User Manual

MXi AMPLIFIER CONTROLLER BOARD REV.2
PUB06-82 Rev 1 July 17, 2007 06-82-39 MXi Amplifier Controller Board Rev.2
The MXi board has a built-in Hall Effect current sensor that is able to measure the current drawn by the RF
amplifier from the +32 volt power supply. The standard MXi low power transmitter would have a 25 amp current
sensor. There are two possible sensors that could be installed in the MXi which depend on the actual current level
that is to be measured. A higher power MXi transmitter would be fitted with a 40 amp current sensor. For higher
power transmitters, the power supply would be external to the amplifier chassis and is fitted with its own current
sensor telemetry that is fed into the MXi board separately and the MXi current sensor would be left unused.
Another consideration for making the current sensor external on higher power MXi transmitters is the gauge of
wire required for these higher current make it difficult to wire into the MXi board connector J4.
The circuit board has winding jumpers E20-E23 that allow a single input winding to the current sensor or a dual
winding input to the current sensor. These winding jumpers allow for lower power transmitters (which require
lower power supply currents) to generate a reasonable output voltage from the current sensor. When the winding
jumpers are in the low current setting (“Lo I” = E20 jumpered to E21), this causes the input current to pass
through the input of the current sensor twice (in two windings) producing double the output voltage than would be
generated by a single winding. This is used for lower power transmitters where the maximum power supply
current draw is under half of the rating of the current sensor.
When the winding jumpers are in the high current setting (“Hi I” = E20 jumpered to E22 and E21 jumpered to
E23), this causes the input current to pass through the input of the current sensor only once (in one winding)
producing half the output voltage than would be generated by a double winding. This is used for higher power
transmitters where the maximum power supply current draw is over half of the rating of the current sensor.
When the internal current sensing is being utilized, the MXi expects the +32V to be applied at connector J4 pin 1.
This voltage is passed through a current monitoring circuit and then fed back out on pin 2 of J4 to the RF
amplifiers. The Hall Effect Sensor U9 has a built in voltage reference of 2.5 volts. The sensing output would be at
this 2.5V level when there is no current through the sensor. When current passes through the sensor, then the
output voltage would either increase or decrease from the 2.5V reference. Whether the voltage increases or
decreases depends on the direction of current flow through the sensor thus making this device bi-directional. In
our configuration, the current only flows one way and increasing current will result in decreasing sensor output
voltages. Jumper E4 selects between the internal U9 2.5V reference or an external current sensor. When using
U9 as the sensor, jumper E4 must be placed in the INT position. The output of E4 is fed into the positive input of
OpAmp U7B with the negative input fed from the U9 sensing output such that the OpAmp effectively removes the
2.5V offset introduced by the U9 sensor. A potentiometer R15 is placed in the feedback path of U7A to allow the
current to be calibrated.
When the power supply is external (usual in higher power MXi transmitters), the actual power supply is external to
the amplifier chassis and will then do its own current metering. In this configuration, the MXi board would not have
its own U9 current sensor installed but would receive this telemetry information from J5 pin 5. Jumper E4 selects
between the internal U9 sensing output or an external current sensor. When using the external sensor, jumper E4
must be placed in the EXT position. OpAmp U7A allows for a gain control through potentiometer R15 so that the
current reading can be calibrated.
The Power Supply voltage is expected to be +32 volts DC +/- 0.5V and can be fed into the MXi board from one of
two points, depending on whether the supply is internal or external to the amplifier chassis.
When the power supply is internal to the MXi amplifier chassis, then the MXi expects the +32V to be applied at
connector J4 pin 1. This voltage is passed through a current monitoring circuit and then fed back out on pin 2 of
J4 to the RF amplifiers. This configuration is used when the MXi is a lower power transmitter that can
accommodate the power supply internally.
When the power supply is external (usual in higher power MXi transmitters), the actual power supply is external to
the amplifier chassis and will then do its own current metering. The +32V sample is connected to J5 pin 6 (which
is in parallel with J1 pin 1). Connector J1 is a higher current contact that would allow the current to be passed
through the internal current sensor U9. For an external supply these are not required and so both U9 and J4 will
most likely not be installed on the board.