500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-6
15 January 2005
2.5.6
RF OSCILLATOR PWB (A4A3)
(see
figure SD-9)
:
The RF oscillator PWB is a dual
purpose low level RF drive source for the exciter. It
contains a crystal controlled oscillator circuit that can
be connected as the RF drive generator and it contains
a buffer/isolation amplifier that will accept and
process an externally produced RF drive at the
assigned carrier frequency. Links on the PWB are
user connected to enable the integral crystal controlled
oscillator and use it as the RF drive source or to
enable the buffer/isolation amplifier when an external
exciter is the RF drive source.
2.5.6.1 Crystal
Oscillator
: The crystal oscillator
consists of transistor Q2, quartz crystal Y1, isolation
amplifier Q3, frequency divider U1, buffer amplifiers
Q4/Q5 and their associated components. When the
crystal oscillator is to be the RF drive source, it must
be enabled and the buffer amplifier must be inhibited.
The crystal oscillator is enabled by linking pad
A
to
pad
B
. Pad
D
must not be linked to any other pad to
inhibit the buffer amplifier. The crystal oscillator
operates within the frequency band of 2.0MHz and
4.0MHz. Its Crystal (Y1) frequency must be 4 (4
ƒ
c),
8 (8
ƒ
c) or 16 (16
ƒ
c) times the carrier frequency,
whichever results in a frequency within the operating
band. Variable capacitor C10 is adjusted to precisely
set the oscillator to the desired frequency. The output
of the oscillator is buffered by Q3 and applied to 12-
bit binary counter U1, as its clock. Pad
E
is linked to
the divider pad which will provide the assigned carrier
frequency (
/
4
when
ƒ
c is between 500 kHz and 535
kHz,
/8
when
ƒ
c is between 250 kHz and 500 kHz or
/16
when
ƒ
c is between 190 kHz and 250 kHz). Pad
F
must not be linked to pad
E
. Q4/Q5, which are
connected as a balanced drive, current amplify the
selected output of U1. The resultant
RF drive
at J1-4
will be a 15 volt peak-to-peak square wave at the
carrier frequency (
ƒ
c).
2.5.6.2 External RF Drive Input:
Circuitry
consisting of Q1, U2, U3, U4 and their associated
compnents are not used in ND radiobeacons.
2.5.7
RF SYNTHESIZER PWB (A4A3)
(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 radio beacon broadcast
band (190 kHz to 535 kHz). 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.
2.5.8
RF DRIVE AMPLIFIER (A4A4):
The
low level RF drive, a square wave switching between
ground and 13.5 volts peak at the carrier frequency, is
passed through connector A4A4J1 and applied to the
primary winding of 1:1 coupling transformer A4T1.
Transformer A4T1 has two identical sets of secondary
windings, one wire of each set is connected to the gate
and one wire of each set is connected to the source of
power MOSFET transistors A4Q1/Q2. MOSFETS
A4Q1/Q2 are connected in a push-pull configuration
with the phasing of their inputs determining which
one will be turned on.
2.5.8.1
When the gate of MOSFET A4Q1 is
positive, the gate of MOSFET A4Q2 will be negative.
A4Q1 will be on/A4Q2 will be off. The negative
voltage
Tx on (B- VDC)
input at J2-1 will be applied
through A4L1 and A4Q1 to A4Q1/Q2's drain/source
junction. During the next half cycle, the polarity on
the gates of A4Q1 and A4Q2 will reverse. A4Q1 will
be off/A4Q2 will be on. A ground will be applied to
A4Q1/Q2's drain/source junction. The output at
A4Q1/Q2's source/drain junction will be a square
wave switching between ground and B- VDC, at the
carrier frequency.
2.5.8.2
The output from A4Q1/Q2 is applied
through A4C3/C4/R1 to A4J2 as a 48-52 volt peak-to-
peak square wave
RF Drive
output for the RF power
amplifiers. It is also peak detected by CR1 and
applied to A4J3 as the
RF Drive Level
reference. This
output is a dc voltage (between 12 and 16 V dc) that is
proportional to the amplitude of the RF drive.
Inductor A4L1 and capacitors A4C1/C2 provide
decoupling for the
Tx On (B- VDC)
input. Diode
A4CR2 provides protection against transient voltages
in excess of 39 volts.
2.5.9
MODULATOR DRIVER PWB (A4A2)
(see figures SD-7 and SD-8)
:
The modulator driver
processes the
voice audio
input and passes it to an
emission mode multiplexer where it is multiplexed
with a
keyed tone
from the keyer, depending on the
emission mode selected (keyed tone or keyed
tone/voice). The modulator driver produces a pulse
width modulated
mod drive
that contains the carrier
