CSU-CHILL Receiver Front End

This article presents a new receiver front-end design for the CSU-CHILL radar.

Overview

The front-end is designed to maximize the RF dynamic range, while at the same time, preserving low noise figure and a reasonably wide RF bandwidth. The front end presents two RF channels, one per polarization.

The existing front end achieves a low noise figure, thanks to the 0.7 dB noise figure LNAs. However, the LNAs have a low output compression point (+10 dBm) and a high gain (33 dB), which results in an input compression point of -23 dBm. Thus, the receiver can saturate when receiving strong echoes from nearby targets. The original design called for the use of two tuned cavity filters of approx. 3 MHz bandwidth between the LNA and the first mixer, in order to improve the mixer noise figure by eliminating noise contribution from the unwanted mixer side band. These filters were removed, however, after they were found to adversely affect the copolar correlation coefficient ($\rho _{co}$, also called $\rho _{HV}$). As a result, the overall receiver noise floor was slightly increased.

In addition to the LNA channels, two additional receiver channels served as the transmitter monitoring channels. These channels include the same mixers used in the main receive channels, but omit the LNAs. They are connected to Klystron outputs via a 50 dB loop coupler and a 40 dB attenuator. These channels are used to sample the Klystron output spectrum. However, transmitter output power calibration is done using a pair of average power meters.

Low Noise Amplifiers

The updated front-end features an LNA with better saturation characteristics: +13 dB output compression point, 25 dB gain, yielding an input saturation point of -12 dBm. The noise figure is 0.65 dB. Thus, the overall linear RF dynamic range of the LNA, for a 1 MHz bandwidth is -12 - (-113.9 + 0.65) = 101.25 dB.

High-speed switch

The output of the LNA is fed to a pair of high-speed switches.

Mixer

The mixers used in the design are high-dynamic range, single-sideband mixers with an unwanted sideband suppression of 25 dB.