User Guide

Technical Documentation
NHD–4
System Module
P.A.M.S
Page 4–28
Original 11/97
CDMA Receiver
CDMA IF SAW Filter (Z2)
For CDMA RX operation the 45 MHz IF signal exits the 1st IF amplifier
(V9) through pin 4 of the diode switch (V10). This signal then enters the
CDMA SAW Filter (Z2), passing through an impedance matching network
comprised of L8, L9, C71, C72, and C73. R22 and R23 supply current to
V9 and V10. R22 also serves to set the Q of L8. The SAW filter itself has
a bandwidth of 1.23 MHz. The output at pin 10 is matched to the next
stage via C70 and L7. Two of the ten pins of this device serve as the RF
input and output, while the other eight are ground. It is extremely
important that all ten pins are well soldered to the PCB.
1CDMA IF LNA Stage (V7)
The received CDMA IF signal goes through another stage of amplification
before being down converted to baseband by the CDAGCR IC (N1). V7 is
this gain stage, a BJT in common base configuration. V8 temperature
compensates the base voltage to V7 at the junction of R19, R20 and R21.
The IF signal leaves this amplifier through the collector, pin 1. It enters
the CDAGCR IC (N1) at pin 8, passing through an impedance matching
network L4 and C69. In CDMA RX operation about 0.60 V should be
found on the emitter of V7, pins 2 and 4. The collector will have about
1.40 V. The collector and base of V8 will be approximately 0.7 V.
CDAGCR – CDMA Receiver IC (N1)
The CDAGCR IC (N1) serves two functions. It controls the gain (AGC) of
the received CDMA signal, and it quadrature demodulates the 45 MHz IF
signal, and at the same time brings the IF down to baseband frequencies.
The dynamic gain of the CDAGCR IC (N1) is controlled by the two
incoming signals RX_IREF and RX_ICT. These two signals are currents
entering the IC at pins 23 and 24, through R17 and R16 respectively.
RX_IREF is the reference current. RX_ICT is the control current. It varies
in accordance with the gain required of the IC.
To measure these currents directly requires that you break the circuit and
input an ammeter. This is impractical. Instead it is suggested that you
simply measure the voltage drop across R17 and R16 to determine if
these signals are correct. The voltage drop across R17 will remain
constant at approximately 360 mV, while the drop across R16 will vary
from approximately 10 mV to 115 mV. A simple test to demonstrate the
functionality of the CDMA receiver AGC is found in the Troubleshooting
portion of this document. This test also provides a table detailing the
voltage drops (currents) generated verse received RF signal levels.
The DC supply VRXD provides bias to this IC. This supply will be
approximately 4.40 V when the phone is in CDMA mode. When
troubleshooting be sure to check all eight pins.
The demodulated baseband digital I and Q signals exit the CDAGCR IC at
pins 26 and 27 and proceed to the BFILCT IC (N2) through C33 and C57.