QUINSTAR TECHNOLOGY has acquired the product line and the capability of Berkshire Technologies, Inc, a small innovative company specializing in the manufacture of ultra low noise microwave amplifiers and systems. Berkshire Products incorporate many years of unique experience in designing GaAsFET and HEMT amplifiers tailored for ultra low noise at spot frequencies. QuinStar offers a standard line of coolable low-noise amplifiers (LNAs) that cover the most popularly used frequencies in the 1 to 18 GHz range. We can also custom-design and build special-purpose coolable amplifiers to customer specifications.

APPLICATIONS
Cooled, low noise amplifiers are used in critical applications that require the ultimate in sensitivity. Cooled LNAs find important uses in radio astronomy, satellite earth stations, radiometers, and electron spin resonance measurements. They can also serve as lF amplifiers for SIS and Schottky diode-based millimeter and submillimeter mixers.

COOLING HEMT BASED AMPLIFIERS
Cooling a HEMT-based amplifier to 20K decreases the noise temperature by a factor of ten (and increases gain by about 3dB) compared to room temperature values. A typical cooling curve showing amplifier noise temperature as a function of physical temperature is shown below. Note that the noise continues to decrease below 20K, and that amplifier performance continues to improve down to physical temperatures of 4K and lower.

An amplifier cooled to 20K physical produces the lowest noise temperature, which can be estimated by the relation 1.0K per 1 GHz of operating frequency plus 1K. That is, an amplifier operating at 8.5 GHz can be expected to have a typical noise temperature of 10K (1.0K * 8.5 + 1K).

CRYOGENIC COOLING SYSTEMS
Cooling the amplifier to 20K or below requires a cryogenic refrigeration system. Components of this system are the helium refrigerator, the dewar and its radiation heat shield, together with the amplifiers and associated waveguide and coaxial components. The amplifiers, which are attached to a 20K refrigerator cold head, are in a vacuum. They are surrounded by the radiation shield (temperature approximately 70K) which impedes the flow of heat from the dewar to the amplifiers. Microwave connections in and out of the dewar are designed to achieve low electrical loss and to provide a large thermal barrier. Input isolators are cooled to 20K to reduce the loss temperature of those components. Cool down time for such a system is several hours; the exact time depends upon the size of the refrigerator.

A 20K cryogenic system of the type described provides nearly the ultimate in noise performance for only moderate complexity and cost. Such systems can be custom built by QuinStar Technology. To exceed its performance, one would have to go to a liquid Helium cooling system, or employ a refrigerator capable of maintaining the amplifiers at temperatures of 4K.

Refrigerator-cooled HEMT systems have been found to be very reliable in field use on antennas. The MTBF for a 20K refrigerator is much greater than one year; amplifier life is extremely long based on the performance of hundreds of units operating in the field.

POWER SUPPLIES FOR COOLABLE AMPLIFIERS
Power Supply Model PS-3D is recommended for use with our cooled amplifiers. This servo-controlled power supply is complete with metering and maintains the proper bias on the cooled FETs (click here for PDF drawing).

AUTOMATED NOISE FIGURE TEST SYSTEMS
The Automated Test Bench measures the noise performance of low-noise microwave amplifiers as a function of frequency and displays the results as a graph and as a table of values. The frequency coverage is up to 18 GHz; measurement accuracy is better than 1 K. The Test Bench is computer controlled.

The system performs noise figure measurements by switching the input of the LNA between a terminated load at ambient temperature, and one at 77K (or other temperature). The Automated Test Bench consists of two units. The RF unit contains a precision test receiver; the D/A unit contains the analog-to-digital converter, relay drivers, and digital-to-analog converters. The Test Bench is controlled by an IBM-PC or compatible computer through the IEEE-488 bus.

MANUFACTURING PRACTICES AND PROCEDURES
All our amplifiers are designed using computer aided techniques to optimize performance over the frequency range of interest. Computer design techniques have enabled us to achieve much wider bandwidths than formerly possible. At 4 GHz, 30 percent bandwidth is available with only a small degradation of noise performance; we produce an octave band amplifier in the 2-4 GHz range. For cooled amplifiers, each stage is supplied with an external regulator circuit which stabilizes the operating point over the large range of temperature changes encountered during cool down. We test each amplifier several times at the appropriate temperatures in the process of manufacture, and adjust for optimum performance in the cold condition.

Noise temperature and gain of our cooled HEMT amplifiers are measured on an automated test system. The inherent accuracy of measurements is a few tenths of a degree Kelvin. With each amplifier shipped, we provide test data showing frequency range of the amplifier, noise figure, small signal gain, and optimum operating biases.

Additional test data over operating temperature and frequency ranges may be available at additional cost. Below is a sample list of some of our amplifier models. Most amplifiers we produce are custom-designed to meet the customer's specific requirements.

The amplifiers are cooled to 20 Kelvin in the dewar, where they are interfaced with low loss gapped waveguide. Low loss stainless steel coax lines are employed at the lower frequencies. Vacuum instrumentation and a cryogenic thermometer are included, and a remote helium compressor is supplied with a set of 10 feet of flexible helium lines. In addition a post amplifier of approximately 20 dB gain can be provided to overcome subsequent filter and mixer losses.

The standard system employs the Cryogenic Technology (CTI) model 22 refrigerator. For faster cool down and greater cooling capacity the CTI model 350 refrigerator may be supplied at additional cost.

We supply complete cryogenically cooled systems containing our line of Ultra-Low-Noise cooled HEMT amplifiers. These systems include the closed cycle helium refrigerator (CCR), vacuum dewar, cooled amplifiers, power supplies and remote control monitor unit.

Standard models are available in the frequency ranges indicated below. Additionally, we supply a line of  Broadband Cryogenic Circulators and Isolators (QCY/QCI Series) for applications in the 4 to 40 GHz range.

Pleases see Automatic web quote request form