To: Joe Greene

From: Jon Hagen

Date: May 7, 2005

Subject: Notes on the modulator file: modulator_clamps.wpd

Floating Deck turns klystrons ON

Buffer Deck turns klystrons OFF

The job of the floating deck is to turn on the klystron beam current by pulling up the mod anodes from 5kV more negative than the cathodes to 50kV more positive and to hold the mod anodes at that high potential during the pulse. At the end of the pulse, the floating deck is turned off. The mod anodes, however, behave as charged capacitors. They must be forcibly discharged to turn off the klystrons.

This discharge is the job of the buffer deck. After the floating deck is turned off, the buffer deck is turned on briefly (a couple of microseconds) to discharge the mod anodes back down to 5kV below the klystron cathode potential. The buffer deck provides another discharge path, a 3-Megohm resistor in parallel with its switch tube. Once the fast discharge ("tail clipping") is done by the tube, the 3- Megohm resistor is sufficient to hold the mod anode low. Thus, between radar pulses, both decks remain turned off. ("Turned off" means that the switch tubes are tuned off, i.e. non-conducting).

Circuit details

The floating deck gets its name from the fact that its chassis (its "deck") does not remain at a fixed potential. The cathode of its switch tube, V301, is connected to this chassis. The plate of this switch tube is connected to a fixed voltage, the half-voltage tap of the beam supply. When the switch tube is pulsed on, the tube pulls its own chassis up (like pulling your boots up by chinning yourself on a bar).

Thus the voltage on the floating deck chassis is variable. (Though isn't given a chance to 'float' in the usual sense of finding its own preferred potential. Instead, it pulls itself up to -50kV or lets the buffer deck pull it down to -105kV. Of course, it is 'floating' in the sense that you cannot test its circuitry with a scope unless the scope chassis is tied to the deck itself. The scope would have to be battery powered, or plugged into the 110VAC supplied to the floating deck by the high-voltage isolation transformer. Sometimes a scope, used this way, has been placed on an insulating board, spanning the open tank. Extreme care must be used. Everything must be off and discharged before it is safe to touch the scope.

The name "buffer deck" seems entirely arbitrary. (The two decks might have been better named the "turn on deck" and the "turn off deck"). In any event, the buffer deck does not float, but is kept at constant and extremely lethal voltage, -105kV. Again, the use of a scope is almost too dangerous to be practical when the beam supply is on.

Both decks are controlled by optical fibers, which, unlike wire cables, can pass through the oil bath to the high-voltage decks without shorting out the high voltage. The optical signal to the floating deck is identical to the desired pulse, for example, a 100usec optical pulse to the floating deck will turn on the klystron beams for 100usec.

The optical pulse to the buffer deck is narrow and always has the same length, about 3 usec. It occurs just after the pulse to the floating deck ends, and causes the buffer deck to provide a 3 usec long fast discharge of the mod anode. Note that it discharges not only the capacitance of the mod anodes (about 75pF per klystron), but at the same time discharges the capacitance formed by the floating deck and the walls of the oil tank, several hundred pF.

Both the floating deck and the modulator deck circuits have totem pole drivers so that the DP11 grids can be pulled up (to turn them on) or pulled down (to turn them off). At first sight, the pull-down tubes seem to be included to discharge the grid capacitance of the DP11s. But this capacitance is small, and the cathode resistors for the paralleled 4-400s can pull the grids down. The purpose of the pull-down tubes in the modulator decks is to prevent a shoot-through condition (both switch tubes on at the same time). This prevention is needed because the switch tubes are triodes; there is no screen grid to shield their control grids from their plates.

Consider the DP11 in the buffer deck: The plate of this tube is pulled high suddenly when the floating deck turns on. The sudden increase of plate voltage produces a current through the plate-to-grid capacitance. Without the pull-down tube ('clamp tube'), the current that flows out of the grid has no low impedance path, so the grid voltage would jump, producing an unwanted turn-on of the DP11.

A pnpn diode circuit is triggered by the appearance of negative grid current; it turns on the clamp tube which then pulls the grid firmly to ground.

Similarly, the DP11 in the floating deck tries to turn on when the buffer deck is pulsed to "tail clip".

The sudden pull down of the floating deck must charge the plate-to-grid capacitance of the DP11 switch tube. Current pulling the floating deck down is sensed by a transformer which applies positive bias to the floating deck's clamp tube.