Row-specific SQ2 feedback (fast switching)

The SQ2-Feedback (SQ2FB) output of the MCE, controlled by a DAC can be set to either a constant value or a row-specific set of values switching at row-visit rate. The SQ2FB output is driven by the card plugged into the BC2 slot of the MCE. Either a Bias Card or an Address Card can be plugged into the BC2 slot.

Bias-card-driven SQ2FB

Hardware requirements

• Bias cards prior to Rev. F have a slow opamp at the output followed by 2 sets of kHz-range RC filters that limit the output switching frequency to few kHz range and prevents the output from switching at the row-visit rate, typically around 500kHz. Therefore, when these bias cards are used in BC2 slot, only a constant SQ2FB value (one value per array-column) can be specified.
• Bias Cards Rev. F and later have faster outputs and they can be switched at row-visit rates up to about 900kHz .

Firmware requirements

The firmware support for switching SQ2FB values at row-visit rate is added starting revision BC 5.0.5.

Relevant mce parameters

• enbl_mux : one value per channel, to enable / disable multiplexing DAC values. Default: mode = 0 (disabled); write 1 to enable.
  • When enbl_mux = 0, DACs are loaded with values specified using flux_fb parameter.
  • When enbl_mux = 1, multiplexing is enabled and DACs are loaded with a new value from fb_col# at each row switch.
• flux_fb: values for each of the 32 flux-feedback 16-bit DACs on Bias Card. These are used for any column having enbl_mux = 0.
• fb_col0, fb_col1, ... to fb_col31: registers, each of which may contain up to 41 sq2fb values. These are only used when enbl_mux = 1 for the column

Muxing can be enabled / disabled on each column independently. For classic, constant FB output, ensure enbl_mux = 0 0 0 0 ... .

Software requirements

• mas/trunk : r510 or later - servo programs will automatically detect the fast bias card.
• mce_script/trunk: r764 or later

Configuration instructions

• When creating mce.cin:
  • $fw_rev["bc"] = 0x5000005 or greater.
  • $card_list should indicate a "bc2" (i.e. not a "bac")
  • mce_status -g should show entries for "bc2 fb_col#" and "sq2 fb_col#" as well as "bc2 flux_fb" and "sq2 fb".
• In experiment.cfg:
  • Set one of
    • config_fast_sq2 = 1; # enable fast-SQ2 switching (and tuning)
    • config_fast_sq2 = 0; # disable fast-SQ2 switching
  • Set
    • hardware_fast_sq2 = 2; # new bias card, supports fast- and no-switching
    • (In older mce_script, the equivalent is to set hardware_bac = 0)

Address-card-driven SQ2FB

See older instructions here: Fast SQ2 Feedback and TES Biasing with an Address Card

Hardware requirements

• No modifications are necessary to the address card.
• Note that the usual BC2 bias/heater line is not available when the biasing address card (BAC) is in use. However, if needed, the address-card hardware can be physically modified to provide the same functionality, for details see: http://www.phas.ubc.ca/~scuba2/sc2mce/system/addr_card/design/ECOs/AC-ECO049-20080508.doc
• The SQ2 FB DAC range is reduced from 16 to 14 bits when using the BAC.

Firmware requirements

• AC 5.0.0 or later

Relevant mce parameters

enbl_mux

const_val

fb_col0, fb_col1, ... to fb_col31

const_val39

row_len

Software requirements

• mas/trunk : r439 or later
• mce_script/trunk : this has worked for a while...

Configuration instructions

• When creating mce.cfg:
  • $card_list must indicate a "bac" instead of a "bc2"
  • mce_status -g should show entries for "bac fb_col#" and "sq2 fb_col#" and no entries for "bc2"
• In experiment.cfg:
  • Set one of
    • config_fast_sq2 = 1; # enable fast-SQ2 switching (and tuning)
    • config_fast_sq2 = 0; # simulate non-fast-SQ2 bc2 by writing the same FB value for each row.
  • Set
    • hardware_fast_sq2 = 1; # biasing address card
    • (In older mce_script, set hardware_bac = 1 instead.)