file: wpfiles\430tx\timinggenereator1.wpt draft Nov. 15, 2001

Arecibo 430 MHz Transmitter

Timing Generator

Operation and Maintenance Manual

Written by Jon Hagen

Nov. 2001

file:wpfiles\430tx\timinggenerator1.wpd	draft: Nov. 15, 2001

430 TX Timing Generator (mounted in the transmitter console)

1.0 Introduction

This console-mounted unit (Figure 1) imposes protective limits or extensions, when needed, on the four external timing request signals, converting them into four equivalent timing command signals for the 430 MHz radar system. These limits or extensions are applied a. when the externally supplied requests violate specified limits and b. in the event of ac power dips. In addition to this protective circuitry, the unit includes an internal timing generator for stand-alone transmitter testing.

2.0 Control Signal Requests

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

1.Receiver Protection Request (IPP)

This pulse turns on the receiver protectors (monoplexers). Normally the computer raises this line about 5 microseconds prior to starting the RF pulse, to be sure that the receiver is fully protected before any RF (leakage) arrives. Protection Request should be maintained high throughout the RF pulse and lowered a few microseconds after the pulse to avoid hitting the receiver with echoes from close-in clutter and transmission line reflections.

2. Beam Request (Pulse)

The klystrons can produce power only when their beams are on. It is good practice to keep the beams off until the receiver protection is active, even though the klystrons produce no RF until the beams are on and RF drive is present. (When only the beams are on, the klystrons produce enough noise power to render the receiver useless and, in the event of a crowbar, the klystrons may produce a potentially damaging noise pulse). Thus Beam Request should be raised after Protection Request is raised and lowered before Protection Request is lowered.

3. RF Request (RF Gate)

This pulse applies RF drive to the klystrons. The RF Request should not be raised until the beam current is fully on, as specified by the klystron manufacturer. Note that RF Request can also be turned off and back on during the beam pulse, in order to transmit a burst of two or more 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 command signals as follows:

1. Receiver Protection Command (sent to the platform-located monoplexers) The

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

a. Protection Request is high (the intended, i.e. nominal form of protection)

b. RF Request is high (automatic protection)

c. Protection Request has been raised in the last 25 microseconds; one can simply pulse the protection request and then rely on automatic protection. (This provides compatibility with the former timing control system).

d. RF Request has been lowered in the last 5 microseconds (extended automatic protection)

e. An ac power dip is occurring

2. Beam Command (sent to the klystron 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 2 usec and b. the length of any output space is at least 900 usec. The maximum output PRF is therefore 1kHz/(0.9 + 0.002) = 1109 Hz. 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. suppresses 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 the Granted Beam Request Normally, requests are legal and the Granted Beam Request is identical to the Beam Request. Finally, the Granted Beam Request, is OR'd with LOCAL.AND.CW, to form the Beam Command. The transmitter has a seldom used CW mode; setting this unit to "LOCAL" and its Duty Cycle selector switch "CW" causes the receiver protection, rf, and beam commands all to be high. (A removable stop on the duty cycle dial prevents accidental selection of "CW").

Note: Copies of the Beam Command are also sent to the Low-Level Modulator Chassis and the Low-Level Amplifier Chassis, the two units that comprise the exciter, where they are used to "unblank" (i.e. enable) the exciter. (The RF Command provides fast turn on and turn off, but, additional blanking is necessary to provide total suppression of feed- through during receive periods).

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

The RF command is the AND of the RF Request, the Protection Acknowledge signal, and a 2 usec delayed version of the Beam Command. This prevents driving the klystrons until the beam is fully on and, of course, prevents driving the klystrons when the receiver protection is not active. The RF Command is also forced low if a power dip is occurring.

4. Phase Command

The Phase Request is modified, if necessary, so that phase states last at least 0.9 microseconds. The resulting Phase Command is sent to the RF exciter unit. Normally Phase Requests will be legal and the 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. In this figure, the normal signal paths are shown in bold. The individual blocks are described below

.4.1 PRF Limiter

Figure 3 is a simplified schematic diagram of the PRF Limiter. 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= 2 uec and T1 = 900 usec. The maximum PRF is therefore 1kHz/ (0.9 + .002) = 1109 Hz.

4.2 Phase Flip Rate Limiter

This block is identical to the PRF Limiter, except that T1 and T2 are both 0.9 usec. The maximum phase code chip length is therefore 0.9 usec.

4.3 Duty Cycle and Pulse Width Limiter

Figure 4, a simplified schematic diagram of this block, 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 be granted. 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 zero and the flip flop will not get turned on. That BEAM REQUEST will be ignored (suppressed) 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, ready to grant the next BEAM REQUEST.

When the GRANTED BEAM REQUEST goes high, the counter begins counting up from zero 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.). If the counter reaches a count of 10496 (2.099 msec of up counting) , it will reset the flip flop to truncate the GRANTED BEAM REQUEST, therefore lowering the GRANTED BEAM REQUEST and truncating the pulse. Truncating the pulse causes the counter to reverse and start counting down, just as if the BEAM REQUEST had been lowered properly..

4.3 Internal Pulse Generator

This unit produces local versions of the four control pulses, and is used for stand-alone testing of the transmitter. Refer to Figure 5, 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).

4.4 Brownout Protector

This circuitry produces a "line voltage fault" signal (1-bit) as soon as any one of the three power
line phases falls below a threshold voltage of XXX volts. The fault simultaneously blanks the transmitter (suppressing the RF Command) and applies receiver protection (turning on the monoplexers). Any sensed brownout is stretched an additional one second, so even the briefest power dip will produce a one second blanking.

5. Installation

After lab testing, the unit can be connected in parallel with the old unit during an ionosphere run. This will verify that the outputs are correct and also that the timing requests, as currently generated, are within specifications. When the unit is installed in the transmitter, it should be supplied with its own UPS, just as each of the two upstairs monoplexer drivers has its own UPS.