Print Date 01/12/09 Page 6 of 24 E2175-00MA-02
The microprocessor on the Main Board provides all control and monitoring of the hardware on the
board, including analogue to digital conversion of power supply levels. It also interfaces, via serial data
links, with other hardware control microprocessors on the Amplifier Control Board and on the Front
Panel module (separate dedicated bus – section 1.1.1).
A separate hardware ‘watchdog’ circuit monitors the running of the microprocessor’s program and
provides warnings (on board led, to Front Panel and to rear panel output) if the program fails to run,
and also automatically mutes the Amplifier’s output. The microprocessor also generates a mute signal
under various circumstances, this signal being directed via the safety ‘Interlock’ relay. This relay will
itself provide an over-riding mute signal if the external Interlock line is open-circuited, irrespective of
any internal Amplifier condition.
1.1.3 : 150/300W Amplifier Module, Control and Power Supply
The 150/300W Amplifier Module is fitted on a fan-cooled heatsink which runs between the front and
rear panels. To reduce ingress of dust, nearly all of the forced air is directed over the heatsink fins with
only a small ‘bleed’ of air over components mounted on the heatsink surface itself. The Module can
provide at least 150W or 300W output from approximately 2-4W RF input. The amplifier itself is an
integrated 50Ωgain block consisting of a dual RF Mosfet transistor with matching sections at its input
and output. This ‘pallet’ is mounted on an aluminium block and is easily replaced as a unit if required.
The maximum output power level is determined by the Power Supply fitted (36V for 150W, 48V for
300W). The output power level is set by the bias voltage applied to the output Mosfet by the Control
Board. A range of approximately 40W to 150W (XA150) or 40W to 300W (XA300) is provided with the
power input specified.
The Amplifier Module feeds a lowpass filter and directional coupler (both in a separate screened
section). The filter is a nine section Tchebycheff, giving a very high level of attenuation at harmonic
frequencies. The module itself also has sensors to measure heatsink temperature and output Mosfet
current. Voltages from the coupler (indicating forward and reverse power) and the sensors are fed to
the Control Board to enable accurate setting of forward output power, with a automatic reduction in
that power if reverse power, heatsink temperature or output Mosfet current becomes excessive.
A low level sample of the output power is taken from the Amplifier Module (after the separate low pass
filter section) to feed the front panel RF MON output connector.
As already described, the Control Board contains (analogue) circuitry to generate a control reference
voltage to set the bias voltage applied to the output Mosfet in the Amplifier Module. This control
voltage is initially set by the front panel SET POWER control to generate a desired power output level.
Excessive reverse power, heatsink temperature or amplifier current, above preset ‘trip’ points, then
automatically adjusts the set control voltage, gradually reducing the output power (‘fold-back’). Various
muting or external interlock conditions also over-ride the set control voltage, instantaneously reducing
the power output to a minimum.
A microprocessor on the Control Board provides all monitoring of the hardware on the board, including
analogue to digital conversion of forward/reverse power, heatsink temperature, amplifier current and
set power level. It also interfaces, via a serial data link, with the hardware control microprocessor on
the Main Board (RS485 bus – section 1.1.2). Digitised versions of all the mentioned levels and various
fault conditions (low power, power fail etc.) are sent to the Main Board and then onwards for display
on the Front Panel or remotely. A separate hardware ‘watchdog’ circuit monitors the running of the
microprocessor’s program and provides a Front Panel PA - CPU led warning if the program fails to
run, and puts the adjacent PA - FWD, REV, TEMP and CURRENT leds all off (their indications would
be invalid in this circumstance).
The Power Supply is a universal input, switched mode design, with a power rating and output voltage
dependent on the maximum output power from the Amplifier. The XA150 has a 36V, 300W supply and
the XA300 has a 48V, 600W supply. Its single output is fed to the ‘12V Supply and Fan Control Board’
(section 1.1.4).
