PEP-II LLRF Direct Loop Panel

• Feedback
• PEPII LLRF Controls

Overview

Based on inputs on this panel, the direct loop coefficients (II, IQ, QI, QQ) on the RF Processing (RFP) module are calculated and updated using these equations:

DAC_counts = Conv Factor * ( 10**((Loop Gain + Gain Offset)/20) )
II and QQ values = ( DAC_counts * cos(Loop Phase * pi/180) ) + offset
IQ and QI values = ( DAC_counts * sin(Loop Phase * pi/180) ) + offset
Notes:
II,IQ,QI,QQ must be within -2047 to 2047 and will be clipped if beyond.
The values are updated whenever direct loop phase or any other input is changed.
The II,IQ,QI,QQ offsets are set during RFP module calibration.

This panel is accessed from the top left of the Feedback panel.

• Loop Control - Opens or closes the direct RF feedback loop.  When activated, a complicated EPICS sequence will slowly ramp up the gain as the baseband IQ references are increased (not a faston sequence).  Before the direct loop can be enabled, it must be configured as instructed in the Station Configuration Procedure. Normally ON.

• Loop Gain (dB) - Direct loop gain setpoint.  Normally 15 dB.

• Gain Offset (dB) - Direct loop gain setpoint offset.  Normally 0 dB.

• Conv Factor - Scale factor which converts desired gain (in volts/volts) to DAC counts.  This value is determined during direct loop configuration which is a part of station configuration. The value of the RF attenuator on the output of the RFP module will affect this scale factor.  Nominal values range from 170 to 240 Counts/(volt/volt).

• Loop Phase (Deg) - Direct loop phase setpoint.  If the station phase has not been moved since the cavities have been tuned, direct loop configuration will find the correct value to be near 0 degrees.  Recently in the LER we have been forcing a -10 degree offset into the direct RF feedback loop to provide additional phase margin to the direct loop integrator.  Nominal value HER = 0,  LER = 350 degrees.

• Gain Delta (dB) - Sets the change in direct loop gain for each iteration during a direct loop closing sequence (not faston sequence).  Normally 1 dB.

• Init Gain Offs (dB) - Sets the initial gain offset during a direct loop closing sequence (not faston sequence). Normally -10 dB.

• Ramp Interval (sec) - Sets the amount of time between direct loop gain changes during a direct loop closing sequence (not faston sequence).  Normally -3 seconds.

• Init Gap Volt (kV) - Sets the initial gap voltage during a direct loop closing sequence (not faston sequence).  Normally - HER = 2000 kV, LER = 1000 kV.

• Init Drive Pwr (W) - Sets the initial drive power during a direct loop closing sequence (not faston sequence).  Normally - 10 Watts.

• Init Stl Time (sec) - Sets the initial amount of time the direct loop gain remains at the initial setting during a direct loop closing sequence (not faston sequence) before the gain is ramped.  Normally 10 seconds.

• Freq Offs Tracking - ON/OFF button for the EPICS loop which adjusts the phase of the direct loop as the cavities detune for beam loading.  The station phase is also adjusted to compensate for the change in direct loop phase to keep the cavity phase constant. The comb loop phase is also adjusted for stability. This option must be turned OFF when integral compensation is ON. The amount of direct loop phase correction is:

  correction (Deg) = average of the cavity frequency offsets (kHz) * Freq Offset Conversion * Group Delay (nsec) * -0.000360

  The correction is added to the station phase. The amount of comb loop phase correction is:

  correction (Deg) = -Direct Loop Phase Correction (Deg) * Correction Conversion

  Both the conversion constants in the above equations, along with the comb loop base phase, can be adjusted from this panel.

• Integral Compenstn - ON/OFF button for the analog integral compensation option for the direct RF feedback loop.  Adds 40 dB additional gain to the direct loop centered at 476 MHz which rolls off to 0 dB at 30 kHz.  This added gain rejects the ripple introduced by the switching aspect of the HVPS.  This option affects the stability of the direct RF feedback loop when the cavities are detuned beyond ~70 kHz.  For full current LER operation this compensation may have to be deactivated.  Presently normally OFF.

• Lead Compenstn - ON/OFF button for the analog lead/lag compensation option for the direct RF feedback loop.  Adds a phase & gain bump which decreases the cavity impedance at frequencies corresponding to full current detuning.  This decreases the maximum driving term for the low order coupled bunch modes by 2 dB and has no adverse effects.  Normally ON.

• Phase Monitor Delta - Sets the deadband for the frequency offset tracking loop. This is the amount the phase must change before the RFP module is updated with new direct and comb loop coefficients. This value should be the same for both direct and comb loop. Normally 0.5 Degrees.

• Group Delay (nsec) - Measured group delay of the direct RF Feedback loop.  Measured and updated during direct loop configuration which is a part of station configuration. Normally ranges from 450 ns to 500 ns.