Friday, August 17, 2001

430 TX Timing Generator (mounted in the transmitter console)

1.0 Introduction

This console-mounted unit (Figure 1) imposes protective limits on the four externally supplied timing signals used in normal operation. It also contains an internal timing generator for stand- alone transmitter testing. When the four timing signals (requests) do not violate duty cycle, pulse length, or receiver protection constraints, they are passed on, unmodified, as commands to the transmitter. Otherwise, they are modified (e.g. pulses are cut short or suppressed entirely). Timing violations turn on red indicator lights. These warning lights remain lit until reset with an adjacent push button.

2.0 Control Signal Requests

Four externally supplied TTL lines are used to modulate the transmitter. These lines (including their traditional names) are listed below in the usual order of activation.

1.Protection Request (IPP)

The IPP pulse turns on the receiver protector (monoplexer). Receiver protection will become active about 2 microseconds after this pulse goes high. Normally the experimenter raises this line about 5 microseconds prior to raising the Beam Request line.

2. Beam Request (Pulse )

This pulse turns on the klystron Beam Command immediately, except that, if the receiver protection is not active, the Protection Command will be turned on automatically and the beam will be held off until the Protection Acknowledge line from the platform signals that the protection is active.

3. RF Request (RF Gate)

The RF Request is kept low to hold off the drive signal until beam current if fully on, as specified by the klystron manufacturer, Litton Industries. The RF Gate request can also used to suppress the RF for within the dc pulse, in order to transmit multiple short pulses.

4. Phase Request (Phase)

This signal flips the phase of the transmitted signal by 180 degrees and is used to apply phase coding to the transmitted signal.

3.0 Control Signal Commands

The request signals are converted into the following transmitter command signals:

1. Protection Command (sent to the platform-located receiver protectors)

The protection command is high if and only if either of the following are true:

a. Protection Request is high or has been raised in the last 25 microseconds

b. Beam Request is high or has been lowered in the last 25 microseconds

2. Beam Command (sent to the transmitter beam modulator)

The Beam Request is first sent through the PRF Limiter, a block which will ensure that a. the length of any output pulse is at least 55 usec and b. the length of any output space is at least 900 usec. The maximum output PRF is therefore 1kHz/(0.9 + 0.55) = 1.047 kHz. The PRF-limited Beam Request is next sent through the Duty Cycle and Pulse Length Limiter, a block that

a. truncates any Beam Request longer than 2.099 msec and b. ignores any Beam Request that occurs so soon after the previous Request as to exceed a 6.25% duty cycle. The output of this block is called the Granted Beam Request. Normally the requests are legal and the Granted Beam Request identical to the Beam Request. The Beam Command is the AND of the Granted Beam Request and the Protection Acknowledge line (the line looping back from the platform to signal that the Protection Command has indeed activated the PIN diode monoplexers).

3. RF Command (sent to the low level RF drive circuits)

The RF command is the AND of the RF Request and the Beam Command. This prevents driving the klystrons until the beam is on. A delay element ensures that the RF drive is not applied until the beam current is fully on.

4. Phase Command

The Phase Request is modified, if necessary, so that phase states last at least 0.9 microseconds. The resulting Granted Phase Request is the Phase Command to the RF exciter unit. Normally the Phase Requests will be legal and the Granted Phase Command will be identical to the Phase Request.

4.0 Overall Circuit Description

The transmitter commands described above are generated by the logic shown in Figure 2, a block diagram. The individual blocks are described below.

4.1 PRF Limiter

This block, shown in Figure 3. The circuit uses two non-retriggerable one-shots. Examination of the circuit shows that the space between output pulses will be no shorter than T1 and that the duration of output pulses will be no shorter than T2. For the PRF limiter, T2= 55 uec and T1 = 900 usec. The maximum PRF is therefore 1kHz/ (0.9 + .055) = 1.047 kHz.

4.2 Phase Flip Rate Limiter

This block, shown in Figure 4 is identical to the PRF Limiter, except that T1 and T2 are both 1 usec. The maximum phase code chip length is therefore 1 usec.

4.3 Duty Cycle and Pulse Width Limiter

Figure 5, a simplified schematic diagram, shows the duty cycle and pulse width limiter.

The circuit has one input, BEAM REQUEST, and one output, GRANTED BEAM REQUEST. Note that the heart this circuit is the flip-flop FF1 and the AND gate U1 at the upper right corner. When FF1 is high, the beam request is granted (gated). In normal operation, the D input of the FF1 will be high when a request pulse arrives, so the leading edge of the BEAM REQUEST pulse will turn on the flip flop and thereby gate itself through the AND gate to become the GRANTED BEAM REQUEST. However, if the BEAM REQUEST pulse arrives too soon (before the elapsed time following the previous pulse is at least 15 times the length of that pulse), the D input will still be low and the flip flop will not be gated on. That BEAM REQUEST will be ignored and the duty cycle will not exceed 1/(1+15), i.e. 6.25%. When a BEAM REQUEST is honored but then lasts more than 2.099 msec, the flip-flop is reset, truncating the GRANTED BEAM REQUEST. Normally, however, the GRANTED BEAM REQUEST falls when the BEAM REQUEST falls.

The operation of the circuit is as follows: The GRANTED BEAM REQUEST is used as the up/down control input of a special up/down counter, the Beam Request Integrator, which is driven from a 5MHz clock. Between pulses, this counter is kept in the down mode. When the counter gets down to zero, it disables itself, staying at zero and thereby keeping the D input of FF1 high and ready to grant the next BEAM REQUEST.

When the GRANTED BEAM REQUEST goes high, the counter begins counting up at a 5MHz rate, integrating the beam time. When the GRANTED BEAM REQUEST goes lo, the counter reverses and integrates down to zero, counting at a rate of 5Mhz/15. (In the down mode, a divide by 15 prescaler suppresses all but one out of every 15 pulses from the 5MHz clock.). The counter will also reverse direction if it reaches a count of 10496 (2.099 msec of up counting). Reaching this limit will also reset the flip flop to truncate the GRANTED BEAM REQUEST, therefore lowering the BEAM COMMAND.

4.3 Internal Pulse Generator

This unit produces local versions of the four control pulses. It is used for stand-alone testing of the transmitter. Refer to Figure 6, the block diagram of the Internal Pulse Generator. This generator is based on two counters: the PRF counter and the Pulse Length Counter. Both counters are free running; they cycle continuously. The PRF counter, a straight binary counter, is clocked at 1MHz. Its successively divided outputs are the inputs to a 10-position PRF Selector switch. The selected output rate is used to clock the Pulse Length Counter, which is a 3-decade counter that cycles continuously from 000 to 999. The same clock is used to clock a D-type flip flop, whose output is the generated pulse. The flip flop is turned on when the counter state is 000 and is turned off when the counter state is 001, 002, ......... 060, which correspond to duty cycles from .001 to .060 (0.1% to 6%). Note that a 0% duty cycle (beam off) is also provided. When the duty cycle selector is set to 0%, the D input of the flip flop is held continuously low.

The output of the flip-flop is delayed by 24 usec by means of a 24-bit shift register clocked at 1 MHz. The shift register output is the LOCAL BEAM REQUEST. This is OR'd with the output of the flip-flop to produce the LOCAL RECEIVER PROTECTION REQUEST, so that the receiver protection goes high 24 usec before the beam is turned on and then stays high until the end of the beam pulse.

The LOCAL RF REQUEST is identical to the LOCAL BEAM REQUEST. The LOCAL PHASE REQUEST is derived from a front panel BNC jack. For most testing, nothing will be plugged into this jack (no phase modulation).