KLYSTRONS OUTPUT CORRECTION: LabVIEW CODE DEVELOPMENT AT SLAC A.Cappelletti CERN, 22 Apr 2009

I Q GHz HYBHYB HYBHYB Other experiment… Delay (2 round trips) + Mode conversion HYBHYB HYBHYB STRUCTURE ATTENUATOR PH. SHIFTER ATTENUATOR - MODULATIONPOWER SUPPLY PULSE COMPRESSION FEEDBACK SYSTEM OVERVIEW  s   AMP – PH DETECTIONAFGLAPTOP

MODULATION ISSUES User input and klystrons output are handled in terms of amplitude and phase, but practical concerns make Re-Im data a better choice. Ideally, the objective is to get both a flat top amplitude pulse to feed the compressor, and a constant phase waveform.

REAL WORLD SIGNALS… We always refer to modulating signals, but it has to be remarked that they are transmitted on a 11.4 GHz carrier  We perform a demodulation… … through the analytical representation s a (t) = s(t) + j H{s(t)} = A(t)e j(  (t)) Where H{} denotes the Hilbert transform.  The main concern is flattening A(t), the phase being coerced to the arbitrary value of  /4 A(t)

The basic idea is a simple point-by-point compensation. If the first input/output relation looks like Then the input could be modified this way: In more detail, the first step is to define an error vector intended to (iteratively) modify the input. Error vector after 1 step

THE CORRECTION ALGORITHM FIND ERROR VECTOR …0 Current (ideal) input Corresponding output FIND ERROR VECTOR + + (Initial) error vector New input… … and corresponding new output … …

The algorithm assumes a 1 to 1 correspondence between the acquired output and the ideal input: a time shift is needed and performed through a cross-correlation. For the same reason, the output has to be properly downsampled (# of pts is different). The error vector has to be opportunely (and manually!) rescaled, otherwise it won’t significantly affect the input, whose level is approximately 3 times higher. CONVERGENCE: each iteration is triggered by a user’s command. In 3-4 steps the correction reaches its best… OTHER ISSUES  = argmax{R in,out }

… and the resulting pulse is already better looking. PERFORMANCES Example 1 (After 2 steps)Example 2 (After 3 steps)

Basic and intermediate level training was complete. Allowed to operate ASTA’s RF power controls if supervised. Choke seen from the boroscope ABOUT ASTA AND THE PETS