Performance Measurements on Active Cold Loads for Radiometer Calibration

 A transistor based cold load for radiometer calibration, a so-called active cold load (ACL), is being developed for future space programs.

Typically, a radiometer needs frequent calibration using a hot and a cold reference point. Ideally these references should encompass the radiometer’s expected range of input brightness temperatures – typically some 100 to 300 K for an Earth sensing system. The hot reference presents little problem and can just be a microwave load at the internal instrument temperature. Cooling a load to below 100 K in space is not easy, and therefore a view to free space (around 3 K), either via a special sky viewing antenna or via a reflector in front of the main feed horn, has typically been applied. But this is not always very practical, hence the desire for a small, low power, low weight, stable cold reference.


Two such ACLs - based on transistors connected backwards, i.e. the gate is connected to the output connector – have been designed and built by Ylinen Electronics, and their performance is being assessed at the Technical University of Denmark. For this first test development a frequency in the X-band has been chosen.


The overall objective of the DTU project work is to design and construct a Test Bed for monitoring the ACLs, and to carry out tests of their performance.


In order for an ACL to serve as a primary radiometer calibration device it must exhibit basic stability and dependence upon external environment equal to or better than classical calibration devices. To this end a test activity has been carried out comprising: test requirements and specifications, design and manufacture of a proper Test Bed, a short term test of thermal dependencies, and finally a long term test of basic stability. A crucial component in this sequence is the Test Bed aiming at measuring the output of the ACLs with adequate precision, resolution, and accuracy. In effect the Test Bed is a stable radiometer designed to measure the ACLs in an optimal way, using a liquid nitrogen target as a basic but occasional reference. For intermediate calibration checks, a built-in loads at 44 °C as well as a noise diode, adding some 80 K to the input brightness temperatures, can be observed at any time.


The test activities comprise two different campaigns:


The varying temperature campaign includes a staircase scheme in which the temperature is increased from 0°C to 50°C in 10°C steps. During this test the ACLs are inside a climate test chamber and the Test Bed outside. The measurements show that the output of the ACLs increase linearly with physical temperature with a sensitivity of some 0.4 K / °C.


During the long term stability test, the Test Bed and the ACLs are left undisturbed in a quiet basement room. The ACLs are not temperature regulated, but will just be at ambient. The measurements rely on stability and internal calibration of the Test Bed, augmented by LN2 calibrations at suitable intervals. During the stability tests a 12 minutes cycle, in which the ACLs are measured alternatively for 5 min. interrupted by 1 min. internal calibration, is used. The validity of this scheme is supported by actual measurements of the Test Bed stability. The LN2 calibration reveals that the output of ACL1 is 73 K and of ACL2 51 K. The first ten months of operation have indicated excellent stability. The drift is around 1 K – mostly during the first half year. The campaign continues in order to reveal if a certain “burn-in” has taken place.