PEP-II LLRF Direct Loop Panel
This panel contains some additional functions and constants to support
the operation of the direct RF feedback loop. Some of
the parameters exist on the Feedback panel but are repeated here for
of the inputs are used in the fast and slow turnon sequences while others
support the direct loop frequency offset tracking option.
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
II,IQ,QI,QQ must be within -2047 to 2047 and will be clipped
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.
- 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
The value of the RF attenuator on the output of the
will affect this scale factor. Nominal values range from 170 to 240
- 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:
The correction is added to the station phase. The amount of comb loop phase
correction (Deg) = average of the cavity
frequency offsets (kHz) * Freq Offset Conversion * Group Delay (nsec) * -0.000360
Both the conversion constants in the above equations, along with the comb
loop base phase, can be adjusted from this panel.
correction (Deg) = -Direct Loop Phase Correction (Deg) * Correction Conversion
- 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
- Group Delay (nsec) - Measured group delay
of the direct RF Feedback loop. Measured and
updated during direct loop configuration which is a part of
Normally ranges from 450 ns to 500 ns.