RF Limiters are devices used in Receiver or Transmit chains to protect the sensitive components in it from excess power flow. RF limiters limit the maximum RF signal power passing through it on the base of its threshold design.
If the input power of the RF signal is within the limits of the RF limiter, it will pass the RF signal with a marginal insertion loss. If a higher RF signal reaches the input of the Limiter above the threshold, the limiter attenuates the signal level and ensure the protection of the RF components following in the system.
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RF and microwave Limiters are designed and developed by using RF PIN diode and Schottky diodes to perform strong limiting action with very limited insertion loss.
PIN diode limiters are incident RF power-controlled variable resistors. During low input signal, the impedance of the limiter diode is at its maximum, hence it will have minimum insertion loss, in the range of 0.5 dB. During high power input signal PIN diode offer impedance mismatch and reflects the majority of the RF input signal back towards the source.
A Schottky diode has stored charges which offer faster recovery time between its forward conducting (ON state) and the reverse (OFF state) cut off state. This fast switching capability makes it ideal for RF and microwave and high-speed switching applications.
There are basically three types of limiters are used for the RF and Microwave applications.
Conventional PIN diode limiters reduce high power inputs signal to a low power signal level that can be tolerated by the following RF component in the chain to protect the sensitive circuit components.
Feedback limiters will have a feedback circuit to protect more sensitive components. Feedback limiters have lower threshold levels and leakage levels than Conventional limiters.
High power RF limiters will handle very higher power levels and are used to protect sensitive receiver circuits from high power.
RF Limiter is a device used to limits the flow of input signal power above a specified limit. The amplitude limit in the radio frequency or RF spectrum range is defined by the manufacturer during the time of design.
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While selecting RF limiter, frequency, Flat Power Leakage, isolation and Power handling is the prime specifications RF designer needs to consider to achieve the desired design goals of the RF and microwave communication systems.
Range of frequency over which RF limiters are designed to operate. Start and stop frequency need to be below and above the required frequency band of operation.
Power handling indicates the maximum input power in watts a limiter can handle at the normal operation without breaking down or getting damaged. Power handling for limiter needs to check for the Max Input Continuous-wave and peek Power. Peak power indicates the max surge an RF limiter can handle.
Insertion loss of the limiter indicates the level of RF power loss when the signal passes through the limiter. If the signal power amplitude level is below the threshold of the limiter, the signal will pass through it, however, a portion of the signal will get attenuated in it. The level of attenuation in the signal is represented as insertion loss. The ideal limiter will have 0 dB insertion loss, however, most of the filters will have about 0.3 to 0.5 dB loss.
Flat leakage is the maximum signal output level from the RF limiter if higher RF power is applied to the input of the limiter. It is the designed RF power output from the limiter. RF designers need to select a limiter with Flat leakage lesser than the input power of the other following RF components in the RF chain.
Turn on time of the limiter indicate the time it required to offer adequate attenuation to the high input RF signal powers after it is exposed to a high RF signal above the limiter threshold. Faster turn-on time offers better protection to the following components in the chain. It is very important to choose an RF limiter with a very fast turn-on time to prevent damage to RF circuits when high power fast raising peak pulses are approaching the input of the limiter.
Recovery time indicates the time RF limiter is required to start passing the input RF signal to the output after the high RF signal energy dissipate. Slow recovery time may lead to the missing of important signals hence the fast recovery time limiters are better for the applications.
As limiters are the first component in the Rx chain, any noise or irregularities due to impedance mismatch of the limiter will get amplified by the LNA. Hence need to select a suitable RF Limiter with a perfect Impedance match.
Limiters will add internal noise to the signal and it will damage the signal quality. RF Limiter with good noise figure performance needs to select for the communication applications.
Limiters are available in Surface mount technology (SMT), components and waveguide packages. The connectorized RF limiters are with coaxial connectors which are generally available in the market. Waveguide RF limiter assemblies are hollow metallic conductors with an elliptical, or circular, rectangular cross-section.
RF Limiters are one of the prime RF components required in an RF receiver chain as many of the RF components are very sensitive to high RF power. Selecting a suitable RF limiter help to protect the RF circuits and improve the performance of the entire receiver chain.
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