RF Limiter

by Sanjay Mishra

RF Limiter

A RF Limiter is used to limit the RF power going into a circuit. Most commonly it is used in RF transceivers. In radio transceivers the transmitter portion and the receiver portion share the same antenna. When the transmitter is operating it delivers power that can range from milliwatts to kilowatts (depending on the radio's output power) to the antenna.

The transmitter output can couple to the receiver's input by antenna reflections, leakage or by other means. Even a a small amount of transmitter power would damage the radio receiver's sensitive input circuitry. A radio receiver normally has a sensitive low noise amplifier at the front end to amplify far off weak signals. A RF limiter would protect the radio receiver by limiting the RF signal which it can see.

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Single Stage RF Limiter

Single stage RF limiters typically comprise of a PIN diode in parallel with a RF choke. Capacitors are used to provide DC blocking

C1

|

 PIN

  Diode  

|

|

Gnd  


C2

|

RF

Choke

|

|

Gnd  


Depending on the design, the power incident on the PIN diode can be largely reflected. If this is the case, a PIN diode with a few hundred milliwatts of dissipation capacity can typically protect against much larger incident RF power. The reflected power can be dissipated by re-radiation through the antenna or by a resistive load connected through a circulator or isolator

Depending on the input frequency and the PIN diode characteristics, the circuit above can typically provide 20-30 db of isolation. If more isolation is required, for example in high power transceivers, multistage RF Limiter must be used.

Multistage RF Limiter

A multistage RF limiter comprises of a single Stage RF limiter with additional PIN diodes in parallel to the ground. The choice and placement of these additional PIN diodes is critical.

The PIN diode with the thicker I layer is placed towards the input of the RF limiter. The PIN diodes are placed a quarter wavelength away from each other.

When a RF wave is incident it passes through the blocking capacitors and impinges on the PIN diodes. The last PIN diode has the thinnest I layer and switches first. A standing wave with its maximum at the PIN diode up the chain is created. This PIN diode now switches.

The spike and leakage characteristics of the RF limiter are determined by the last PIN diode while the power handling capability is determined by the PIN diodes up the chain.

Most applications use 1, 2 or 3 stage RF Limiters.

Extreme Protection

If the receiver input circuitry is very sensitive even a small amount of RF input power can damage it. In this case a multi stage RF Limiter can be used with its last PIN diode biased (by a RF coupled Schotty diode or similar circuit) so that the PIN diode trips even for a small amount of incident RF power. This trip point can be adjusted depending on the 

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