Home Featured Articles The Role and Trending Requirements of RF Limiters in Multifunctional AESA Radar

The Role and Trending Requirements of RF Limiters in Multifunctional AESA Radar

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by Fairview Microwave

In the past few years, active electronically scanned array (AESA) radar have transitioned from cutting-edge radar technology designed to be deployed on next-generation multi-role fighters, to capability and life extension systems for older aircraft and legacy radar designs. There are an increasing number of radars replaced with AESA systems, with one of the major demands being multifunctional capability to meet multi-mission and multi-role expectations for modern radar. These new expectations present substantial design difficulties, especially for the receive signal chain in a transmit/receive module driving a high number of AESA antenna elements. The design constraints of high antenna element AESA radar are further complicated when the AESA is forced into footprints catering to a previous generation of mechanical radar. These factors are putting greater strain on receiver technology and greater emphasis on RF limiters to protect these sensitive downstream components from higher transmit and jamming signal powers.

Multifunctional AESA Radar Trends & Challenges

There are an increasing number of news releases presented by companies and military organizations around the world announcing new Multifunctional array radar (MFAR), or multifunction phased array radar (MPAR), active-electronically scanned array (AESA) capability and contract wins [1.1 – 1.7]. It is a growing trend to replace legacy radar technology with the latest MFAR systems. This trend extends from multi-role fighter aircraft, military helicopters, naval vessels, and even to land mobile systems. Many of these platforms include expectations for these AESA radar systems to fit in the footprints of the mechanical radar they replace. The result of these expectations is the requirement to fit tens to hundreds of antenna elements and transmit/receive (TR) modules (TRMs) in a space that places significant constraints on the size of the antenna array and the size/shape of TRMs.

The latest AESA radar receivers not only have to perform in an extremely cluttered spectrum rife with jamming and interfering signals, but now also have to contend with demands for higher transmit power to meet certain mission specifications. The desire for greater multifunctional capability further extends this design challenge, as there is a trade-off between radar range, which requires higher transmit power, and receiver sensitivity [2.1]. Due to the reflections from air interface at the antenna elements and impedance mismatch at the antenna ports from coupling with other nearby antenna elements, duplex transmit and receiver radar systems suffer from high reflected power at the input of the receiver signal chain. 

Separate antenna systems for transmit and receive could be used to limit this effect, but such a system would require double the antenna elements, physical separation, and more complex computing and digital signal processing.

Figure 1: A high-level diagram of a transmit/receive module (TRM) in an AESA

Typical methods of reducing the impedance mismatch from coupling antenna elements are less effective in MFAR, which may have several separate beams active at the same time. This is because the impedance matching mechanisms used with AESA radar generally can’t account for every configuration of the radar, where some nearby antenna elements may be in transmit mode while others are in receive mode performing different functions. Hence, achieving higher ranges with MFAR, or greater low range accuracy, is limited by the isolation circuitry.

A switched radar doesn’t suffer from the same requirements, but it also may not be able to provide continuous multifunctional capability if a transmit-receive switch is used [2.2]. However, even switched receiver protection doesn’t protect against interference, jamming, and antenna coupling to nearby active antenna elements. Also, large clutter returns from near-range objects could be significant enough to desensitize a receiver unless there is automatic variable attenuation built in to the receiver chain. To enable simultaneous multifunctional capability there is an incentive to overcome these challenges without relying on switching.

Importantly, circulators tend not to provide as much isolation as transmit-receive switches. Generally, circulators that are employed in compact TRMs only have isolation specifications around 20 dB to 30 dB. With transmit powers for MFARs pushing above 10 watts per element and receive signals in the microwatts, that means that additional receive protection is necessary to avoid desensitization and/or saturation of the receiver. 

Moreover, the latest jammers that use antenna arrays are able to generate extremely high incident power to a radar antenna. Though much of the jammer power may be mitigated by frequency agility features, jammer technology and routines are also becoming more capable at targeting even frequency agile radars. During targeting and sweep modes, a radar may be more difficult to jam, but some multifunctional roles may lead to parts of an antenna array being dedicated to functions that potentially mitigate the benefits of AESA narrow antenna directivity. If the directionality aligns well enough with a jammer, or the jammer power is high enough, substantial jamming energy can still be picked up by the antenna array elements. Alongside high transmit powers, TRM receive circuitry also has to contend with increased interference and more sophisticated jamming technology, and possibly all at the same time.

Figure 2: A PIN diode limiter circuit with DC blocking capacitors and RF choke

Use of RF Limiters in AESA Radar Applications

For the previously mentioned reasons, many AESA and MFAR radar designs are employing limiters at the input of the receive signal chain to combat high input powers. A key component of radar receiver protection is the RF Limiter. These devices, usually comprised of diodes, present a high impedance at low signal power levels and a much lower impedance at high signal power levels. An RF Limiter is typically placed between the receiver circuitry and circulator in a TRM. 

The effect of an RF Limiter is to reduce the overall signal energy to ensure that the signal power doesn’t exceed the input power limitations at the input of the receiver. The impact of a limiter exposed to high signal energy is to attenuate signals across the entire bandwidth of the limiter. Though it is not ideal to reduce the signal energy to the receiver, which reduces the receiver sensitivity, the alternative is to allow high signal energy to pass through to the receiver, potentially damaging sensitive receive components.

Moreover, even signal energy below the damaging threshold may push the receiver into saturation, which degrades the linearity of the signal. Reduced receive linearity has substantial impacts on radar performance, and could lead to degraded dynamic range, and nonlinear artifacts masking important target data [2.3]. With highly nonlinear radar receivers, large radar signatures could produce substantial harmonics that either appear as false targets or obscure smaller or longer range targets. Hence, the value of an RF Limiter in the signal chain.

Key RF Limiter Considerations for Multifunctional AESA Radar

RF power limiters, or signal limiters, come in a variety of shapes and styles. A common implementation is the use of PIN diodes specifically chosen to yield desired specifications of the limiter. Additional RF chokes and DC blocking capacitors may also be used in a limiter to provide additional receiver protection to DC and low frequency phenomena, such as ESD. The RF choke completes the DC current path for the PIN diode, but must be carefully chosen as the non-ideal behavior of an RF transformer/inductor can have substantial impact on the performance of an RF Limiter.

Figure 3: A Digital radar receiver diagram with limiter receiver protection

As with other high-reliability (Hi-Rel) applications, it is essential that RF Limiters meet the environmental and physical ruggedness requirements of the application military standards for the type of missions the MFAR will undergo. A test method used for RF Limiters, and other military-grade electronics, is MIL-STD-202F, which contains humidity, shock, vibration, and altitude test methods.

Insertion Loss, VSWR, and Frequency Range

As with most RF components used in receiver signal chains, the insertion loss, VSWR, and frequency range of operation are all prime considerations when selecting RF Limiters. The insertion loss and VSWR can be of significant concern, as excessive insertion loss and VSWR may limit the dynamic range of the radar receiver. It is desirable to have these parameters as low as possible. With connectorized RF Limiter packages, the insertion loss and VSWR performance of the connectors has a major impact on the resulting insertion loss and VSWR, and should be high precision and low loss designs for radar applications. For frequency agile radars with high bandwidths of operation, it is also important to select RF Limiters that can also achieve these high bandwidths and provide good insertion loss and VSWR performance across the entire desired frequency range.

Leakage Power and Maximum Input Power

The leakage power, often given in continuous wave (CW) and peak power, is the maximum power that the limiter will output if exposed to the maximum input power. It is typically good practice to select a limiter that has a leakage power at least a few dB below the input power limits of the low-noise amplifier (LNA) at the input of the receiver. This can help prevent saturation of the LNA and ensure linear response of the receiver signal chain. 

Suppression of Even-order Harmonics

As an RF Limiter operates in a diode’s saturation mode, there is generally a nonlinear response to a limiter. It is important for a limiter to be designed to ensure that the harmonics don’t extend into the operating frequency of the receiver. One factor to consider is the suppression of even-order harmonics by the limiter, which may be specified by the manufacturer.

Figure 4: VSWR and Insertion loss plots of a commercially available RF limiter

Turn-on Time, Recover Time, and Spike Leakage

After exposure to a high enough RF signal to force charge carriers into the diode’s (PIN diode) I layer, electrons from the N layer, and holes from the P layer, the resistance of the limiter begins to decrease. Though fast, there is a finite time before the limiter turns on enough to adequately attenuate high input signal powers. If a pulse of even has a rise time faster than the turn-on time of the limiter, then a spike leakage will occur. Hence, it is important to choose a limiter with a fast enough turn-on time to prevent potential high-rise time pulses.

After the high RF signal energy dissipates, it takes some time for the charge carriers to leave the I layer, or recombine within the I layer. At this point, the resistance of the limiter is still low and begins to rise again, reducing the attenuation of the RF signal over time. The length of time it takes for the limiter to “recover” is important, as a too slow recovery time may limit the near-range of the radar after the limiter turned on to protect the receiver from the transmit pulse. The DC resistance of the RF choke also has a detrimental impact on the recovery time of the limiter, and must be minimized.

Added Noise Figure

Radar receiver sensitivity is of the highest concern, hence the added noise figure of an RF Limiter, or any component in the receive chain, is essential to keep low. The intrinsic resistance of the diodes, choke, and capacitors in the limiter may produce some added noise, but methods of enabling good performance in other aspects of the limiter should also help in reducing the added noise figure of the limiter. 

Conclusion

RF Limiters are one of the few choices to improve the isolation of radar receivers from the increasing transmit powers of MFAR. When seeking to enable the most flexible and capable AESA, RF Limiters provide enhanced receiver protection without the need for transmit-receive switching. As more legacy radars are upgraded with AESA and MFAR technology, it is likely that more RF Limiters will be employed to address the challenges of radar receiver protection in an increasingly complex combat theater.

Resources

AESA News

1. Raytheon to Supply AESA Radar for Marine Hornets

2. The Air Force has Given a Test F-16 a Powerful APG-83 AESA Radar

3. Raytheon Selected for B-52 AESA radar Upgrade

4. Germany Seeks AESA Radar for Tranche 2-3 Eurofighters, Plus Additional Aircraft

5. Russia’s Ka-52M Combat Helicopter to Receive AESA Radar

6. Contract for Ground Master 200 Multi Mission Radar Brings Modern 4D AESA To the Dutch Army

7. Bangladesh Orders AESA Air Defense Radars From Leonardo

AESA Technology

1. Survey of Systems Engineering Approaches for Design and Development of AESA Radar for Fighter Aircraft

2. Bistatic Radar Concept Demonstrator System Development and Verification

3. EITN90 Radar and Remote Sensing Lecture 9: Radar Transmitters and Receivers

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