Radar limiters are designed to protect the components, mainly in the receiver, from damage due to high-level pulses. Large signals can come from a variety of sources, including the radar’s own transmitter and electromagnetic (EM) bombs.^1-6 Filtering should be provided for these undesired signals, which may differ from useful signals by power level, polarization, spectrum distribution, and direction of arrival (DOA). This report will review the numerous types of limiters used to eliminate these undesired signals because of their excessive power levels.^7-11

In most radar systems, limiters work with other circuits to perform several functions, including with polarizationnonselective reflectors to limit the segment of open space from which undesired signals are received, and polarization-selective radiating elements to eliminate signals not complying with a radar system’s design polarization and filters, especially bandstop filters.¹³

Most studies on protecting receivers with limiters have focused on the use of a single limiter in the receive path (Fig. 1), although this approach does not protect all sensitive components in the receive chain, including duplexer circuits. A practical protection solution should include at least two limiters. The effectiveness of the protection is related to the level of power that leaks to the limiter’s output, via flat or spike leakage (Fig. 2). But by using limiters with suitably fast response times, such leakage can be minimized.

The basic limiter used in many radar receivers is the diode limiter, which can be analyzed as an active and passive device.^7,10,11 As an example, Fig. 3 shows a three-stage passive PIN diode limiter. The PIN diodes are nonlinear elements controlled exclusively by input signal power. At zero bias, the diode’s impedance is greater than 1 kç, dropping as low as 1 ç with forward bias.¹⁰ For low-amplitude signals, each diode’s impedance has a minimum value, which translated to minimal signal loss but minimal isolation as well. For the time that the diodes make a transition from a maximum to minimum impedance value, limiter output signal decreases according to the amount of isolation^{8, 12}:

I ≈ 20log[1 + n(Z₀/2R)] (in dB)
where
n = the number of diodes and R << Z₀ for equivalent serial resistance of the diode.
The power absorbed by each diode, Pabs, normalized to input power, Pin, is¹²:
P_abs/P_in ≈ (4R/n²Z₀)

A single-stage diode limiter provides about 20-dB isolation, handling several hundred milliwatts of power without damage.^8,11 With a large signal, current through such a limiter in saturation can result in destruction of the diode. To prevent damage in the presence of large signals, multi-stage limiters are used. In a multi-stage limiter, limiting diode D₁ has a thicker intrinsic (I) layer than the I layer of a switching diode D₃. During the leading edge of a large-amplitude input signal, the impedance of cleanup-stage diode D₃ changes first, since the transit time for carriers across the thinner I layer is shorter than for the thicker I layers of diodes D₁ and D₂. This causes a standing wave, in the plane of D₂. This enhancedamplitude signal forces the carriers to flow into the I layer of diode D=ν, decreasing the impedance of D₂. This change of impedance of first D₂ and then D₁ into a low-impedance state results in high combined isolation. The level of the power leaking to the limiter output is 2 to 4 dB higher than the cleanup stage threshold level. The typical acceptable power level for the input stage of the receiver is +10 dBm, while the threshold level for the thinnest I layer diode is around +7 dBm.⁸ As a result, energy leaking from the limiter, especially during a spike, can damage sensitive receive circuits.

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