RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-12
01 November 2003
the
mod %
line (J3-6). For 100% adjustment on the
test meter, the test switch on the front panel is set to
MOD-REF
, the output on U6B will be double the
output of U6D to compensate for the return line
through the meter from the output of U6D.
2.2.6.13
Bypass Control
: Under normal operating
conditions the
bypass
input (J4-4) will be at ground
potential. The bypass circuit will have no influence.
If the ground potential is removed from J4-4,
transistor Q7 will have no effect on
inhibit
output J4-
7 regardless of the output from comparator U4A.
The shutdown circuit will be inhibited and the
transmitter will remain operating.
2.2.7
HARMONIC FILTER (A7)
(see figures
SD-8A and SD-8B): The harmonic filter assembly is
a pass-band filter, which attenuates the harmonics on
the square wave output from the modulator/power
amplifier circuits. The subject filter has a flat
response characteristic over the operating bandwidth.
The filter components include inductors L1 and L2
(A4L1 through A4L6 for frequencies between 651
and 1250 kHz and between 1600 and 1800 kHz) and
capacitors A1C1 through A1C12 and A2C1 through
A2C12. The band-pass is selected through external
link connections to TB1 through TB4 (refer to
section 3 for details). Provision is made to
automatically inhibit the RF output from the main
transmitter and select the RF output from the standby
transmitter (optional feature, for dual transmitters
only), in the event that the main transmitter fails.
There are two variations of the harmonic filter
assembly. The /01 and /02 variations (see figure SD-
8A) are used when the carrier frequency is between
190 kHz and 650 kHz. The /03 and /04 variations
(see figure SD-8B) are used for extended frequency
operation between 651 kHz and 1250 kHz or between
1600 kHz and 1800 kHz.
2.2.7.1
Current Probe
: The current probe circuit
consists of transformer T1 and associated
components. The
RF in (main TX)
input (J1) is
applied across transformer T1 and passed through a
series inductance, frequency select capacitors A1C1
through A1C12, across transformers T3/T4, applied
through
RF out
connector J2 and passed to the
antenna system. A sample of the RF input current is
applied through transformer T1's secondary, resistors
R1/R2 and through
RF current sample
terminals
TB5-1 and TB5-2 for use within the overmodulation
circuitry in the monitor PWB A5.
2.2.7.2
Forward/Reflected Power Probe
: The
forward/reflected power probe circuit consists of
transformers T2/T3 and associated components.
Transformer T2 senses the output voltage and
transformer T3 and resistors A3R1/A3R2 sense the
output current. When the output voltage and current
are in phase, the
refld pwr
output (TB6-1) will be
zero. The
fwd pwr
output (TB6-3), a DC voltage
detected by diode A3CR1 representative of the
transmitter forward power, is passed to circuits
within monitor PWB A5. If the output voltage and
current are out of phase, a DC voltage detected by
diode A3CR2 representative of the transmitter
reflected power, is applied through TB6-1 and passed
to monitor PWB A5.
Relay K1 is for use on a dual transmitter only. If the
main side of a dual transmitter fails, the
+24 VDC
input (TB6-6) will be inhibited, relay K1 (normally
energized) will de-energize. The
RF in (stby TX)
input (J3) will be applied through
RF out
connector
J2. The
RF in (main TX)
on connector J1 will be
inhibited by the contacts of relay K1.
Current transformer T4 and a separate secondary
winding of transformer T2 provide voltage or current
waveform outputs through TB6-4/5 (selected by
RF
MON
switch S1) for monitoring purposes on
RF MON
connector J1 on the front panel.
2.2.7.3
RF Monitor Switching:
RF MON
switch
S1 allows selection of either the voltage or the current
waveform to be monitored on RF monitor BNC
connector J1 on the transmitter front panel. The
following description provides the necessary
information for the setting of the
RF MON
switch S1.
2.2.7.4
The high capacitive reactance of the
antenna is tuned at the carrier frequency by the ATU
loading coils to produce a series resonant circuit.
The resulting net antenna resistance is then
transformed to 50 ohms in a matching transformer to
provide the transmitter’s required load impedance.
When the antenna is very short compared with the
wavelength of the operating frequency, the series
resonant circuit has an extremely high Q. Under
these conditions a perfect match may occur at the
carrier frequency but the sidebands may be badly
mismatched causing a standing wave on the feeder
cable at the sideband frequencies.
