word represent the total number of “BIG STEP” RF amplifiers
ON at any time. For example:
a. 010111/XXXXXX = 23 BIG STEPS ON
b. 101111/XXXXXX = 47 BIG STEPS ON
c. 000000/XXXXXX = 0 BIG STEPS ON
The six least significant bits, B7 through B12, each control a
“BINARY STEP” RF amplifier. If the bit is a “1”, the associated
amplifier is ON. For example:
a. XXXXXX/000001 = B12: ON
b. XXXXXX/000011 = B11, B12: ON
c. XXXXXX/010011 = B8, B11, B12: ON
The RAISE and LOWER controls on the transmitter front panel
set the DC component of the (Audio + DC) signal. Under ideal
conditions, for 25 kW carrier, the Analog to Digital Converter
samples the DC component of the (Audio + DC) signal and turns
23 “BIG STEP” RF amplifiers ON and all the “BINARY
STEPS” are OFF:
•
010111/000000
When audio is applied to the transmitter, the audio component
of the (Audio + DC) signal is sampled by the Analog to Digital
Converter and the “BINARY STEP” amplifiers begin turning
ON until the six least significant bits of the 12-bit digital word
are all “LOGIC HIGH”:
•
010111/111111
As the audio component keeps increasing, the “BINARY STEP”
RF amplifiers turn OFF and another “BIG STEP” RF amplifier
turns ON:
•
011000/000000
This process repeats until the positive modulation peak is
reached. For 100% modulation of a 25kW carrier, this will
require twice the number of “BIG STEP” RF amplifiers ON as
are required for carrier, or 47. At that instant, the digital word
would be:
•
101111/000000
At this point, the audio component of the (Audio + DC) signal
begins to decrease, and RF amplifier modules begin turning
OFF.
Refer to SECTION J, Analog Input Board, and SECTION K,
Analog to Digital Converter, for additional information.
4.9.4
Amplitude Modulation in the DX-25U
The transmitter Power Amplifier stage uses 64 solid-state RF
amplifier modules. Each RF amplifier can be switched on and
off very quickly. The RF voltage outputs of the 64 RF amplifiers,
or “steps”, are combined to produce the total transmitter RF
voltage output. The output of an AM transmitter is an RF voltage
which varies according to the audio modulating signal input.
Each RF amplifier provides a fixed voltage, and the RF output
can be changed by switching the appropriate number of ampli-
fiers ON. A fixed number of amplifiers are switched ON for
carrier. If the audio signal increases, more amplifiers are
switched ON. If the audio signal decreases, amplifiers are
switched OFF. As the audio signal changes from instant to
instant, the number of RF amplifiers that are ON changes.
a. For carrier only, no modulation, only enough RF amplifi-
ers to produce the required RF voltage for carrier power
are switched ON.
b. A positive modulation peak requires a high RF voltage at
the transmitter output and a large number of amplifiers are
switched ON.
c. For a 100% negative modulation peak, which corresponds
to zero RF voltage output, all the amplifiers are switched
OFF.
SUMMARY: Amplitude modulation in the DX-25U is accom-
plished by turning on only enough RF amplifier modules at any
time to produce carrier and the audio modulating signal at that
moment.
4.9.5
Power Amplifier Stage
The transmitter Power Amplifier stage contains 64 identical RF
amplifier modules. The amplifier modules are used as 58 equal
RF voltage “BIG STEP” amplifiers, and six fractional RF volt-
age “BINARY STEP” amplifiers. The six “Binary Steps” are:
a. B7: = 1/2 “Big Step” RF voltage
b. B8: = 1/4 “Big Step” RF voltage
c. B9: = 1/8 “Big Step” RF voltage
d. B10: = 1/16 “Big Step” RF voltage
e. B11: = 1/32 “Big Step” RF voltage
f. B12: = 1/64 step “Big Step” RF voltage
As the 58 “BIG STEP” amplifiers are turned ON and OFF, the
RF output changes in equal VOLTAGE steps, not in equal power
steps, because of operating characteristics of the output com-
biner. For a 100% positive modulation peak, the RF output
voltage of the combiner must double. For a 100% negative
modulation peak, the RF voltage of the combiner must be zero.
For a typical carrier of 25 kW, approximately 23 “BIG STEP”
RF amplifiers are ON. As audio is applied, 47 “BIG STEP” RF
amplifiers turn ON for the 100% positive peak and all modules
turn OFF for the 100% negative peak.
The “BINARY STEP” amplifiers are switched in sequence to
provide smooth transitions between the equal RF voltage steps
created by the “BIG STEP” amplifiers. When all “BINARY
STEP” amplifiers are ON, their total amplitude equals 63/64 of
a “BIG STEP” voltage.
4.9.6
Summary: Digital Modulator
The amplitude modulation process in the transmitter takes place
in three steps.
First, The audio input signal is converted into a digital data
stream, a series of 12-bit digital words, by an Analog to Digital
Converter. This digital data stream is the “digital audio” signal.
Second, the digital data from the Analog to Digital Converter is
encoded by the Modulation Encoder to provide the control
signals required by the Power Amplifier stage.
DX-25U
4-10
888-2297-002
Rev. AA: 8-9-00
WARNING: Disconnect primary power prior to servicing.
