A Spectrum of Radar Solutions
By Dr. Douglas Carlson, Director of Business Development, M/A-COM Technology Solutions

Abstract
Radar applications require a diverse set of RF components supporting a range of radar architectures. From transmit power generation to signal management, the industry requires cost effective solutions that meet the system requirements.

Introduction
Radar systems encompass a broad array of functional specifications. They can span frequencies from UHF to millimeter wave and have power levels that range from milli-watts to kilowatts. They are utilized in antenna technology, both dish or phased array, and applications that include everything from over-the-horizon volume search to automotive autonomous cruise control. This broad application and performance space creates unique challenges and opportunities for components suppliers addressing this market.

At its most basic level, the block diagram of a radar system can be represented as shown in Figure 1. In the transmit chain, there are functional blocks dedicated to signal generation, phase and amplitude control and power amplification. The receive chain, in general, has functional blocks dedicated to amplification, amplitude and phase control, and analog to digital conversion. Often, the receive chain is protected with some sort of limiter or receiver protector. The signal from the transmitter is routed to the antenna through a circulator or Transmit/Receive (T/R) switch, isolating the transmit channel from the receive channel. The functional blocks for both the transmit and receive channels, can be realized in a variety of technologies, depending on system requirements. For example, the entire T/R function can be implemented in a single integrated circuit or each functional block can be a hybrid assembled subsystem. In the following sections, possible implementations of the various functional blocks will be reviewed, highlighting design trade-offs.

Circulator vs. Switch Technology
Signal routing from the transmit chain to the antenna and back from the antenna to the receive chain can be accomplished by a number of technologies. The choice of solution is primarily driven by the maximum power the system must handle. In order of power handling capability, from highest to lowest, candidate solutions are:

• Ferrite Based Circulators: 1 KW CW; 2.5 KW pulsed
• PIN Diode based Switches: to 100W CW; 1.5 KW pulsed
• MMIC Switches: to 10W

Circulators (see Table 1) are three-port devices that offer low insertion loss, high port-to- port isolation and high power handling capability (to 2500 W under pulsed conditions).

Under medium power conditions, PIN diode technology provides effective solutions for both parabolic and phased array apertures. The power handling capability of PIN diodes is limited by thermal management and the ability to keep the device below a critical junction temperature for reliable operation. M/A-COM Technology Solutions has demonstrated PIN diode based switches capable of handling 50 W CW and up to 1500 W under pulsed conditions. Utilizing M/A-COM Technology Solutions’ unique Heterolithic Monolithic Integrated Circuit (HMIC) process—which combines high performance silicon diodes with high performance passive components built on a glass/silicon composite substrate—diode-based MMIC switches have been designed and are commercially available. These products are available in both PQFN plastic packages and surmount formats. Technical advances are continually expanding CW power handling capabilities. Coupled with the low insertion loss and high isolation that can be achieved in this technology, PIN diodes are an ideal solution for high power switching applications where reduced size and weight are a consideration while maintaining high performance. (See Figure 2).

Lastly, GaAs based MMIC switches offer an ideal solution for lower power T/R modules (<10W). PHEMT based MMIC switches offer very high switching speeds, good insertion loss and very high isolation coupled with high linearity. Available in both die and PQFN plastic packaged formats, MMIC switches are a cost effective high performance solution.

Transmit Power Amplification
Transmit power can be achieved with both vacuum tube and solid state technology. In this article, the focus will be on solid state solutions. For applications under 3.5 GHz, silicon power transistors provide effective solutions for high power, relatively narrow band applications. Individual, packaged, power transistors can be assembled into a complete output chain (see Figure 3) where matching and combining is the primary responsibility of the full amplifier designer.

As an alternative, transistors are available in a pallet format (Figure 4). Pallets offer 50 Ω matched and partially matched building blocks, simplifying the use of high power transistors. Using this approach, complete high power output stages can be constructed.

Receive Chain Amplification
Effective solutions for the amplification of the received solutions must keep the front end noise figure as low as possible, maintaining dynamic range, while being capable of withstanding jamming, either intentional or unintentional. Intentional jamming levels can reach very well into the kilowatts range in CW modes and higher in bursts to try to disable radars. Unintentional jamming can come from the radar’s own transmit signal, if not properly isolated, or from nearby radars, or even return signals from nearby objects. The challenge is that technologies which exhibit extremely low noise figures typically also are easily damaged by large signal. Often limiters are used as receiver protectors; though effective at protecting the receive chain, these devices can introduce an unacceptable recovery time into the system performance.

An ideal solution would be a low noise amplifier capable of withstanding significant input power. Gallium Nitride HEMT based LNAs offer this potential. At M/A-COM Technology Solutions, we have measured GaN LNAs with 600 µm gate periphery capable of handling greater than 34 dBm at 15 V Vdd. GaN LNAs offer the promise of the ideal device for receive chain amplifiers.

Signal Control
Phased array applications require amplitude and phase control of each T/R channel. This can be accomplished mechanically or electronically through either the use of discrete phase shifters and attenuators or more integrated solutions. Integration, at the IC and board levels, provides significant opportunity for size and cost reductions. Examination of the Transmit (Tx) and Receive (Rx) sections of the block diagram reveals several features:

1. There is significant opportunity for integration of functionality both within each chain as well as between the Tx and Rx chains.

2. Each control component requires logical input to determine its correct state: switches, attenuators, phase shifters. This requires that designs must satisfy both RF and digital functionality.

Partitioning of the block diagram is a key element to drive cost reduction through functional integration. Appropriate selection of functional boundaries will drive cost and performance. On-chip integration is a clear path to lower cost when the volume of the opportunity is sufficient to warrant the custom design effort. Cost reduction through integration is parallel to the strategy followed by the silicon industry. In the case of digital processors, cost and performance is driven through integration and compaction reducing board component count, assembly complexity while dramatically increasing system functionality.

M/A-COM Technology Solutions exploits high volume, commercial pHEMT and CMOS processes to realize the required RF and digital functionality. The appropriate choice of process technology allows the on-chip integration of control components, switches, phase shifters and attenuators, with amplifiers (low noise and power). An example of this functional integration is shown in Figure 5.

Custom CMOS controllers are effective solutions for managing the many control states required in a T/R module. CMOS provides very fast switching speeds, serial to parallel conversion, and data buffering with minimal current consumption and low cost. Depending upon the power and noise specifications of the module, significant RF functionality can be realized in either Si CMOS or SiGe BiCMOS technologies.

High performance, cost effective solutions enabling a broad class of radar system are available for all major RF blocks. The optimal choice of device among various technologies is dictated by many system requirements.

M/A-COM Technology Solutions
www.macomtech.com
TXTLINX.COM90
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