For the two receive paths (the downlink), the input signal feeds to a high-rejection bandpass ceramic filter (diplexer) via the A1 and A2 ports. The input signal passes through a series of amplifiers, phase shifters, attenuators, and SPDT switches before reaching the receiver ports (Rx1 and Rx2). The total gain across the receiver band is 30 dB.

Two diplexers are required for maintaining optimum performance. The transmit side of the diplexer filter, inserted after the transmit amplifier, prevents wideband noise from entering the receiver and degrading performance. The diplexer’s receive section prevents the coupled transmit signal from degrading the linearity of the receive LNA. This ceramic filter consists of four resonators in a coaxial structure and produces a 1 dB insertion loss in the receive band. A six-section (six resonator cavities) transmit filter has a low-pass structure with 0.5 dB of loss and a rejection of 65 dB at the crossover frequency of 2.15 GHz. The T/R module, with associated control circuitry, is 6.75 in. long and 3 in. wide; its thickness, including housing, is 0.75 in. (see Figure 3).

To minimize costs, the AFRL/ industry team evaluated several commercial off-the-shelf (COTS) main components for the transmit and receive channels. For a receive frequency range of 2.2-2.3 GHz, the team evaluated several silicon germanium LNA devices and finally selected one based on its low cost and ease of use. They also selected COTS gallium arsenide HBT MMICs for the transmit channels’ power amplifier to ensure power output capability of 32 dBm. The amplifier provides an overall gain of 20 dB, with >40% added power efficiency across the 1.7-2.1 GHz band.

Researchers based the phase shifter design on a single MMIC chip with two back-to-back SPDT switches in a surfacemounted package.4 By consolidating two separate SPDT switches, the device design reduces the component count from 8 devices per phase shifter (32 per T/R module) to 4 devices per phase shifter (16 per T/R module). The team measured the insertion loss of the phase shifter at 4.5 dB, observing a total change in insertion loss of 0.4 dB in all phase states.