GBT K-band Focal Plane Array Monitor and Control Interface, Cryogenic LNA Bias System February 27, 2008

Existing Bias Technology KFPA Requirements Cryo LNA Bias Cards –4 or 6 bias channels/card, screwdriver adjusted Vdrain and Vgate, servo Idrain. –Card caged, each card 8.5 x 4.5 x 0.5”, 28 in 3 sp. – 19” Rack. –2 wires per amp stage plus ground via hermetic feedthrough. KFPA will have 61 pixels, 2 polarizations, 4 amp stages: –488 Vdrain, 488 Vgate, 122 LED lines, plus grounds. –82 – 6 channel cards, 12 rack spaces (21”) –1220 pin hermetic feed through including LEDs and grounds Decision: –Bias regulators inside the dewar, adjust them remotely. –LNA Bias becomes more dependant on M+C

Existing NRAO M+C Technology VLBA Standard Interface Board –256 Control bits, 256 Monitor bits, 64 Analog channels 11 MHz 57k GBT Gregorian Receiver Interface –48 Control bits, 50 Monitor bits, 56 Analog channels, 8 Analog sources 11 MHz 57k –RF attenuating enclosure, filtered connectors, 12 x 7 x 4” –Analog channels are 12 bits

KFPA Monitor and Control Needs ~2000 Analog monitor points –488 Cryo LNA stages, Vd, Id, Vg –61 Noise cal Id, plus overall Vd, Vg –61 I-LED –61 Down Converter LO levels, Amplifiers bias –15K, 50K, 300K, Vacuum, Power Supplies >7000 Digital M+C points –61 Down Converter Attenuators x2, 4-5 bits –488 Cryo LNA stage bias settings, 8 bits –61 LED On/Off –Cryo state, S/R, Cal State… Thousands of wires, 1-2m long.

KFPA Monitor and Control Needs Increase Monitor and Control distribution –Build M+C capability into individual receiver modules. –Use serial lines for communication with modules. –Ethernet to GBT Network with COTS single board as go-between. Benefits –Fewer dewar feed throughs. –No long line problems. –COTS: Inter-Integrated Circuit (IIc, I2C, I 2 C). Disadvantages –New for GB Receiver –Interference, Clock runs at 400 kHz

Cryogenic LNA Bias, ALMA

Cryogenic LNA Bias

ALMA Bias regulator circuit. 8 - Dual, non-volatile, 8 bit, IIc potentiometers controlling voltage references to create Vdrain-set and Idrain-set for regulator. Two 10 bit, 12 channel, IIc A/D converters, scaled, monitor Vd, Id, Vg, I-LED (drop two Vg measurements on later amp stages). Each pixel enabled or isolated by IIc bus buffer, selected by IIc I/O Expander external to the pixel. Bias and Monitor for both polarizations fits on 2 x 3” PC card, 61 cards and mother board fit in 9 x 9 x 4”. Bias card array is contained in a pod off the dewar.

Monitor and Control

Each module contains IIc A/D’s and I/O Expanders and is enabled or isolated by IIc bus buffer, selected by IIc I/O Expander external to the module. Additional IIc M+C boards constructed for non-module monitor and control functions. Multiple IIc busses isolate subsystems and decrease M+C scan time.

Summary and Comments Thousands of Monitor and Control points distributed over entire receiver chassis communicating via a serial bus instead of points wired to a central unit. COTS single board computer interfaces between M+C nodes and GBT M+C System and is located outside the dewar. RFI Mitigation: –We strongly advise not running M+C scans during observing scans, rather in between scans. –A/D converters will be set for external (bus) clock, single board computer is enclosed in an RF attenuating enclosure, low pass feed throughs will be used.