Characterization of the MMIC 85-115 GHz HEMT Amplifiers
Note: Scroll down for pictures of the test set up
The signals received by radio telescopes are often extremely weak, of the order of 1e -15 to 1e -20 Watt. For a radio telescope to be effective, it needs very sensitive and stable receivers and amplifiers. The SZA amplifiers use 4 cryogenically cooled InP High Electron Mobility Transitors (HEMTs) coupled with Monolithic Microwave Integrated Circuits. They have very low quantum noise in the 4 to 100 GHz frequency range.
This page contains a brief description of the testing of the high frequency amplifiers for gain and system temperature. This work has been carried out by graduate students Dan Siegal and Ryan Hennessy as well as undergraduate student Cristina Tcheyan. Professor John Carlstrom supervised the testing. A detailed report written by Dan Siegal and is available as a PDF file by clicking here.
The amplifier testing was done in an existing laboratory dewar. This dewar had been used for the DASI experiment (which was centered around 30 GHz), hence it needed some modifying to be able to test the performance of the amplifiers in the 90 GHz band. In the original implementation of the lab dewar, the 10K stage plate was directly bolted to the refrigerator cold finger. This thin-walled cold finger was then the only structural support for the plate, and pressure applied to the edges of the plate could permanently damage the refrigerator. To reduce stress on the cold finger, both the 10 K stage and the 50 K stage of the dewar were slightly modified. The 10 K stage is now supported by the 50 K plate and separated from it with four insulating stand-offs located in the corners. A copper shim and spacer between the cold finger and the 10 K stage create the thermal link between them.
To get the microwave signals to the device being tested within the dewar, a waveguide feed-thru was built. The waveguide section was made of stainless steel for good thermal insulation (the temperature at the outer and inner ends of the feed-thru is around 300K and 10K respectively), and the inside was electroplated with gold for low-loss microwave transmission. The middle of the waveguide section was cold-strapped to the dewar's 50K stage.
Additional dewar modifications include a new hermetic feed through and cables, built to connect the amplifiers to an external bias control card, and improved insulation between the 300K and 50K stage. Large holes in the 50K radiation shield, with allowed 300K radiation to warm the amplifiers were covered with superinsulation. The dewar was also helium-leak checked and leaky seals were replaced.
To test the amplifiers, a source of 90 GHz microwaves had to be created from the laboratory's existing HP 83650B sweep generator. The maximum output of this source is 50 GHz, hence, a frequency tripler had to be used to reach the 75-115 GHz test range. Some amplification between the source and tripler was necessary as was filtering and attenuation after the frequency was increased. The HP 8757D scalar network analyser was used to measure absolute incident power reflected and transmitted by the SZA amplifiers. The anlayser also measured the power of a reference signal. A block diagram of the entire system is available here. Labview was used to control the microwave source and the network analyser over a general purpose interface bus.
A few images of the test set up are shown below: