PEP-II LLRF Drive Power Control Panel

Overview

This panel contains the various constants necessary to control the klystron drive power.  This is an interesting task since the actuator changes depending on if the direct RF feedback loop is closed or open.  With the direct loop open, the actuator is the baseband reference IQ voltages (either from the gap module or from the RF Processing (RFP) module if it is configured to operate without a gap module).  With the direct loop closed, the HVPS output voltage controls the drive power.

The drive power panel can be accessed from the Feedback panel or some of the other buried panels.  The drive power control panel is broken into two panels, the second half covering the ON_CW state with the direct RF feedback loop OFF but both will be explained here. 

At the top of the panel is the drive power error calculation:

Err (W) = Setpnt - Actual

The error calculation is straight forward but there is a caveat.  The parameter, "Lower Limit on Err (W)", limits the error from becoming too negative (Nominal value = -1.0 W.) This restriction was added during the development of the slow direct loop turnon sequence and prevents the HVPS voltage from getting too large.  Recall that with the direct loop closed, the HVPS voltage controls the drive power.

TUNE State

In TUNE state, the baseband RF reference is generated by two DAC channels in the RFP module. The following equation converts the drive power error into DAC counts.  These counts are then converted into I and Q counts based on the "Station Phase (Deg)" setting and loaded into the DACs.  When the drive power loop is enabled (in Tune state), the "Initial Counts" value is preloaded into the loop output.  There is also a restriction on how many counts the loop can change on each iteration:

DAC counts = Err * Gain Factor * Conversion

ON_CW State - Direct Loop OFF - GVF Module Missing

The RFP module can be configured to generate it's own baseband reference (no Gap module necessary) by moving two jumpers on the motherboard.  The jumpers allow using a pair of DAC channels normally used to null an offset to provide the IQ baseband reference voltages instead.  The controls are very similar to the TUNE state controls.  The parameters are listed for completeness:

ON_CW State - Direct Loop OFF - GVF Module Available

This is the normal ON_CW direct loop hardware setup. All completed PEP-II RF stations have a Gap module to produce the baseband IQ references.  The drive power loop will determine the average value of the references and the DSP in the Gap module will add fast modulations to follow the beam induced gap transients.

ON_CW State - Direct Loop ON

This is the normal mode of operation for the stations.  Note that with the direct loop closed, changing the drive power requires altering the HVPS voltage (not the baseband IQ reference).  The basic philosophy for configuring the operation of this mode of the drive power loop is as follows.  We make the assumption that the baseband IQ reference (which is the setpoint for the gap voltage when the direct loop is closed) does not normally change since the gap voltage is held constant.  The HVPS voltage is what is needed to change as the klystron power requirement increases with added beam.  If the drive power is above the set point, the HVPS voltage must be increased to reduce it.

To configure the direct loop closed option of the drive power loop, first open the "Drive PWR Plot" on the upper right of the panel.   The station must be at normal operating voltage with the direct loop closed.  Create a step change in the loop by changing the drive power setting by 1 Watt.  The response viewed on the plot should be a smooth non-oscillatory ramp.  In the past we adjusted the gain by manually changing the "Conversion" factor but one could calibrate the conversion factor and adjust the response with the "Gain Factor".  It is also prudent to put a step change into the gap voltage set point and verify the drive power loop does not misbehave.