Basis for the FOFB is the real time operation mode of the digital BPM
(DBPM) system where each BPM electronics continuously delivers data at
a rate of 4 kHz. This rate is the result of down conversion and
decimation in the digital receivers with their 31.2 MHz ADC clocks
locked to the 500 MHz ring RF frequency. Presently, the 4 kHz data
streams of the 72 digital BPMs are not synchronous to each
other. Thus, the FOFB has to wait for the latest data acquisition
before a new orbit correction can be calculated although the transfer
time for BPM data between the different sectors takes only 8
s. The asynchronous data rate introduces an additional delay of
maximal one feedback cycle which amounts to 250 s. This delay
will be eliminated by a DBPM firmware upgrade. The passband width of
each BPM is set to 2 kHz by means of programmable filters on the
digital receiver. It results in a resolution of 1.2 m and typical
group delays of about 300-600 s. The numerical controlled
oscillator (NCO) frequencies on the DDCs are adjusted to the main RF
frequency. An automatic loop on the BPM low level control system
tracks the ring RF frequency and reprograms the NCO frequencies to
keep the BPM signal in the passband width of the DDCs. This becomes
important when even smaller passband widths of the BPMs are envisaged
in order to reduce noise and group delays in the DDCs and hence to
increase the bandwidth of the feedback loop. Frequency changes are
necessary since horizontal path length effects are taken into account
by the FOFB as well. Off-energy orbits are not corrected by the
steerer magnets but by adjustments of the RF frequency. It is
therefore necessary to subtract the dispersion orbit from the measured
orbit before a correction is applied. The dispersion orbit is
extracted on the DSP level by a one dimensional SVD fit on the
position data from three sectors, which corresponds to 18 BPM
readings. Whenever the fitted RF frequency error exceeds 5 Hz
(equivalent to dP/P
) a high level application on
the beam dynamics model server [4] corrects the RF
frequency.
The start of the fast orbit feedback is performed in a sequence where
the central high level orbit correction application (former 'slow
orbit feedback, SOFB') corrects the electron beam to the
required reference orbit within 5 m and adjusts the RF frequency.
Subsequently, all necessary feedback parameters including the inverted
response sub-matrices and PID control parameters are downloaded to the
DSPs at the twelve BPM stations. A global trigger from the timing
system starts the FOFB on all BPM stations synchronously. Since the
same number of correctors and BPMs are used to constrain the orbit to
the ``Golden Orbit'' at each of the 72 BPM locations, it is
indispensable to rely on each single position reading. BPM pickup
cross checks of the four RF buttons have therefore been implemented on
the DSP in order to detect BPMs with spurious bad readings. If the
sums of the two diagonal BPM button raw values do not agree within a
predefined level (default 20%) the reading is considered to be
faulty. In such a case, the DSP disables the BPM and stops the
feedback in this particular sector because the structure of the
``inverted'' response sub-matrices are not appropriate anymore. The
halt of the feedback loop on one BPM station also directly affects the
two adjacent sectors which do not get position readings over the fiber
links anymore and consequently skip the correction cycles with 'data
timeouts'. The localized structure of the fast orbit feedback allows
this type of asymmetric operation mode. Nevertheless the feedback is
then automatically stopped by the EPICS control system which
permanently monitors the status of all sectors. The high level orbit
control application reloads a new set of sub-matrices where the
particular faulty BPM is disabled and finally restarts the
feedback. This scenario has already been successfully tested during
machine development shifts.