isn’t unlike troubleshooting
the AMPS receiver on any other Nokia phone. In fact, NHD–4 uses the
same FM receiver IC as many other Nokia phones. So long as the
NHD–4 phone is in an AMPS RX mode, this receiver will be on. The
following tests detail ways to make the AMPS receiver operational and
test its functionality.
Tests and Quick Checks
The Service Software software package contains a quick test for the
AMPS receiver. When selected, the software prompts you to send a
modulated carrier into the phone at a –65 dBm signal level. Hitting the
”Test” soft key prompts the software to read the RSSI voltage level that
the CDRFI IC has A/D converted into a digital signature. If the digital
value falls within the range which a –65 dBm signal should, it is a good
bet that the AMPS receiver is working properly. There is still a chance
that the demodulated baseband ANARX signal (the audio) is corrupt, but
you do know the rest of the AMPS receiver is functional.
If the RSSI values are less
than the ranges stated, than there is an AMPS
RX sensitivity problem. Most likely a component or solder joint along the
AMPS RX receiver path has failed. The possibility exists that the error is
in the baseband A/D conversion. To ensure that the error is in the RF it is
recommended that the remaining tests below be performed until the error
is uncovered.
If the RSSI values are greater
than the ranges stated than additional RF
energy above the stated input level is entering the RX path. Check the
level of the signal generator used for the test. It may be sourcing more
RF power than was requested by the test. Ensure that cable losses have
been properly accounted for.
Setting Up the AMPS Receiver Test
To troubleshoot the AMPS receiver first remove the shields. Apply power
and service cables to the phone and turn it on. Initiate the AMPS
Troubleshooting Mode from Service Software. Then feed a –65 dBm
881.52 MHz signal modulated with 8 kHz FM and 1 kHz audio into the RF
port of the phone via the bottom connector. A stand alone RF signal
generator should do the trick, or a CDMA test box with this functionality.
Do not forget to account for the signal loss between the signal generator
and the phone.