What is an RF Limiter?

RF Limiters are used in Transmit/Receive modules to prevent damage to the low noise amplifier due to excessive power at the input of the system. LNA's and other components in the receive chain of a wireless systems are very sensitive, as they are usually designed to accept small signal levels which are then amplified.

In ideal situation where the input power to a system is within the limits of the LNA, the limiter which is placed right before the LNA is in a low loss state, i.e it passes the RF signal to the input of the LNA with a marginal insertion loss. However, when there is a more powerful signal incident at the input of the LNA, one that could damage the LNA, the limiter attenuates the signal level to ensure that the LNA or other components in the Rx chain are not damaged due to the excessive input power.

Insertion Loss: When the incoming signal level is below the limiting threshold in amplitude, the signal passes through the limiter with a low loss. The loss in this case is known as the insertion loss of the limiter. This is usually very low. In an ideal case this would be zero.

Flat Leakage: Flat leakage is the designed RF output power level limit for a given limiter. So for example, if the maximum power an LNA can handle is 20 dBm. One should select a Limiter than has a Flat Leakage of less than 20 dBm. It can be thought of as the limited CW power that passes through the limiter (after clamping down the signal power level) upon application of a high-power signal to the input of a limiter.

Max Input Power (CW): This is the maximum input power (CW) that a limiter can handle during normal operation of a system. 

Max Input Power (Peak): This is the maximum input peak power that a limiter can handle without breaking down or getting damaged. This is not a continuous power, it is the max. surge in power that a limiter can handle.

Turn On Time: The turn on time of the limiter is very important. It is the time taken by the limiter to start limiting the input signal. The faster the turn-on-time the better.

Spike Leakage: If the RF input pulse has a faster rise time that the turn-on time of the limiter, there will be a spike in RF power that gets through till the limiter turns on. This is called Spike leakage. If the limiter turns on faster than the Rise time of the RF input signal, there will be no spike leakage. Spike leakage is often represented in units of energy, not power. For instance, if the RF limiter allows a spike with a power of 1 Watt for 10 nanoseconds, then the spike leakage would be 10 nano-joules.

Impedance matching is very important when it comes to limiters, as this is usually the 1st component in the Rx chain, right before the LNA. Any noise or irregularities in the signal will get amplified by the LNA.

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