User Guide

Technical Documentation
NHD–4
System Module
P.A.M.S
Page 4–18
Original 11/97
The gain of the CDMA transmitter is controlled by two devices, the
CDAGCT IC (N100) and the AT–109 variable attenuator (V106).
The TX_OFFSET voltage will fall somewhere between 0.0 and 3.15 V,
read at C703. This circuit can be found on the CDCONT schematic. The
resultant voltage is found at the CDCONT IC (N201) at pin 7. The
CDCONT IC interprets this voltage and generates the TX_ICT and
TX_IREF currents.
The gain of the CDAGCT IC (N100) is controlled by the two incoming
currents TX_ICT and TX_IREF. These two signals are currents entering
the IC at pins 24 and 25, through R116 and R115 respectively. The gain
of the IC is set by the ratio of these two current levels. TX_IREF is the
reference current. It stays constant at about 1.0 mA. TX_ICT is the
control current. It varies in as a function of the TX_Gain voltage at the
CDCONT IC.
To measure these currents directly requires that you break the circuit and
input an ammeter. This is impractical in a diagnostics environment.
Instead it is suggested that you simply measure the voltage drop across
R115 and R116 to determine if these signals are correct. The voltage
drop across R115 will remain constant at approximately 100 mV, while the
drop across R116 will vary from approximately 0.0 mV to 100 mV,
depending upon the level of gain required by the AGC system. Below is a
table depicting some sample TX_GAIN and TX_ICT values corresponding
to CDMA TX output power levels.
CDMA TX Output
RF Signal Level
(dBm)
TX_GAIN Voltage
at C213
(V)
TX_ICT Control
Current
(mA)
TX_ICT
(as voltage drop
across R116)
(mV)
23 1.72 0.860 86.0
15 1.78 0.700 70.0
10 1.80 0.560 56.0
–5 1.87 0.298 29.8
–20 1.93 0.171 17.1
–35 2.00 0.093 9.3
The Service Software provides a manual control mechanism which
provides the ability to test this transmitter control functionality. This
mechanism is called CDMA TX Manual Gain Control and is discussed in
the Troubleshooting section of this manual.
The amount of attenuation provided by the AT–109 (V106) is controlled by
the control voltage VC. VC is a function of the AGC_REF PDM via the
circuit centered around the op–amp N202. The N202 op–amp can be
found on the CDCONT schematic. VNEG is used at the inverting input of
N202. VNEG will remain constant, typically at –4.1 V. For minimum
attenuation the AUX AGC PDM voltage is typically 0.75 V, resulting in 3.25
V for VC. For maximum attenuation the AUX AGC PDM voltage is
typically 10.0 mV, resulting in 1.50 V for VC.