User Guide

Technical Documentation
NHD–4
System Module
P.A.M.S
Page 4–17
Original 11/97
Introduction
NHD–4 uses the same transmitter to up convert, amplify and filter the
analog AMPS and the digital CDMA signals. The key differences between
analog and digital transmission are the Power Amplifier (PA) bias levels ,
attenuation levels of the variable attenuator, and operation of the RF
transmitter ASIC (CDAGCT). It is important to keep in mind that the
AMPS and CDMA signals are significantly different. The AMPS signal is
distinct FM modulated carrier with a channel bandwidth of 30 kHz. CDMA
modulation is spread spectrum. A CDMA signal is 1.23 MHz wide and
appears noise–like.
Aside from this introduction, the Functional Description describes the
various signals entering and exiting the NHD–4 transmitter circuit, as well
as the DC voltage supplies that bias it.
TX Gain Limiting
TX Limiting is a control feature for CDMA TX operation. In some
conditions the AGC loop of the phone may call upon the transmitter to
provide more output power than is recommended for healthy operation.
The TX Limiting circuit places a ceiling or limit on the output power of the
CDMA transmitter. Transmitting above the limit might put the CLY–10 PA
(V113) out of its linear range of operation.
In CDMA operation the TXI_REF PDM stays fixed at a tuned voltage level.
This tuned level corresponds to the TX output power limit. The tuned
TXI_REF PDM line will be approximately 1.0 V. The detector voltage, TXI,
directly reflects the output power of the TX PA chain (V110–V113). For
maximum CDMA output power TXI is approximately 1.0 V DC at Pin 2.
For minimum CDMA output power TXI is about 2.26 V.
When TXI equals TXI_REF, the LIM_ADJ line goes logic low to
approximately 0.0 V. A way to test CDMA TX Limiting Control is to probe
the LIM_ADJ line with an oscilloscope and maximize the gain of the
transmitter. When the TX output power reaches the limit the LIM_ADJ line
will toggle continuously, appearing as a square wave 3.2 Vpp (read at
R840) with an approximate frequency of 400 Hz.
CDMA TX Gain Control
A fundamental requirement for proper CDMA system operation is that
received signal power levels reaching the digital demodulators remain
constant. This is true for both the mobile unit and the base station. The
mobile unit must dynamically adjust the gain of its receiver to ensure that
the down converted baseband I & Q signal levels delivered to the CDSB
ASIC are always constant. The mobile must also dynamically adjust its
transmit output power so that the base station always receives the same
signal strength. The amount of gain needed at the mobile unit receiver is
used to determine how much gain to provide the mobile unit transmitter,
thus they are linked in a loop.