Bias Step Rebias

For various reasons, it may be beneficial to bias or rebias detectors into their transition using bias-step functions instead of IVs, since - bias steps can be much faster than IVs - bias steps don’t require overbiasing

Yuhan Wang has developed the bias-step rebiasing function to find a suitable bias-point using just bias steps, which is described in greater detail in his spie proceeding.

The general control flow is as follows:

  1. Take initial bias steps bsa_0

    will rebias if there are any bias lines where R0 is all nan If >10% of dets are have Rfrac < 0.1, marks bg as overbias_needed If >50% of dets have Rfrac > 0.9, marks bg as normal (and drop from normal needed)

  2. Overbias overbias_needed bgs

    runs overbias_dets on bgs that need overbiasing, and waits For biaslines that have been overbiased, set the DC bias voltage to testbed_100mK_bias_voltage Run bias steps

    (no bias retry here)

    Check normal condition (above) and add updated bias lines to bg_detectors_normal array if they meet it

  3. Drop from normal

    For each normal bg, step voltage by 0.5(V_norm - V_SC) determined from IV Run bias steps

    update bsa_0 so it’s always the last bias step ran These bias steps don’t have a retry

    Check normal condition. If any bgs are still normal, repeat. After exiting loop, the resulting DC biases are initial_dc_biases

  4. Find new bjas voltage and take bias steps:

    vspread is diff between median normal and median SC voltages based on IV New bias voltage is init +/- 0.15*vspread where +/- is determined by whether or not the median Rfrac is > or < than 0.5 Take bias steps (bsa_1) and retry if needed

  5. Determine new bias point for each channel:

    v_estimate = (v0 (target - Rfrac_1) + v1 (Rfrac_0 - target)) / (Rfrac_0 - Rfrac_1) vnew = median(v_estimate[bg]), or if all are nan, vnew = (v1 - v0) / 2

  6. Set bias to vnew and retake bias steps bsa_2 (with retry if needed)

    BG is successful if abs(Rfrac - target) < 0.05

  7. Fine tuning unsuccessful bias groups

    Step 0.15 vspread in the appropriate direction Take bias steps (with retry) v_estimate2 = (v0 (target - Rfrac_1) + v1 (Rfrac_0 - target)) / (Rfrac_0 - Rfrac_1)

    with v0= prev estimate and v1 = prev estimate + delta

    Apply v_estimate2, retake bias steps.

  8. Return final bias step and bias voltages