Pulsar LPA100, Системное руководство

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Page 8–2 June 2001
50W / 100W Linear Power Amplifier System Manual
Technologies, Inc.
8.2 12.5W PA Circuit
Description
The function of the 12.5W PA Module (see Figure
8-3, Schematic 1606C33S) is to amplify a 0 dBm
(1 mW) input to an output power level of 10 W.
The input from pins C28/A28 passes thru a 700
kHz low pass filter (LPF) consisting of L1 and C1.
Potentiometer (R53), labeled “INPUT LEVEL
SET” on the front panel, is used to adjust the
power level to 10 W output with 1 mW applied at
the input.
The 12.5W PA Module operates in a 30 to 535
kHz range without tuning. The amplifier has a
maximum gain of approximately 49 dB (class A,
complementary symmetry push-pull stage).
Negative feedback is used to derive a nominal
output impedance of 50
Ω
.
All bypassing is done to common (pins A30/C30,
A32/C32). Transistors QN1, QN2 and QN3 are 14
pin DIPs, each containing four individual transis-
tors; QN1 is PNP, while QN2 and QN3 are NPN.
The LPF output drives the amplifier QN1 and
QN2. QN1A/QN1B and QN2A/QN2B are config-
ured as a differential amplifier, while QN1C and
QN2C are constant current sources. The input
signal is applied to the bases of QN1A and QN2A.
Negative feedback is applied to the bases of
QN1B and QN2B. At the positive side (QN2), the
differential output from QN2A and QN2B is
amplified by QN2D and Q2. At the negative side
(QN1), the differential output from QN1A and
QN1B is amplified by QN1D and Q1. The
positive side power output transistor (Q6) is
driven by Q5; the negative side power output tran-
sistor (Q7) is driven by Q4.
The no-load feedback is from transformer (T1)
back thru the RC network of R21, C7, C2, C5 and
R18 to the junction of R16 and R17, for the
purpose of stability. The loaded feedback is
derived from a sampling resistor (R33, R35, R36,
R37, R38, and R39, all in parallel) and fed back
thru C28, C29 and R23. The overall no-load
voltage gain is approximately 282. The overall
loaded voltage gain is approximately 141. The
partial loaded gain, between C28/A28 and the
primary of T1, is approximately 38.
The alarm circuit (loss of RF signal condition)
consists of QN3, Q8, K1 and associated compo-
nents. The RF signal is monitored by C22, at T1
pin 1. The signal sample is amplified in QN3A and
fed to QN3B and QN3C (QN3B and QN3C are
configured as diodes). A voltage doubler is formed
from C30, QN3C and QN3B. The output of QN3B
drives QN3D, via R44 and R45. QN3D is
saturated for an input of 1W to C22 (with
reference to T1 secondary). As QN3D saturates,
Q8 conducts, driving the front panel LED (D6,
power monitor), causing K1 to energize (or de-
energize), indicating loss of signal condition.
Jumper JU1 allows the selection of an open circuit
or a closed circuit for the loss of signal condition.
The +20 Vdc line (leading to the alarm circuit,
etc.) is filtered by C10, C11, L2, L4, C19, C20 and
C21. The -20 Vdc (leading to C2/C4) is filtered by
C12, C13, L3, C16, C17, C18 and L5.
8.3 12.5W PA Troubleshooting
To check individual transistors, e.g., Q1 thru Q8,
QN1, QN2 and QN3, remove them first from the
PC Board. Ohmmeter measurements of the tran-
sistors while in the PC Board are misleading
because of other paths on the board.
You may remove the heat sink by unscrewing the
four (4) corner screws and the hold-down screws
of Q1 thru Q8. The 12.5W PA Module can operate
at no-load conditions without the heat sink for
short periods of time while you are trou-
bleshooting.
!
CAUTION
THE 12.5W PA IS AN OP-AMP PROVIDING VERY
HIGH GAIN WITH NEGATIVE FEEDBACK. TRAN-
SISTORS Q1 THROUGH Q5, Q6, & Q7 ARE
THERMALLY CONNECTED, I.E., THEY ARE
MOUNTED ON THE SAME PART OF THE HEAT
SINK. ANY FAILING TRANSISTOR MAY AFFECT
OTHER TRANSISTORS. CHECK EACH TRAN-
SISTOR SEPARATELY. IF NO FAULTS ARE
FOUND, CHECK OTHER COMPONENTS.
BE CAREFUL NOT TO MISPLACE SCREWS,
SPRING WASHER OR INSULATING WASHER
USED TO MOUNT Q1 – Q8. DAMAGED SCREWS
OR INSULATORS SHOULD NOT BE USED.