A miniature L-band transceiver that operates at a carrier frequency of 1.25 GHz has been developed as part of a generic radar electronics module (REM) that would constitute one unit in an array of many identical units in a very-large-aperture phased-array antenna. NASA and the Department of Defense are considering the deployment of such antennas in outer space; the underlying principles of operation, and some of those of design, also are applicable on Earth. The large dimensions of the antennas make it advantageous to distribute radio-frequency electronic circuitry into elements of the arrays. The design of the REM is intended to implement the distribution. The design also reflects a requirement to minimize the size and weight of the circuitry in order to minimize the weight of any such antenna. Other requirements include making the transceiver robust and radiation-hard and minimizing power demand.
Figure 1 depicts the functional blocks of the REM, including the L-band transceiver. The key functions of the REM include signal generation, frequency translation, amplification, detection, handling of data, and radar control and timing. An arbitrary-waveform generator that includes logic circuitry and a digital- to-analog converter (DAC) generates a linear- frequency-modulation chirp waveform. A frequency synthesizer produces local-oscillator signals used for frequency conversion and clock signals for the arbitrary- waveform generator, for a digitizer [that is, an analog-to-digital converter (ADC)], and for a control and timing unit. Digital functions include command, timing, telemetry, filtering, and high-rate framing and serialization of data for a high-speed scientific-data interface.
The aforementioned digital implementation of filtering is a key feature of the REM architecture. Digital filters, in contradistinction to analog ones, provide consistent and temperature-independent performance, which is particularly important when REMs are distributed throughout a large array. Digital filtering also enables selection among multiple filter parameters as required for different radar operating modes. After digital filtering, data are decimated appropriately in order to minimize the data rate out of an antenna panel.
The L-band transceiver (see Figure 2) includes a radio-frequency (RF)-to-baseband down-converter chain and an intermediate- frequency (IF)-to-RF upconverter chain. Transmit/receive (T/R) switches enable the use of a single feed to the antenna for both transmission and reception. The T/R switches also afford a built-in test capability by enabling injection of a calibration signal into the receiver chain. In order of decreasing priority, components of the transceiver were selected according to requirements of radiation hardness, then compactness, then low power. All of the RF components are radiationhard. The noise figure (NF) was optimized to the extent that (1) a low-noise amplifier (LNA) (characterized by NF < 2 dB) was selected but (2) the receiver front-end T/R switches were selected for a high degree of isolation and acceptably low loss, regardless of the requirement to minimize noise.
The filter specifications were chosen to minimize the sizes of the filters, thereby placing the baseband higher in frequency than would otherwise be necessary. This is an acceptable trade-off inasmuch as (1) the consequent requisite digitizer bandwidth is still realizable by use of commercial devices and (2) the decimation performed by the digital filters eliminates excess bandwidth. The receiver band-pass filter (BPF) is placed in front of the LNA in order to limit radio-frequency interference. A programmable attenuator is included to provide adequate dynamic range in the event that the amplitude of the radar echo varies significantly. Care was taken to minimize cost by minimizing the number of parts and the number of different types of parts: in particular, the amplifiers and mixer used in the up-converter are also used in the down-converter.
The packaging of the L-band transceiver was designed in recognition that the different types of electronic devices used must be mounted and connected in different ways. The packaging approach was to place circuits that perform different functions in separate cavities in the module housing, coupling the DC signals through the walls by use of filtered connections only. This approach provides shielding from noise leakage. Thus, the down-converter (receiver) chain, the upconverter (transmitting) chain, and the control and power-supply circuitry are each located in separate cavities of the housing. The active RF components (e.g., amplifiers) are on one side of the module, while the passive RF components (e.g., attenuators and filters) and the control and power circuits are on the opposite side. The RF functional blocks are further separated, according to frequency, onto individual substrates and into individual cavities.