Refer to the following sections for additional information on the
Power Amplifier:
a. SECTION A: RF Amplifier Module.
b. SECTION G: RF Combiners
4.7.5
RF Samples from the Output Combiner
Four RF samples from various points on the RF combiner are
distributed to various circuits elsewhere in the transmitter. Refer
to Sheet 4 of the DX-25U Overall Schematic, 839-7855-151, to
locate the following sample transformers.
4.7.5.1
T9: Bandpass Filter VSWR Detector
An RF current sample from T9, near the RF ground point in the
combiner, is fed to the Output Monitor and compared to the RF
voltage sample at the output of the Bandpass Filter section of
the Output Network. During a VSWR condition, a phase shift
in the RF current and voltage samples will be detected by the
phase angle detection circuitry. Refer to SECTION H, Output
Monitor, for additional information on VSWR protection.
4.7.5.2
T6: Oscillator Sync Signal
During a VSWR condition, all power amplifier modules are
quickly turned OFF. Because of the resonant circuits, “ringing
currents” will continue to flow in the output network, and in the
RF combiner secondary, for several RF cycles. For maximum
MOSFET reliability during this condition, Q1/Q12 and Q2/Q9
in all PA Modules must switch in phase with these output
network ringing currents. This is accomplished with the Oscil-
lator Sync circuitry on the Oscillator.
A current sample from ferrite inductor transformer T6 at the
combiner output is fed to J3 on the Oscillator. A phase shift
network, amplifier stage and an analog switch are used to
synchronize RF drive phase with output network ringing current
during VSWR shut-downs. Refer to SECTION A, Oscillator, for
additional circuit information, and to SECTION V, Mainte-
nance, for adjustment procedures.
4.7.5.3
T1: Neutralization Board
T1 on the Neutralization Board is a ferrite toroidal transformer
similar to the ones used in the RF combiner and is only used for
IQM reduction for AM Stereo operation. The transformer is
wired to the Predriver output splitter T8.
4.8
Output Network Description
The Output Network of the DX-25U is comprised of a Bandpass
Filter and Pi Matching network and contained in the Output
Network cabinet.
4.8.1
Bandpass Filter
The bandpass filter/output network serves as both an impedance
matching network and filter and consists of L1, C1A, and C1B.
At the very high end of the medium wave band, 1500kHz and
above, C6 is also a part of the bandpass filter. Vacuum variable
capacitor C1A is brought out to the front of the transmitter as
the TUNE control. The TUNE control is adjusted for a peak in
output power. The combiner output impedance is low, approxi-
mately 8 Ohms, and is matched to approximately 50 Ohms.
The bandpass filter also “smooths” the small steps in the output
signal that remain after the Digital to Analog conversion by the
Power Amplifier stage. Any other harmonic and spurious signals
in the RF output are also attenuated by the bandpass filter.
Refer to SECTION V, Maintenance, for information on tuning
and adjustment of the bandpass filter if required by a frequency
change or major component failure.
4.8.2
PI Matching Network
The PI Matching Network consists of C2A, C2B, L3, C3A,
C3B, C4A, C4B and C5. Parallel capacitors C2A and C2B
comprise the first leg of the
π
network. Inductor L3 and capaci-
tors C3A, C3B, and, at some frequencies, C5 are adjusted and
tuned to 3f
c
. The parallel resonant circuit provides further
attenuation of the 3rd harmonic to FCC specifications while
passing the carrier frequency. VACuum variable capacitor C4A
is brought out to the front of the transmitter as the LOAD
control. The LOAD control adjusts the amount of Power Am-
plifier current.
4.8.3
Spark Gap, E2
A spark gap at the output of the transmitter protects against high
transient voltages caused by lightning or electrostatic discharge.
This does NOT substitute for proper DC grounding chokes, ball
gaps, and other protection at the towers. Set E2 at .090".
4.9
Digital Modulation Principles
4.9.1
Digital Terms and Concepts
The discussion of Analog to Digital and Digital to Analog
Conversion includes terms, abbreviations, and concepts which
may not be familiar to some Broadcast Station engineers and
technicians. Most terms will be explained in the discussion, but
a summary is included for review or reference.
a.
ANALOG
refers to a continuous range of values. Examples
include audio signals from a microphone, a turntable
cartridge, CD, etc.
b.
DIGITAL
is related to digits, or discrete quantities. An
analog signal changes continuously, but a digital signal
changes in steps. An analog signal has an infinite number
of possible values, and a digital signal has a finite, or
limited, number of possible values.
c. BINARY: Has only two possible values. A BINARY
NUMBER is represented using only the digits 0 and 1.
This is useful because a circuit can be two states, either
ON or OFF.
d.
BINARY
can also refer to a series where each step is either
multiplied or divided by two to get the next step. An
example in the transmitter are the Binary RF amplifier
steps: the 1/2 step; 1/4 step; 1/8 step; 1/16 step; 1/32 step
DX-25U
4-8
888-2297-002
Rev. AA: 8-9-00
WARNING: Disconnect primary power prior to servicing.
