LFB system computing farm - measures the phase of each bunch and calculates a correction signal (kick). A ~4 MHz lowpass version of the kick is digitized in the LFB back-end module and broadcast over a dedicated fiber optic link to the RF stations.
Fiber optic link - sends 10 bits of data and is updated 72 times per turn (9.814 MHz). There are two fibers for the HER - one goes to region 8 and the other to region 12. The LER has only one fiber to region 4. In each RF region, the fiber daisy chains through the Gap modules so that a loss in signal or failure of a module will prevent the woofer kick data from being transmitted to stations further down the chain.
Gap voltage feed forward module - Generates the baseband IQ references for the RF station. Uses data from the woofer link to modulate the reference based on a programmable look-up table following a preset ~1 turn delay. Retransmits the woofer kick signal to the next RF station in the woofer chain.
-Put the gap module in Load state using the button at the bottom of Gap module panel.
-Type in the new phase offset value and be sure to hit the return key.
-Load in the new look-up table using the Load button to the right of the phase offset entry.
-Return the module to Run state using the button at the bottom of Gap module panel.
Repeat this procedure until opening/closing the woofer loop introduces no phase offset. Once the proper value of the phase offset entry has been determined, this value will be valid for every RF station in that ring (HER or LER). You can then bring up the Gap module panel for each if these stations and make a new look-up table using the above procedure with the same phase offset. The offset value WILL be different for HER and LER stations.
LR44 - 1700 ns
LR45 - 1900 ns
HR81 - 6100 ns
HR83 - 6000 ns
HR85 - 5900 ns
HR21 - 5200 ns
HR23 - 5200 ns
The value of the 1 turn delay should not need to be altered unless there is a hardware change which would affect the transmission delay by at least 50 ns. This would require altering the length of a fiber optic cable by 30 feet. The resolution of the delay adjustment is 100 ns since the delay FIFO clocks at 9.814 MHz.
E0CF = /tbl/gvfHERdetun.tbl - HER woofer equalizer file - same for all HER stations
E0CF = /tbl/gvfLERdetun.tbl - LER woofer equalizer file - same for all LER stations
Make sure the Equalizer Bank (lower right on Gap module panel) is set to "Bank 0" using the right mouse key to select. Bank 1 corresponds to no equalization and could be used to measure the non-equalized channel response in the future.
The second parameter is Max Phase (Deg Peak-to-Peak). The nominal setting is 10 degrees. This limits the maximum kick magnitude to prevent large phase modulations to generate huge phase modulations which might cause the station to fault.
When any of the woofer table parameters are modified, the table must be regenerated using the following procedure:
-Put the gap module in Load state using the button at the bottom of the Gap module panel.
-Type in the new look-up-table parameters and be sure to hit the return key.
-Load in the new look-up table using the Load button to the right of the phase offset entry.
-Return the module to Run state using the button at the bottom of the Gap module panel.
To measure the transfer function with the network analyzer in region 4, start by inserting the network analyzer into the loop by disconnecting a SMA cable which carries the woofer info to the back-end module, placing the network analyzer in series. Make sure the network analyzer is calibrated to remove any additional cable length required to make the connection. Next inject beam and adjust the stimulus level to result in a clean response measurement around the first revolution harmonic. Do not measure the 0-mode due to the presence of klystron ripple which could confuse the measurement. The transfer function around the first revolution harmonic should clearly show the two synchrotron sidebands and 180 phase near each peak. If this is not observed than the bunch-by-bunch filter coefficients in the LFB system are not correct and need to be optimized. There is no possible way for the RF system to introduce the magnitude of delay error needed to produce large phase errors at 136 kHz. I mention this because we experienced this before. Once the transfer function is correct for mode 1, observe the higher harmonics and adjust the 1-turn delay to make the phase correct. Gain can also be observed but be careful about driving the system into saturation while measuring.