2
6
2
and intermodulation. The UHF Synthesizer supplies a programmable LO to the IF/Converter mixer
where the LO and IF signals are combined to create the desired UHF frequency. The UHF signal
from the Converter module is then passed through the UHF Bandpass Filter (FL1) to remove the
unwanted LO and sum products from the conversion process while passing and amplifying the
desired difference signals. The signal is then amplified approximately 25dB by the 2 Watt UHF
amplifier (A1) and passed through the Metering Coupler (CP1). Here, a forward RF sample is
provided for use by the Metering Detector (A3) while the main signal is passed to the output of the
drawer.
Metering and control functions are provided by the Metering Detector, the Display/Monitor board
(PC3) and the Control/Interface Board (PC4). The Display/Monitor Board has several status and
diagnostic LED indicators which are presented on the Exciter’s front panel. The Control/Interface
board provides for various controls and logic circuits for proper operation of the transmitter. Dc
power for the Exciter is supplied by a small, efficient multioutput switching power supply PS1 which
furnishes ±28, ±15 and ±5 volts.
2.2a
Linearity Corrector:
Schematic Diagram 40404011/Rev 55
t
A2PC2
IF IN (J1)
11dBm typical
IF OUT (J2)
12dBm typical
Gain with S1 ENABLE/DISABLE (J1-J2)
0 to
2dB typical
Current Draw
610mA @ +15Vdc
The Linearity Corrector is a six-stage, unity gain circuit which compensates for nonlinear distortions
generated in the transmitter's Class AB 500 Watt Power Amplifier drawer. When properly adjusted,
it provides correction to the transmitter’s output signal for sync amplitude, differential gain,
differential phase, ICPM and intermodulation. Corrector amplifiers U1 through U8 are all
monolithic amplifier stages providing approximately 12dB of gain per device. A phase correction
network is centered around amplifiers U2 through U5 while the circuitry surrounding U7 and U8
corrects for differential gain and sync compression. When properly adjusted, the phase and gain
correction networks will collectively reduce intermodulation distortion.
Input amplifier U1 is biased through resistor R1 and inductor L1 which acts as an RF choke.
Capacitor C2 is utilized as a B+ bypass with coupling capacitors C1 and C3 advancing the input
signal to the matching T-attenuator made up of resistors R2, R3 and R4. At hybrid splitter DC1,
the IF carrier is divided into two equal amplitude signals which are 90
(
out of phase. At the
splitter’s
90
(
port, a negative phase-shift network composed of inductor L3 and capacitors C15
through C18 adds another
22.5
(
shift to the signal (
112.5
(
total) before amplification by U3.
Conversely, connected to the 0
(
port of the splitter is a positive phase-shift network comprised of
inductor L2 and capacitors C4 throughC7. Here the 0
(
signal is shifted in the positive direction by
22.5
(
resulting in a 135
(
total difference between the two signals. In order to keep the amplitude
of each signal similar, the circuits surrounding amplifiers U2 through U5 are essentially identical
with the same biasing, bypassing and coupling described for U1. Transformers T3 and T4 on the
negative shift side of the circuit are 2:1 step-up types identical to T1 and T2, but necessary to
efficiently drive gain expansion diodes CR1 and CR2. Through adjustment of slope potentiometers
R15 and R25 and threshold (cut-in) pots R21 and R30, the amplitude of the negative shifted signal
can be varied to add to or subtract from the positive shifted signal at in-phase combiner CP1.
When adjusted properly, this circuit can then correct the differential phase, intermodulation and
ICPM distortions created by the transmitter’s power amplifier. At the output of combiner CP1 the