RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-8
01 November 2003
pulse duration corresponding to the carrier level plus
tone modulation (MCW). The B+ to modulator drive
source voltage will also be varying at this rate and
will effectively control the output of the power
amplifier. If operating in the CW mode, the B+ to
modulator drive source voltage will not vary and the
output of the power amplifier will remain constant.
2.2.5.2
Power Amplifier
: The power amplifier
circuit is comprised of power MOSFETs Q2 through
Q5, 2:1 step-down transformer T1, output
transformer T2 and associated components. The
amplifier operates as a class 'D' (switched mode)
amplifier. Under normal operating conditions, a 48-
volt peak-to-peak
RF drive
input on connector J2 is
applied through the primary windings of T1 to four
identical secondary windings. Each secondary is
connected between the gate and source leads of its
respective MOSFET. The power MOSFETs are
connected in a series push-pull configuration with the
phasing of their inputs determining which pair
(Q2/Q5 or Q3/Q4) are turned on.
With power MOSFET transistors Q2/Q5 forward
biased (turned on) and Q3/Q4 reversed biased (turned
off), current will flow from the B+ line through the
source/drain junction of Q2 and the primary of
transformer T2, through the source drain of Q5 to the
less positive voltage at the modulation drive source.
During the next half cycle, transistors Q2/Q5 will be
reversed biased (turned off) and Q3/Q4 will be
forward biased (turned on). Current will flow from
the B+ line through the source/drain junction of Q4
and the primary of transformer T2 (in the reverse
direction), through the source drain junction of Q3 to
the less positive voltage at the modulation drive
source. The RF output on connector J3 will be a
square wave at the RF carrier frequency level. The
maximum and minimum undistorted RF carrier
output the power amplifier can produce is dictated by
the magnitude of the B+ modulator voltage.
2.2.6
MONITOR PWB (A5)
(see figure
SD-6): The monitor PWB monitors the critical
parameters of the subject transmitter and produces
local control signals which turn on lamps on the
transmitter's front panel when these parameters are
not met or a status condition of the transmitter is to
be known. Potentiometers provide adjustments for
establishing the carrier/modulation thresholds,
shutdown time delay (seconds) and calibration of the
forward/reflected power metering circuits. The
assembly produces an additional unm15V
[+15V (A)], for internal use. Remote outputs include
battery status (AC or DC power source), audio
monitoring, shutdown condition (transmitter has
shutdown) and SWR condition (transmitter's RF
output is being cutback).
2.2.6.1
Forward Power/Fwd Pwr Cal
: The
forward power monitoring and calibration circuit is
comprised of buffer amplifiers U1A/U1D,
FWD PWR
potentiometer R21 and associated components. The
fwd pwr
input (J1-3), a DC voltage representing the
forward power of the transmitter with a
superimposed AC voltage proportional to the
modulation depth, is applied across smoothing
capacitor C19, through loading resistors R2/R9 and
applied to the non-inverting gate of U1A-3.
Capacitor C3 filters the modulation component while
charging to a DC level that is representative of the
carrier level.
The output on U1A-1 is applied through resistor R17
and
FWD PWR
potentiometer R21 and passed to
fwd
pwr
output (J1-4) to
TEST
meter M1 on the
transmitter’s front panel.
FWD PWR
potentiometer
R21 provides a calibration adjustment for the
forward power indication on
TEST
meter M1 (refer to
special calibration procedures in section 5). The
output on U1A-1 is also applied to the non-inverting
input of U1D and passed to
fwd pwr (remote)
output
(J1-5).
2.2.6.2
Audio Monitoring
: The audio monitoring
circuit is comprised of buffer amplifiers
U1B/U2B/U2A and their associated components.
The
fwd pwr
input (J1-3), a DC voltage representing
the forward power of the transmitter with a
superimposed AC voltage proportional to the
modulation depth, is applied through loading resistor
R2 and applied to the non-inverting gate of buffer
amplifier U1B. The detected modulated audio signal
is applied to the non-inverting input of audio
amplifier U2B. Resistors R28/R29/R30 and
capacitor C8 set the gain of U2B. The output on
U2B-7 is coupled through capacitor C9 and passed to
audio
output (J1-6) and is used to drive speaker LS1
on the transmitter's front panel. Buffer amplifier
U2A-1 provides a filtered
audio (remote)
output (J1-
8) for external use.
