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Technical Manual
5.2.4 RF Power Control
Forward power is controlled by the microprocessor through two mechanisms. Based
on pre-programmed per channel adjustments the microprocessor sets the digital to
analog converter output to a reference setting. IC28B serves as a comparator and, with
the non-inverting input connected to the output of the digital to analog converter, is set
up with a reference voltage. The detected actual PA forward power is fed to the
inverting input of IC28B. The error voltage at the output of IC28B is fed to the exciter
output power control circuit via SKB-4 and the action of the control loop is to set the
RF power such that the actual detected volts equals the reference volts. The digipot
setting is static for each channel unless the required forward power is changed.
The voltage to the top of the digipot is set up by the microprocessor through the Pulse
Width Modulator output PWM1. On PTT the ON duty cycle of the PWM1 output is
progressively increased and the filtered result of this forms a ramp to the top of the
power control digipot. Once 100% duty cycle is reached full power is produced. This
results in a fast but controlled RF power rise characteristic.
5.2.5 User Interface
All user interfaces to the MX920 except the TX RF connections is made by way of the
Main Controller board.
♦ Output latch IC4 drives the six LED indicators on the front panel. The function
is described in Section 1.1.1.
♦ An input port on IC2 serves to accept the programmed status of the 8-bit BCD
channel select input from the front panel Pushwheel switches.
♦ IC31 is a 3 Watt speaker amplifier, which drives the internal speaker.
♦ Microphone audio is into the front panel RJ45 connector. In addition to RX VF,
Monitor control, Voltage output and PTT. The connector functions are
described in more detail in Section 3.3.
♦ IC13 provides CMOS to RS232 conversion for the serial port. This is also
located on the Microphone RJ45 connector.
♦ The front panel adjustment Pots control Volume and Mute Squelch settings.
5.3 Power Amplifier Module
RF from the Exciter passes via a coaxial cable to the input of the PA Module and is
first attenuated by a 50 ohm pad, which is used to provide a good 50 ohm source
impedance for the first LDMOS driver amplifier. The RF is amplified to around
5 Watts at the driver output, and is band dependant. Note: this point does not have
50 ohm impedance and the drive power cannot be measured directly with a 50 ohm
Wattmeter. The signal from the driver is then matched by a broadband network to
drive the low input impedance associated with the final transmit LDMOS power
amplifier transistor. The transistor's low Drain impedance is then also matched back to
50 ohms by a broadband matching network covering a very wide bandwidth. Prior to
transmission, a low loss 13 element elliptical low pass filter, filters out the unwanted
harmonics to less than –90 dBc.
© SPECTRA ENGINEERING 2003 Revision 1.1.4
Installation and Operation
19
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