PEP-II LLRF Ripple Loop Panel
This panel contains some additional functions and constants to support
the gain tracking option of the direct RF feedback loop.
The gain tracking loop lowers
the gain of the forward path of the direct RF feedback loop to correct
for the gain increase of the klystron as the HVPS voltage is raised.
This panel is accessed from
the top left of the Feedback panel.
- Ripple Loop Control - Opens or closes the
DSP based ripple loop on the
RF Processing (RFP) module.
Presently this loop
is used to keep the gain and the phase shift across the forward path (including
the klystron) constant. The DSP code is capable of canceling modulations
at power line harmonics up to 10 kHz through the use of different coefficient
files but this was never fully commissioned.
Before the ripple loop can be enabled, ripple loop parameters must be
setup as instructed in the
Station Configuration Procedure.
- Gain Tracking Control - Opens or closes the
gain tracking option of the DSP based ripple loop.
An EPICS loop measures the klystron gain and lowers the gain of the ripple
loop baseband modulator as the klystron gain increases beyond the klystron
"Gain Setpoint". Normally ON.
- Gain Setpoint (dB) - Measured klystron gain
at the no beam, normal operating gap voltage condition.
This parameter is measured and set during direct loop configuration which
is a part of station configuration.
Normally ranges from 40 to 45 dB.
- Max Delta Gain (dB) - Slew rate limiter for
the gain tracking loop. Prevents large changes
from occurring on a single iteration. Normally 0.2 dB.
- DC Coeff Control - Using the mouse,
the user can manually set the DC gain of the baseband drive modulator if
the "Gain Tracking" loop is deactivated. A left mouse button click
selects the slider and the setpoint can be dragged. Up/down arrow
keys also work. A right mouse button click allows setting manual
- Max Delta - Slew
rate limiter for the gain control of the baseband drive modulator.
Prevents large changes from occurring on a single iteration. Normally
- Min Delta - Deadband
for the gain control of the baseband drive modulator. Prevents tiny
changes from occurring on a single iteration. Normally 0.005 volts.
- Max Calc - Sets the
maximum value of the calculated baseband drive modulator control voltage.
The calculation of the control voltage is based on this maximum value.
This parameter allows lowering the "Maximum" parameter without skewing
the calculation. Normally 0.900 volts.
- Maximum - Setpoint
for the maximum allowed control voltage for the baseband drive modulator.
If any of the multipliers in the baseband drive modulator is overdriven,
the drive phase may abruptly change. This parameter prevents this
from occurring on the control inputs (may still occur on the signal inputs).
Normally 0.900 volts.
- Minimum - Setpoint
for the minimum allowed control voltage for the baseband drive modulator.
As the gain of the baseband drive modulator decreases, the signal inputs
to the modulator must increase to keep the klystron drive power constant.
If the IQ baseband drive voltage inputs to the modulator exceed 1 volt,
the drive phase may abruptly change. This parameter prevents the
input voltage from growing beyond the specified 1 volt limit. The
"plot_drive.m" Matlab script will measure the present modulator input voltage
and predict its maximum value as the gain tracking loop reaches the "Minimum"
limit. This Matlab tool is activated using the "Plot Drive" button
on the MoreMATLAB panel.
Also note that changing the RF attenuator
on the output of the RFP module
will affect the baseband drive
voltage level necessary to produce the required klystron drive power.
Lowering the value of the attenuator also lowers the baseband voltage but
raises the saturated drive power, so there is a trade off. Ideally
the range of gain adjustment (20*log10(Maximum/Minimum)) should provide
sufficient range to cover the no beam klystron power to 90% of the full
rated klystron power (to allow for modulation headroom). Clearly
the LER is the more difficult case since the no beam power is half that
of a HER station. Normally HER = 0.600 volts, LER = 0.500
- Initial - Setpoint
for the initial value of the baseband modulator gain.
We have experimented with setting this to the "Minimum" value and
the "Maximum" value.
Presently we set this to "Maximum" which is the normal no beam state and
consistent with the fast turnon philosophy. To improve the reliability
of the fast turnon, this parameter can be fooled with but I think the "Maximum"
value is the correct one.