The Design and Evaluation of a High Performance Ku-band Downconverter for Spaceborne Interferometric Radar Paul Siqueira, Michael Tope † , Karthik Srinivasan, Edin Insanic, Harish Vedantham, Sumanth Pavaluri, Razi Ahmed, Gerry Walsh † Microwave Remote Sensing Laboratory (MIRSL), Dept. of ECE, University of Massachusetts 113D Knowles Engineering Building, Amherst, MA 01003-9284 † Radar Science and Engineering, Jet Propulsion Laboratory, Pasadena CA 91109 Abstract – This paper discusses the results from the creation and testing of a high precision two-channel Ku-band downconverter development by the University of Massachusetts and the Jet Propulsion Laboratory. The Ku- band downconverter is a critical part (in terms of performance) of interferometric radar systems. The designed Ku-band downconverter is built and tested over temperature ranges and included in a rooftop deployed radar looking at snow covered ground. Lessons learned from the Ku-band development are currently being applied to a similar system operating at Ka-band. I. INTRODUCTION Interferometric radar works by measuring the differential phase measured by two antennas separated by a baseline. If this baseline is oriented in the direction of antenna motion (along-track interferometry), the system is primarily sensitive to the velocity of observed targets. If the baseline is oriented in the cross-track direction (Fig.1) the phase difference is proportional to the topography. In either case, the precision to which the differential phase can be measured affects the accuracy of either the velocity or the topographic measurement. The basic equations for this treatment, and their importance to the development discussed here, are discussed in [1]. Fundamentally however, an increase in the carrier frequency allows for a proportional reduction of baseline length if all other variables are held constant. Since resolution is proportional to bandwidth, higher frequencies also have the advantage of being capable of achieving higher resolution. Hence, in order to reduce the dimension and mass of spaceborne structures for single-platform, single-pass interferometers, and achieve good spatial resolution, there is a general trend to achieving interferometry at higher frequencies (e.g. Ku- and Ka-band). Challenges associated with working at these high frequencies are associated with the ability to measure phase accurately and thermal effects which affect the differential path length for signals within the downconversion chain. The downconverter from RF to baseband is an important component in the overall development of high frequency interferometers (Fig. 2) and is the topic of work supported at the University of Massachusetts by NASA’s Earth Science Technology Office through the Advanced Component H B A 2 A 1 α θ dz Fig 1. Observing geometry of a cross-track interferometric radar. Technology Program. This paper summarizes progress made in building and characterizing a two-channel, two-stage downconverter, initially at Ku-band and subsequently at Ka- band. II. BASIC DEVELOPMENTAL COMPONENTS The basic developmental components of the downconverter design, can be broken down into three parts. That is, from the carrier frequency (Ku- or Ka-band) to an intermediate frequency (L-band), downconversion from L-band to baseband, and finally, in creating a measurement system capable of characterizing the system to high precision in terms of phase and amplitude as a function of frequency and temperature. The intermediate stage at L-band is necessary for providing sufficient room for image rejection and for setting the noise bandwidth. Design performance characteristics for the Ku-band downconverter are given in Table 1, with a similar set of characteristics applying to the Ka-band system (RF from 34.985-35.015). An block diagram of the Ku-band version of the downconverter is given in Fig. 3, with an image of the completed downconverter given in Fig. 4.