To: file monplx_mkII.wpd

From : Jon Hagen

Date: Jan 29, 2003 , April 23, 2003 COMMENTS AUG 25, 2003

Subject: redesign of monoplexer drivers

8-276-03

Have schematics made for MKII driver. Page 1 has the actual switching circuitry. Breadboard this part on old board and test for stability, timing, current draw, and shoot-through.

Must orerder 2n5657, 2n3439 (10 each)

The previous drivers have been modified to the point that their pc boards are unreliable. In addition, several new features should be added.

0. A bias value of 500 ma is sufficient to bring the ac resistance of each diode down to 0.1 ohm.. 1 amp might get this to .08 (not worth the extra current, especially given the series inductance that will remain the determining factor). The reverse bias voltage, if greater than 100V, produces lowest insertion loss. A reverse voltage of 20V approximately doubles, the loss, according to the orignal measurements done by Sylvania. WE'LL STICK WITH THE 130-150 V REVERSE BIAS AND .500 AMP FORWARD BIAS.

1. Rise and fall times should be adjustable (ie. each controlled by the value of a particular capacitor or resistor. Still, the switching transistors should be snapped on and off, avoiding any deliberate ramping through their active regions.

2. The turn on and turn off should be independently controlled by the logic circuitry, rather than the "automatic totem pole" arrangement used previously, in order that the rise and falls not affect each other. STILL GOING TO KEEP THE AUOTMATIC TOTEM POLE - AS LONG AS THERE IS NO SHOOT-THROUGH THAT WOULD PUT ADDITIONAL DRAIN ON THE POSITIVE SUPPLY.

3. The unit should be completely short circuit proof - so that an open or short in the monoplexer will not harm the driver. SO LITTLE CURRENT IS DRAWN FROM THE POSITIVE SUPPLY (APPROX 2 MA MAX ) THAT IT CAN BE PROTECTED BY A SIMPLE SERIES RESISTOR OF VALUE APPROX. 10K (TO LIMIT THE VOLTAGE DROP TO 20V MAX).

4. The acknolwledge signal should be the AND of the command signal and the diode current sense signal rather than just the latter. This way, a failure in the command line will lower the acknowledge immediately, turning the transmitter RF off before the monoplexer driver times out. Without the AND connection, a failure in the command line could allow the monoplexer to time out long before the tranmitter pulse. (The 25 usec extension on diode current would be retained).

DONE

5. The circuit should take into account the nature of the mismatched load (the monoplexer) so that reflections do not cause destructive transient spikes. (Reducing rise and fall times will also reduce such spikes by any amount desired). SPICE ANALYSIS INCLUDING 20FT OF 50OHM CABLE SHOWS SPIKES ARE NOT PROBLEM IF RISE AND FALL TIMES ARE MODERATED.

6. The current supplied by the monoplexer might well be regulated in that the PIN diode impedance is a function of temperature. No, better to supply a fixed voltage to the diode/resistor pairs to avoid a diode failure cascade. YES, RETAIN THE FIXED VOLTAGE OPERATION

7. ack signal should be based on both current to monoplexer and voltage applied. A shorted diode will decrease the voltage and an open diode will decrease the current. To avoid transients, the voltage and current checks could be done, say by a gate occuring maybe 10 usec after the device is turned on. Voltage is hard to monitor (5v goes to 150), so maybe upper and lower current limits.

7a. A latched red LED on the panel should light if the diode current falls below threshold during the gate interval. As with the latched fault LEDs on the timing generator, cables can be tugged and flexed; if they are intermittant, the LED comes on.

8.Circuit design is complicated by characteristics of the pin diodes, i.e. the amount of charge they store while in the fwd biased state. If the rf by-pass capacitor is large enough (or has additional capacitance shunted across it) this capacitance can hold much more charge than that on the diodes and will determine the response of the circuit. THIS IS THE CASE WITH OUR 8-DIODE MPXRS (NEW AND OLD) THE NEW UNIT HAS 8 FEEDTHROUGH CAPS, EACH 1500PF FOR A TOTAL OF 0.012 MICROFARADS.

9. It seems important that we install incident power monitors on both monoplexers. These should provide several signals to downstairs: average power (for monoplexer on and off) and peak power (for monoplexer on and off). Can they be used for fast protection?

This allows us to measure the isolation of the turnstile and to detect timing errors. Additionally, there should be a way to measure the actual isolation (and recovery) of the monoplexer.

4-31-03

Consider independent control of all 8 diodes: 8 totem pole drivers. Each driver can supply a regulated 500 ma diode drive, so resistors aren't needed at the monoplexer. Failure of any diode is easily detected. TOO COMPLICATED

Receiver Protection

Blown out 430MHz radar recivers have been a chronic problem at Arecibo. The basic protection scheme is the following. When the monoplexer has been commanded to turn on AND the total monoplexer PIN diode current is sufficient, the monoplexer returns an "Acknowledge" signal to the transmitter. Until the acknowlege signal is high, the transmitter disables RF drive.

There is protection agains the situation where the command pulse to monoplexer fails during the pulse. When the command pulse falls, a one-shot inside the monoplexer driver maintains the PIN diode current for 25 usec. The acknowledge pulse, however will go low, since it is the AND of the command and the diode current. When the timing generator senses that "Ack" has fallen while "RF Request" is high, it immediately truncates the remainder of that pulse and lights a front panel "Ack error" LED . The 25 usec extension on the monoplexer diode current should protect the receiver while the pulse is being truncated.

There remain several burn-out scenarios:

1. Insufficient isolation in the turnstile junction - this could from improper tuning of the turnstile, or it could be a temporary or momentary condition due to some object near the feed horn reflecting power back into the turnstile to the receiver port.

2. Insufficient isolation in the monoplexer (partial or complete monoplexer failure). The monoplexer uses either 6 or 8 PIN diodes. Only the total diode current is measured at present.

The best - though inconvient - system would be to monitor separately the current in each diode. The acknowelge signal would remain low if one or more diode currents is outside the correct range.

3. Monoplexer drive pulse feed through - If the monoplexer current rises too rapidly, a transient is produced on the receiver's input line. The rise and fall rates of this current can be deliberately slowed, but a slow fall rate limits the speed with which the receiver recovers and hence restricts the minimum range from which signals can be received.

3.Crowbar-related reciever burnout

Reciever failures and crowbar events often occur together. We have assumed that this association arises from the general havoc caused by the crowbar; glitches are induced into nearly any analog or digital circuit, so timing errors could be induced in the receiver protection command/acknowlege loop. Yet, if the crowbar event occurs while the pulse is on, an induced turn-off of the receiver protection should be covered by the 25 usec delay built into the monoplxer. What about crowbar events when the..............