Limiters Prevent Front-End Damage

Often taken for granted when they function properly, microwave limiters are sorely missed when they don’t. They are most often relied upon to protect sensitive components such as low-noise amplifiers (LNAs) in receivers where high-power signals may be present. Many limiters can survive power levels of a few watts at best. For those applications where protection is needed from larger signals, however, Herotek (www.herotek.com) has developed several lines of PIN diode limiters for input levels as high as 40 W CW. They feature fast recovery times, lacking penalties in other key electrical parameters (such as VSWR and insertion loss) that can degrade an LNA’s purpose of boosting a receiver’s sensitivity. Receiver damage can come from many directions, but the usual culprit is the transmitter in the same system. Whether a high-power pulsed radar system or a modulated communications system, a diode limiter can protect a receiver from damage caused by its own transmitter. RF/microwave limiters, which are usually based on PIN diodes, can also be fabricated with Schottky diodes or field-effect transistors (FETs), even in monolithic-microwave-integrated-circuit (MMIC) form.

In all cases, they are designed to protect a receiver or other sensitive receiver by switching to a high-loss state when impacted by a high-level signal. Different models are usually compared by means of a handful of specifications such as recovery time, spike leakage, limiting threshold, and power-handling capability. For example, the recovery time is the time from the end of an incident RF pulse to where the insertion loss of the limiter returns to a value within 3 dB of the quiescent value.

Spike leakage refers to the characteristic common to limiters to momentarily pass more power than when it is under normal saturated signal conditions. This rise in power is usually a spike on the leading edge of the leakage energy. A limiter’s power-handling capability usually assumes that the limiter is being used in an impedance-matched setup, to minimize the opportunities for standing waves that could reflect energy back through the limiter’s active device and compromise power-handling capability.

Herotek has developed a number of different types of limiters, including the LP Series rated for 1 W CW from 0.5 to 18.0 GHz, the low-limiting-threshold LS Series limiters for 10 MHz to 18 GHz, the low-leakage LL Series for 10 MHz to 18 GHz, and the low-leakage LS Series from 10 MHz to 6 GHz.

The company offers limiters for use at frequencies from 100 kHz (and usable at frequencies as low as 45 kHz) with power-handling capability of 1 W CW and 200 W peak power for short pulses (1-μs pulse width and 0.1% pulse duty cycle). As an example, model LS0000130A covers 100 kHz to 3 GHz with +16 dBm leakage for frequencies below 100 MHz and typically +18 dBm for frequencies below 1 MHz. At higher frequencies, the firm’s model LP1-26A spans 1 to 26 GHz while the model LP26-40A is for 26 to 40 GHz. Both feature typical response time of 10 ns. The former has insertion loss of 4.0 to 4.5 dB with maximum VSWR of 2.0:1, typical limiting threshold of +9 dBm, and leakage of +18 dBm when handling 1 W CW input power.

The firm’s LL Series coaxial limiters are designed for low leakage levels (to -10 dBm), available from 10 MHz to 18 GHz with specified leakage levels of -10, -5, 0, and +5 dBm. These limiters handle 1 W CW and 100 W peak power (for 1 μs) with typical recovery times of less than 10 μs. Depending upon frequency range, the maximum insertion loss varies from 0.5 to 4 dB. The maximum VSWR ranges from 1.30:1 to 2.0:1.