PEP-II LLRF Feedback Panel

• Main Station
• PEPII LLRF Controls

Overview

Direct Loop Controls

• Loop ON/OFF - Normally ON - Works only when station state is ON_CW and cannot be turned off while the station is operating (must go to station OFF first). Gain is ramped up when loop is activated (no ramping during fast turn-on sequence). Gain ramping parameters are Gain Delta (1db) and Inital Gain Offset (-10dB). Loop gain and phase are setup during Station Configuration. The Direct Loop panel has more detail. Nominal gain is +15dB.

Comb Loop Controls

• Loop ON/OFF - Normally ON - Works only when direct loop is ON. Gain is ramped up when loop is activated (no ramping during fast turn-on sequence). Gain ramping parameters are Gain Delta (5db) and Inital Gain Offset (-15dB). Loop gain and phase are setup during Station Configuration. Nominal gain is +17dB.

Tuner Loop

• Loop ON/OFF - Normally ON - Works in all station states except ON_FM (for vacuum processing cavities) where the tuners are simply set to their 'home' position. Cavity phases are nulled during Station Configuration which is done when RF electronics or cavity cables have been changed. The Tuners panel has more detail.

Loading Angle Offset

• Loop ON/OFF - Normally ON - Adjusts cavity tuner phase offsets to balance individual cavity voltages during heavy beam loading. Now also used to lower voltage on individual cavities which are succeptible to arcing or vacuum bursts. Desired cavity voltage distribution is set on the Tuners panel and must add up to 100%. This loop is also used as a tool to adjust waveguide network.

HVPS Loop

• Loop OFF/PROCESS/ON - Normally ON - PROCESS mode is used for vacuum processing. Sets the required output voltage from the high voltage power supply. When direct loop is ON, this loop controls the klystron drive power level through interaction with the direct feedback. When direct loop is OFF, this loop controls the gap voltage. The Main Station panel has more detail.

DAC Control Loop

• Tune Mode ON/OFF - Normally ON. Enables EPICS loop which sets the klystron drive power level and station phase when station in 'TUNE'.
• ON_CW Mode ON/OFF - Normally ON. Enables EPICS loop which sets RF reference to the direct loop which determines the station gap voltage. If direct loop is OFF and station is ON_CW, this loop sets the klystron drive power level and station phase.

Direct Loop Options

• Frequency Offset Tracking - Must be ON for beam current >200mA - Corrects direct loop, comb loop, and station phase for cavity detuning. This option must be turned OFF when integral compensation is ON. If the integral loop is not used, this loop should be activated to allow reaching design beam currents.
• Integral Compensation - Normally OFF - Increases gain of direct loop near 476 MHz to cancel HVPS ripple. It has been shown that this option will limit the maximum beam current in the LER to ~1.7 amps. For higher LER beam currents, this loop will have to be deactivated and the frequency offset tracking option activated.
• Lead Compensation - Normally ON - Increases performance of the direct loop and adds 2dB of gain for free.
• The Direct Loop panel has more detail.

Ripple Loop

• Loop ON/OFF - Normally ON - Currently used to keep the phase shift across the klystron and drive amplifier constant as the klystron high voltage is changed due to beam loading. This loop has potential to also cancel power line harmonics to reduce klystron ripple but is not used for that purpose at this time. Ripple reduction is achieved by the integral compensation in the direct loop.
• Gain Tracking - Normally On - Measures the large signal gain fo the klystron and adjusts the DC gain in the ripple loop to maintain constant gain in the direct RF feedback loop. Large signal klystron gain increases as the klystron cathode voltage is raised.
• The Ripple Loop panel has more detail.

Gap Voltage Feed-Forward Loop

• Loop ON/OFF - Must be ON for large beam currrents (>100mA) - Adaptively learns the cavity transients caused by uneven beam fills and the ion clearing gap and then modulates the station reference to follow the disturbance. This prevents the direct RF feedback loop from trying to fight the gap transients.

Longitudinal Feedback Woofer

• Loop ON/OFF - Required for beam currents > 750 mA - Uses kick signal from longitudinal feedback system to phase modulate the station RF. More detail and instructions for setting up the woofer are provided in the Woofer Configuration Procedure.