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500 WATT RADIOBEACON TRANSMITTER

ND2000A-02x-xx0

Page 2-4

15 January 2005

positive, the gate of MOSFET A2Q4 will be positive,
the gates of MOSFETS A2Q2/Q3 will be negative.
MOSFETS A2Q1/Q4 will be turned on, MOSFETS
A2Q2/Q3 will be turned off. A hot carrier diode,
placed across the source/drain junction of each
MOSFET, ensures switching transients do not damage
the MOSFET transistors.

2.4.2.1

The secondary windings of transformer

A2T1 are configured to ensure the RF voltage applied
to MOSFETS A2Q1/Q4 and MOSFETS A2Q2/Q3
are in phase. The voltage applied to A2Q1/Q4 is 180
degrees out of phase with the voltage applied to
A2Q2/Q3. When A2Q1/Q4 are turned on, A2Q2/Q3
will be turned off. Current will flow from the
modulation voltage through the drain/source junction
of MOSFET Q1, through the primary windings of
transformer T1, through the drain/source junction of
Q4 to ground. During the next half cycle, A2Q1/Q4
are turned off, A2Q2/Q3 will be turned on. Current
will flow from the modulation voltage through the
drain/source junction of A2Q3, in the reverse
direction through the primary windings of transformer
T1, through the drain/source junction of A2Q2 to
ground. The resultant RF output (125 watts
maximum) on the secondary of transformer T1 is
essentially a square wave at the RF carrier frequency.
This output is combined with output of the three
associated power amplifiers (A4/A6/A8) and applied
to connector J1 (500 watts maximum).

2.4.2.2

When the temperature in a module reaches

C, thermal switch S1 opens and inhibits the mod

drive input on TB1-3. Thermal switch S1 closes
when the temperature falls below 82

C. The

B- DC

SUPPLY

lamp shall turn on when the B- voltage is

present.

EXCITER MODULE
2.5

There are two identical exciter modules

(A4 and A5) in the transmitter. Only one will be used
during operation. Exciter A4 will be selected when
the

SELECT MAIN TX

switch is set to

, exciter A5 will

be selected when it is set to

. An exciter module

contains a keyer (A1) which provides the modulating
audio; a modulator driver (A2) which produces a 70
kHz, variable pulse width, mod drive output
containing the carrier level and modulation
information; an RF frequency generator (A3) which
may be a fixed frequency, crystal-controlled RF
oscillator or a DDS RF synthesizer; an RF drive

amplifier (A4) which increases the RF drive to the
power level required by the modulator/power
amplifier stage and reg15 V and -15 V power
supplies. See figure SD-5 for electrical schematic and
figures MD-13 through MD-16 for assembly detail.

2.5.1 +15 VDC REGULATOR:

The +15.0 VDC

regulator contains regulator A4U1 and its associated
components.  If regulator A4U1's output exceeds
+16.0 VDC, current will flow through resistor A4R7
to the gate of thyristor A4Q1.  Thyristor A4Q1 will
turn on clamping the +24 V dc input at A4J2-2 to
ground.  The reg15 V dc can be checked on
the

TEST-Volts/Current

meter.

2.5.2 -15 VDC REGULATOR:

The -15.0 V dc

circuit contains zener diode A4R1 and its associated
components. When the

switch is turned on, the B-

voltage is applied through A4J2-1 to diode A4CR1.
Zener diode A4CR1 ensures the -15.0 V does not
exceed the required limits. The regulated -15.0 V can
be checked on the

TEST-Volts/Current

meter.

2.5.3

KEYING/MOD SWITCH:

Keying/mod

switching circuit contains

KEYING

switch A4S1 and

MOD

switch A4S2. During normal operation, both

switches will be in their

position. When the