To: file newmpxr.wp6

From: Jon Hagen

Date: May 11, 2000

Subject: Monoplexer Driver

Two identical new monoplexer drivers are being built for the 430 MHz radar system: one for the receiver in the carriage house and the other for the receiver in the Gregorian dome.

Theory of operation (Refer to the attached schematic diagram):

The driver comprises three circuit blocks: a switch block, a 140V power supply block, and a logic block. It also contains a commercial 5V power supply.

Switch Block

The Switch block contains totem pole circuit with a pull-up transistor Q2 and a pull-down transistor Q5. Q5 pulls the monoplexer down to -5 volts. While Q5 is off, diode D2 opens, letting R6 turn on Q2 to pull the monoplexer up to 140 volts. A push-pull complementary pair, Q3 and Q4, drives the gate of Q2.

140V Supply

The 140V supply simply back-biases the monoplexer. It needs to supply no more than about 7 ma. This supply uses a straight-forward linear regulator.

Logic Block

The monoplexer is commanded to the transmit state (receiver input shorted to ground) by illuminating the optical fiber input Command In. For compatibility with the old drivers, a -3 volt logic Alternate Command In is provided. If the Command Input is used, the Alternate Command input should be left disconnected or at zero volts. When the Alternate Command input is used, the Command In should be disconnected or left un-illuminated.

A fiber Command Out is provided so that the command can be daisy-chained to the second monoplexer from the first monoplexer (the "first" is the one that receives the command from the control room and sends the Acknowledge signal back to the control room). For compatibility with the old drivers, a -3V Alternate Command Out is also provided.

A fiber Acknowledge Out and an -3V Alternate Acknowledge Out are provided to signal the transmitter that receiver protection is active (current is flowing in the monoplexer PIN diodes). R19 sets the threshold of the current detector. It should be adjusted so that, with normal PIN diode current (1/2 amp/diode), the differential voltage at the input of U7 is about 1/10 volt.

As in the old monoplexer driver, a one-shot is provided to stretch the receiver protect pulse by approximately 20 microseconds - though it seems that this added protection could better be done downstairs at the transmitter console. This stretch can later be defeated easily by grounding pin 9 of U5. If this is done, the Ack Out from the first driver can be used as the command from the second driver, eliminating one cable between the two drivers.

When both drivers are used, the second one (slave) passes its acknowledge signal back to the first driver where it is ANDed to produce the acknowledge signal fed back down to the transmitter. Inspection of the schematic diagram on the front panel will show that the switch on the first driver must be set in the Master position while the switch on the second driver must be set to the Slave position

5V Power Supply

A commercial linear 5V , 4A power supply is used to supply -5 volts. One of our present monoplexers uses 6 PIN diodes and requires 3 amps of forward current. Our other monoplexer uses 8 diodes and requires 4 amps.