CryoElectronics

McGill CryoElectronics Fridge Controller

The McGill CryoElectronics system is designed to monitor and control all things cryogenic inside the telescope receiver. We use it primarily to control sub-Kelvin absorption fridges such as the Simon Chase Research 4He-3He-3He fridges we use in our mm-wavelength telescopes.

This system was developed in the McGill Cosmology Instrumentation lab as part of our commitment to providing excellent scientific instrumentation training for students. Engineering and physics students at the undergrad and grad level were involved with the design, development, commissioning, and programming of the board.

The board has the following features:

• • 16 channels of precision differential drive and sense that can be programmed individually to either read out diodes (such as Lakeshore calibrated diodes that require a 10uA bias) or precision Resistive Thermometry Devices (RTD, such as Lakeshore Cernox sensors with drive currents of 1 nA or lower).

    • All thermometry readouts are 4-point connections. You can program each channel individually to provide either (1) a DC current bias appropriate for Diodes, (2) a true-sinusoidal bias appropriate for RTDs, or (3) a square-wave bias which can be used for either.

    • You can mix-and-match sensors: for example, you can choose to configure 5 of the channels for RTDs, 10 for Diodes, and use the last 16th channel to monitor something else, like a LHe level sensor.

    • Typically 1 ohm RMS noise when reading out RTDs, which corresponds to about 0.25 mK noise for a Lakeshore Cernox device, and much smaller noise for a GRT.

    • Calibration files for each sensor can be uploaded to the board and stored on non-volatile flash.

  • 8 heater channels that can each supply up to 120 mA of current through 200 ohms. Each heater has an analog switch that disconnects it from the cryostat when not in use, so that there can be no current leakage. LEDs on the front-panel of the board are illuminated when the heaters are in use to provide clear indication to the user (information about the heater currents/status is available from the webserver as well).

  • A Field Programmable Gate Array (FPGA) is the heart of the board, processing/demodulating the data and synthesizing the drive currents. It includes a embedded processor that runs ucLinux (a compact version of the popular Linux operating system).

  • The board contains a webserver that displays its status, logs temperatures, and allows user control through standard platform-independent webpages. Programming interface to the board is provided through standard TCP/IP. 

    • You can use the board with any web-browser capable device (iphone, mac, windows, linux, whatever). There is no complicated setup, specialized software to install, or interface hardware. (streaming temperature data to a remote computer, as would normally be done while operating a telescope, requires a compact piece of software that received data packets over the network).

    • The data format ("dir files") from the board can be read directly with Canadian KST data visualization software and easily read into matlab, Python, or IDL.

    • Users can log into the Linux embedded processor and execute arbitrarily complex scripts written in Python or choose to run those scripts on a remote computer communicating with the board over ethernet with TCP/IP.

    • Example scripts are supplied and pre-tested for Simon Chase 4He-3He-3He 'sorption fridges.

  • Designed for low noise precision measurements: The board is layed out as three separate "islands" of electronics, each connected together with optical isolators, so that the "islands" share no grounds or copper connections. The islands are: (1) digital control, (2) analog RTD and diode monitoring, and (3) high current heater outputs.

  • Timestamps: the board has an RS-485 port for inputing an IRIG-B encoded GPS signal (this is the timestamping system we use for the South Pole Telescope, POLARBEAR, and APEX-SZ experiments). If the port is used, the FPGA attaches these timestamps to the data that is streamed off the board for archiving.

  • Flexibility: since the heart of the board is an FPGA performing digital signal processing on differential analog drive circuits and differential ADC sense circuits, it is easy to reprogram the board to perform other tasks, such as reading out optical encoders or hall sensors on a half-wave-plate or monitoring a LHe level sensor.

The board is designed to either operate stand-alone in an enclosure, or to be placed in a 6U VME card cage. It is compatible with the back planes and power supplies for the McGill Digital Frequency Domain Multiplexed Bolometer Readout (DfMux).

The board draws about 5-10W of power (depending on whether current is being supplied to the heaters) and needs +5V, -5V, and +20-36V power supplies.

We occasionally provide electronics systems to our collaborators on a cost recovery basis.

Key contacts: Matt Dobbs, James Kennedy (PhD Student) and Graeme Smecher (firmware engineer).