9/15/06
888-2586-001
4-17
WARNING: Disconnect primary power prior to servicing.
Section 4 Overall System Theory
Platinum Z16HD+
4.4.4
PA Modules
Each of the 16 PA Modules contain two PA Amplifier boards mounted on opposite
sides of the PA Module heat sink. The modules are numbered 1 through 8 with the top
module in the front being #1 and counting to #4 at the bottom. The top rear module is
#5 counting down to #8 at the bottom rear, see Figure 4-4 for the front view and Figure
4-5 for the rear view. The PAs are numbered as A1/B1 -A8/B8, C1/D1 - C8/D8 etc...
with the “A” side PAs being plugged into the “A” Z Plane, and the “B” PAs plugged
into the “B” Z Plane etc... From the front of the transmitter, the “A” Z Plane is on the
right, then B, C and D in order to the left. Keep in mind this is reversed when viewing
from the rear of the transmitter. All DC, signal and RF connections to a PA board are
through one edge connector.
4.4.4.1
Power Amplifier (PA)
The RF drive from the IPA enters the PA Amplifier Boards at Pin C. The RF input level
for each amplifier can range from 1 - 15 Watts. TL1, C1 and C2 allow the input
impedance to be matched to 50 ohms and permit the phase shift through the module to
be trimmed to a nominal value which allows all modules to combine in-phase. The RF
signal is then sent to T1, a coaxial 9:1 transformer which also splits the phase providing
0 degree and 180 degree outputs to drive a pair of push-pull RF power MOSFETs (Q1
and Q2). The control signal to Q1 and Q2 (labeled Vg for Gate Voltage) is connected
through the center of the secondary winding of T1.
The bias circuit is stabilized for both voltage and temperature variations. The zener CR2
and resistor R11 generates a stable voltage reference, immune to a high degree to
variations of the drain supply voltage. Thermistor RT2, and resistors R13, R10 and R9
act as variable voltage divider that compensate for heatspreader temperature variations
and to cancel the remaining variation that the drain voltage has on the zener voltage.
4.4.4.1.1 PA Bias Control
For any HD operation, the PA is being biased as class A/B amplifier. The gate control
voltage (Vg) is -18V to turn the amplifier off (or mute), and 0V to turn the amplifier
fully on.
For FM operation, the PA is being biased as class C amplifier. The gate control voltage
(Vg) is -18V to turn the amplifier off (or mute), and -5V to turn the amplifier fully on.
4.4.4.1.2 IPA Bias Control
In any modulation format using an HDx-FM exciter and/or BoostPro, the IPA is a
19dB fixed gain class A/B amplifier that is held fully on unless a mute condition exists.
Summary of Contents for Platinum Z16HD+
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