SRK Electronics FMA501, Руководство пользователя

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SRK Electronics FMA501 manual
 2007 SRK Electronics Pty Ltd 20
The signals from the rotary encoder are filtered and then passed to pins 33, 34 and 35.
Pins 22 and 23 provide an I²C bus which allows communication with the real time clock (U3)
and the flash memory (U5).
The LCD is controlled by the signals on pins 24, 26 and 27, as well as the data on pins
1,37,3,4,5,6,7 and 8.
The crystal connected to pins 28 and 29 provides an internal 1Hz clock for U1.
The two error conditions HIGH O/P and HIGH SWR are monitored on pins 20 and 21. These
signals are derived from the comparators U10 and U11. These devices monitor the forward
and reflected power control loops formed by U9B and U9C. If the output of U9B drops to less
than about 10V, this indicates that an over O/P power condition exists. Likewise U9C and a
high SWR condition.
U9B and U9C constantly compare the actual forward and reflected power with thresholds set
by VR3 and VR6 respectively. VR3 is set to the equivalent of about 550W of forward power.
VR6 is set to the equivalent of about 50W of reflected power. Either U9B or U9C can reduce
the bias voltage to the amplifier, thus reducing the output power. In this way, elegant and
seamless power and SWR protection are provided.
The forward and reflected RF voltages from the directional coupler are accurately converted
to DC levels by U7 and U8. U9A and U9D are used to scale these voltages so that 1000W
passing through the directional coupler provides 5.0V to the MCU.
Small value resistors in the supply lines of the MOSFETs on the amplifier palette generate
voltages proportional to the drain currents. These voltages are amplified and level shifted by
U12, Q2 and Q3. VR11 and VR12 are adjusted so that 20.48A of drain current produce 5.0V
into the MCU.
Gate bias for the MOSFETs is set by VR14 and VR15.
The AC line voltage is monitored by measuring the frequency of the relaxation oscillator
formed by R12, 14 and 15, C39 and D11. These pulses are passed via the optocoupler U13
into the MCU. As D4, C38 and R12,14 and 15 form a reasonable approximation to a constant
current source whose value is proportional to the peak AC voltage, the frequency of these
pulses is a reasonably accurate representation of the AC voltage.
Telemetry is provided by the on chip USART in the MCU, with U2 providing level shifting to
RS232 levels.
+15V for this board is provided by the small SMPSU, U16. U14 pr5V from this for the
digital circuitry and U15 generates a nominal -9V for the analogue circuitry.
6.4
Amplifier palette
The amplifier palette performs the actual RF amplification. Please refer to the appropriate
schematic.
In brief, the RF is split into two paths, each consisting of a 250W class B amplifier (amplifier A
and amplifier B). The outputs from these amplifiers are combined and filtered to provide an
overall output power of 500W. A more detailed description follows:
RF enters the palette on J1 and passes through directional coupler U1. This provides a small
sample of the input power across R17 and R18. This is rectified by D1 which thus provides a