Controls Upgrade of the RF Systems of the CERN PS
R. Garoby, M. Gourber-Pace, S. Hancock, F. di Maio, N. de Metz-Noblat
PS Division, CERN, CH-1211, Geneva 23, Switzerland

Abstract
The latest upgrade of the control systems of the CERN PS Complex has seen the replacement of all components of thecontrols interface to the RF systems of the PS machine. New hardware offers features which have been exploited not only byconcomitant software developments but also by changes in the philosophy underlying the operation of those RF systems. Inaddition, a novel measurement scheme has been installed which provides a display of RF parameters by processing a wide
variety of signals continuously sampled at 1 kHz. New timing diagnostics have also been implemented.
This paper reviews the main advantages of the new equipment and the evolution in operational principles which it hasafforded.

1. INTRODUCTION
The PS (Proton Synchrotron) Complex comprises three linear accelerators and six circular ones, all interlinked. At its heartlies the thirty-six year old PS machine which handles a variety of particle beams on a cycle-to-cycle (so-called "PPM") basis. A
rejuvenation of the control system of the PS is foreseen in two stages, the first of which has been completed and has affected allcomponents of the interface to the three RF systems of the machine.
Figure 1 is a greatly simplified schematic diagram showing the main features of a PS RF system. Three types of control
variable are employed: analogue functions (RF voltage, servo loop gains, etc.); timing pulses; and bit patterns for quasi-staticparameters which are only refreshed once per machine cycle. The corresponding hardware comprises a total of 6 VME,2 CAMAC and 21 G64 crates.
Longitudinal Pick-ups

Radial Pick-ups RF Cavities
BeamTimings &Gains

Signal Processing
BEAM CONTROLRF Source

Power Amplifiers
Tuning Power Supply
Auxiliary Systems

Measurements(Frequency, phase,amplitude, etc...)Voltage&Tuning
servosVoltageprogrammes

Actuation
Figure 1. Basic RF layout.

2. HARDWARE
2.1 Function Generators
New VME-based function generators [Ref. 1] have increased by an order of magnitude the number of vectors available for theproduction of analogue signals. The need for extensive combinations of generators and summing amplifiers to provide detailed
functions has thus been removed. However, the tremendous flexibility of the principal RF system of the PS derives from a
combinatorial philosophy which has, therefore, been extended in this particular case. In order to avoid the transmission of
analogue signals over long distances, a galvanically-isolated serial link carries the instantaneous digital values of the generatedfunction either to a simple DAC for normal applications or to a multiplier plus DAC which converts the product of two such
digital inputs. The latter permits different cavity voltage programmes to be generated within the same machine cycle as variantsof a common fundamental programme.
In addition to more vectors, a function may contain multiple internal stops to hold the output at a certain value until a restartpulse is received and processing of the vector table is resumed. This facility is used extensively to synchronise transitions in theindividual cavity voltage programmes with each other and with specific events (e.g., the opening or closing of the corresponding

cavity short-circuits). The overall number of RF timings has been reduced since the wealth of vectors allows the duration of aflat first vector to determine the effective start of many functions. This trades off a dedicated start pulse against just one extravector. Further economies have been possible because PPM means that restart pulses may be "re-used" at different times ondifferent machine cycles.
A function may also be regenerated ad infinitum by processing the vector table in a loop. This permits the generation of alow frequency (< 20 kHz as the minimum vector duration is 5 µs) periodic function burst for modulation purposes from a
description of one or more periods in the vector table.
2.2 Timing Generators
New VME-based hardware has permitted a change in the philosophy of timing generation. The majority of RF timing
pulses are now generated in absolute time (with respect to the start of a machine cycle) rather than as delays from other timings.
This is an advantage in large timing systems since there are no cabled links. Indeed, the very notion of timing structure has
merely been emulated by establishing logical links between timing channels. A hierarchical timing cascade, which can be
structurally different from cycle to cycle, may be maintained at application program level even though the hardware generates
pulses entirely independent of each other. A further advantage is that a negative interval may be programmed between a linked
timing and its logical predecessor without the need for an advanced prepulse (see Figure 2).

Figure 2. Linked timing application program.
Underlying the new system [Ref. 2] is a master timing generator (MTG) which distrubutes timing information on a single
cable to all timing receiver (Tg8) modules [Ref. 3]. Each Tg8 receives the information in the form of timing frames which may
subsequently initiate locally programmed actions. An action may add a delay (specified as a count of either an internal[10 MHz] or external clock train) before producing an output pulse, a VME interrupt, or both. Different actions may pertain tothe same output channel and this feature is exploited to produce multiple restart pulses for function generators. Up to eighttiming frames are transmitted during a 1 ms time slot and are validated by a "1 kHz event" at the end of this time. Thus,absolute time is readily described in terms of a number of Tg8 internal clock pulses after a particular millisecond from the startof a machine cycle. Time delays may be expressed with respect either to some other MTG event or to an external start.
The 1 kHz frequency of the MTG is derived from a 10 MHz rubidium gas oscillator which is, in turn, updated by a radio
link from an atomic clock. However, the encoding of frames by the MTG and their decoding by a Tg8 are performed using freerunning 4 MHz oscillators with the result that the 1 kHz events are subject to a jitter of ±250 ns. This is entirely sufficient
for all except a small number of beam-related timings which require external starts and external clock trains.
2.3 High-power Equipment Interface
The high-power drive electronics of the twenty-one cavities in the PS are extensively protected by interlocks. The actuationof these systems is controlled and their status monitored via a completely new, G64-based interface [Ref. 4].

3. DIAGNOSTICS
3.1 Sampled RF Measurements
A powerful digital system has been assembled which provides a measurement of the essential RF parameters eachmillisecond during the active part of every machine cycle. The 1 kHz sampling frequency is derived from the same oscillator onwhich the MTG is synchronized. Various real-time tasks control the acquisition of: the beam revolution frequency and selected
RF frequencies; the phase with respect to a revolution frequency reference of each of the principal RF cavities; the harmonic
number of each of those cavities and all programmed and detected cavity voltages. These data are made available during the deadtime at the end of a cycle for processing by an additional real-time task [Ref. 5] which computes the resultant voltage and phasecomponents of the principal RF system by summing vectorially over all the cavities which are on the same harmonic (seeFigure 3). It also computes the phase error, with respect to the phase sum for the appropriate harmonic, of each principal
cavity. Both the raw and processed data can be accessed for display or further treatment.

Figure 3. Total detected voltage and corresponding harmonic number measurements.
3.2 Timing Surveillance
Among the most difficult tasks in the operation of an accelerator is the diagnosis of timing problems. A timing surveillance(TSM) system [Ref. 6] has been developed to monitor simultaneously some 150 timing channels. All pulses, including
multiple ones, are recorded with a resolution of 100 ns and an exhaustive list may be produced for an entire supercycle of themachine.
The precision of the TSM measurement is a consequence of its reliance on the widely-distributed 10 MHz clock describedabove. However, it is insufficient to monitor the beam-related timings that are required to synchronize the PS with its
neighbours and client machines for injection and extraction purposes. To this end, a transfer timing surveillance (TTSM)
system [Ref. 7] has been developed to measure time differences between key signals with a resolution of 1 ns. Eight different
types of beam transfer may be monitored and warnings are issued for tiings that are out of tolerance.
3.3 Fault History
Any problems with the high-power RF systems of the machine are automatically logged so that a complete record of faults[Ref. 8] is available to specialists.

4. ACKNOWLEDGEMENTS
The success of the controls upgrade of the RF systems of the PS machine is due in large part to the dedicated technicalsupport of P. Maesen, J.-P. Terrier and J.-L. Vallet. Important software contributions were made by A. Campbell, J.M. Nonglaton and A. Gagnaire.

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