RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-13
01 November 2003
2.2.7.5
Depending upon the length of the feeder
cable, the impedance at the sideband frequencies may
appear higher or lower than 50 ohms at the
transmitter output. If the sideband impedance
appears low, the current waveform should be selected
by
RF MON
switch S1 to prevent excessive current
overloading of the transmitter. If the sideband
impedance appears high, the voltage waveform
should be selected by
RF MON
switch S1 to prevent
excessive voltages occurring at the sideband
frequency. The correct setting of
RF MON
switch S1
is made by choosing the waveform that displays the
greater modulation depth.
2.2.7.6
The phenomenon that results from the use
of an inefficient antenna produces a bandpass filter
effect that reduces the modulation depth of the
radiated signal.
2.2.7.7
Under no circumstances should the
modulation level, as measured on either the current or
voltage probe, be adjusted beyond 95 percent in an
effort to offset the sideband attenuation that occurs in
the antenna. This will cause excessive dissipation
and distortion to occur and would create spurious
emissions that do not comply with national and/or
international specifications.
2.2.8
RF SYNTHESIZER PWB (A9)
(see RF
synthesizer PWB manual, following section 10 of this
manual)
:
The RF synthesizer PWB uses direct digital
synthesis (DDS) to generate the assigned carrier
frequency within the radiobeacon and AM broadcast
bands (190kHz to 1800kHz). The output of a digital
synthesizer integrated circuit with internal high-speed
12-bit digital to analog converter is low-pass filtered
to provide a sinusoidal continuous output. The sine
wave is digitized and divided by a factor of four to
obtain the carrier frequency.
2.2.9
24-48V POWER SUPPLY PWB (A10,
Optional)
(see figure SD-9): The 24-48V power
supply PWB is a boost type switching power supplies
that provide a regulated 48V supply voltage to the
transmitter when a 24V battery is acting as the power
source. This assembly is not required when a 48V
backup battery is used.
A 24V supply is applied to the drain of switching
FETs Q1 and Q2 through inductor L2. The gate of
FETs Q1 and Q2 is driven with square wave pulses
from pulse width modulation switching power supply
controller U1 through push-pull buffer stage Q4/Q5.
U1 is a fixed-frequency, pulse width modulation
control circuit, incorporating the functions required
for the control of a switching power supply. The
device contains an internal sawtooth oscillator that is
set to a nominal frequency of 100 kHz by external
components R6 and C6. The output pulse width
modulated control pulses to the gate of FETs Q1 and
Q2 are generated by the comparison of this sawtooth
waveform with a feed-back voltage sample from the
junction of U2-cathode and C8(+)/C12(+)/L5-2 and
the bias voltage created by
VOLTAGE CONTROL
potentiometer R3. The resultant variable pulse
width, 100kHz square wave at U1-8 (
E
) is applied to
the gate of FETs Q1 and Q2 (through Q4/Q5) and
turns it on and off with the appropriate pulses to
maintain the desired set DC output voltage. Inductor
L5 and capacitors C9 and C10 provide filtering of
the DC output voltage.
2.2.10
REMOTE INTERFACE PWB (A11)
(see figure SD-1): The remote interface PWB accepts
all of the transmitter’s remote connections (inputs
and outputs). It provides remote interfacing via
terminal blocks (TB1 and TB2) and a 25-socket D-
sub connector (J4). An
EXTERNAL RF INPUT
connector is provided on BNC connector J3. Two-
position, shunt post connector E1,
RMT PWR TRIM
, is
provided to enable (
ENBL
) or disable (
DSBL
) the
remote power trim (
+15VDC Ext
) input (TB1-23).
2.2.11
SHUNT TUNING INDUCTOR (L2):
For frequencies between 850kHz and 1250kHz, and
between 1600kHz and 1800kHz, inductor L2 is used
to tune the RF drive applied to the power amplifier/
modulator (A6). It shunts the RF drive output when
the quick-connect lead is connected to L2-C (between
850kHz and 1250kHz) or to L2-B (between 1600kHz
and 1800kHz). For frequencies below 850kHz, the
quick-connect lead is connected to L1-A, removing
L1 from the circuit.
