The Low-Noise, Integrated Transformer Helium-4 Dipstick Insert Sang Lin Chu Georgia Institute Of Technology 837 State Street N.W. Atlanta, GA 30332 gte813m@prism.gatech.edu, sanglinchu@hotmail.com December 29, 2003
1 Introduction The low-noise dipstick with an integrated low temperature transformer was designed in the summer of 2003 from June to early August by Sang Chu and Alexey Bezryadin as part of a summer research project for the Research Experience for Undergraduates at the University of Illinois at Urbana-Champaign. The intent of developing such a Helium-4 insert, or dipstick, was to measure very low resistances with a low noise floor much better than the previous implementation which had a noise floor of a few Ohms (microvolt and nanoamp resolution driving signals). There are three stages of filters on all lines in the dipstick. The first consists of a compacted stainless steel powder filter, the second of Thermocoaxial MgO powder filter cables, and the third of silver-paste applied to the insulation of thin copper wires directly connected to a 16 pin chip carrier. The bottom of the dipstick where the sample is connected is enclosed by an electromagnetic shield of copper. The transformer is also inside the same copper cell. The transformer setup was designed so that the dipstick could be used with or without the transformer in measurements. The dipstick can achieve a base temperature of 1.45 K when pumped inside a Helium-4 dewar. Ideal usage is for temperatures below 77 K due to ground leaks from the thermocoaxial cables. 2 Vacuum Design The highest part of the dipstick consists of Quick-Flange type fittings. Most of it is comprised of QF-40 sized flanges. A Fischer brand receptacle (part number DBEE-105A-80 24 pin) is mounted on a blank QF-40 flange machined to form a vacuum seal using the supplied o-rings on the receptacle. The low temperature part of the dipstick is sealed forming a vacuum connection with a 1.5 nominal outer diameter stainless steel 316 tube with a welded QF-50 fitting. A small vacuum valve used for pumping and insertion of Helium-4 exchange gas uses QF-16 quick flange fittings. The inner tube is a thin walled 1/2 outer diameter stainless steel tube attached to the center of the vacuum assembly by 5/8 compression fittings connected to a 3/8 male pipe thread extension. A picture of the top portion of the vacuum assembly is shown with the BNC box connected to the Fischer connector receptacle. 3 Electrical Wiring and Filtering All measurement leads are connected at room temperature with female BNC panel mount connectors contained inside an aluminum project box. The BNC box is fitted with the matching female mating connector to the Fischer receptacle (Fischer part number S105A-80 24 pin female plug). The female plug had to be 2
modified to be mounted inside the project box as it is normally used in cable assemblies and not for panel mount installations. From the Fischer connector receptacle, we use Lakeshore low temperature quad twisted pair (part number Quad-Twist Cryogenic Wire QT36) wires which have two pairs of twisted pair leads. These are ideal for low interference four probe measurements. The wires are made of non-ferromagnetic 36 AWG phosphor bronze wire with Formvar insulation and are quite easy to solder. These wires then go straight into the compact powder filters using the ends of an old-style resistor to fit tightly into the female receptacle of the SMA connector (note that the resistors were later removed and the wires were directly connected to the filters). More information on the compact powder filters can be found in the document titled Compact Powder Filters. Each compact powder filter is labeled with numbers corresponding to the pin numbers on the Fischer connector. From the compact powder filters, thermocoaxial cables using SMA connectors are connected. The thermocoaxial cables have 34 AWG enamel coated insulated copper wires which go inside the copper cell through silver paste and directly to the chip holder. More information on assembly and filtering characteristics on the thermocoaxial cables can be found in the document Thermocoaxial Cables as a Low Temperature High Frequency Filter. The following picture shows the connections from the twisted pair wires to the low temperature parts of the dipstick. The sample chip mount is a 16 pin integrated chip carrier with a slight notch on the upper left-hand corner to indicate pin 1. Two pins, labeled by an X, are not used and covered by the use of a Cernox X2024 thermometer (calibration was made in the LabVIEW file Cernox R to T X20424.vi). The pin layout of the chip carrier is labeled in the diagram below. The pin numbers correspond to the Fischer receptacle connector (part number DBEE105A-80 24 pin) used at the room temperature part of the dipstick. Twisted pairs are denoted by arrows connecting them. 3
The input of the transformer is connected on pins 12 and 13. A counterwound heater made of Stablohm resistive wires is connected on pins 20 and 21 with an end to end resistance of 573 Ω. The following table summarizes the resistance from room temperature BNC connectors to the pins on the chip carrier for each pin. The compact powder filters contribute anywhere from 50 80 Ω due to contact resistance and the Advance wire inside whereas the thermocoaxial cables contribute 50 80 Ω due to contact resistance and Nickel-Chrome (80/20) inner conductor. Room temperature leakages (as measured on August 9th, 2003) are also reported. Note that the leakages disappear when cooled near Nitrogen (77 K) temperature. Naturally, there is an associated error with measurement at room temperature on the order of mega-ohms due to this leakage. It is suggested to keep the dipstick under vacuum while not in use to prevent further leakage due to water absorption of the thermocoaxial cables. Resistances were measured with a Keithley 187 True RMS multimeter. 4 Transformer The basic concept behind the transformer is to use an inductively coupled setup so that the signal is amplified by a high turn ratio between the pickup loop and the sample. The following circuit diagram shows the wiring of a typical sample when connected to the transformer. R s denotes the sample resistance, R z the impedance of the transformer, G the gain of the transformer and V the voltage source driving the current I through the sample and transformer connected in parallel. 4
Pin Lead Resistance [Ω] Ground Leak Resistance (300 K) [M Ω] 1 149.56 Overload 2 146.11 >39.7 3 139.55 Overload 4 152.18 Overload 12 166.20 >18.1 13 181.88 Overload 14 157.04 >17.3 15 159.45 Overload 16 148.19 >3.97 17 205.81 >4.95 18 180.09 >4.3 19 166.06 >22.9 20 Heater + >11.79 21 Heater - >11.79 22 180.08 >6.3 23 136.45 >4.95 Table 1: End-to-end resistances (from RT to the base temperature chip holder). The transformer has four primaries primaries P1, P2 is on the yellow/red cable while primaries P3, P4 are on the yellow cable. The secondary S is on the red cable and is used to connect directly to room temperature electronics. The table below summarizes the gains, resistances and nominal turn ratios for each primary as measured at room temperature. At a temperature of 4 K, the resistances of the copper windings of the transformer are expected to be approximately 70 times smaller. Pins 12 and 13 from the chip carrier have a 4 pin removable connector which can be connected to the transformer or directly to the room temperature connections. When connected to the transformer, alignment of the black line on the connected to the yellow/red transformer cable results in a connection to primary P1. Rotation by 180 degrees results in connection to primary P2. Alignment of the black line on the yellow transformer cable results in a connection to primary P3 and similarly, a rotation of 180 degrees results in a connection to primary P4. The table below summarizes the connections to the transformer. The following diagram shows how the connections on the transformer were made. 5
Primary Nominal Turns Resistance [Ω] (300 K) Turn Ratio 3V 1 khz on S P1 300 24.0 1:30 100 mv P2 90 7.67 1:100 30.2 mv P3 30 2.68 1:300 9.9 mv P4 9 0.953 1:1000 2.8 mv S 9000 1780 - - Table 2: Summary of the transformation gains, turn ratios and resistances. Primary P1 P2 P3 P4 Alignment with Black Marker on Connector Aligned with black marker on yellow/red cable from transformer Anti-aligned with black marker on yellow/red cable from transformer Aligned with black marker on yellow cable from transformer Anti-aligned with black marker on yellow cable from transformer Table 3: Wiring chart for the transformer. 6