Microwave Limiters: Application Notes

DEFINITION OF PARAMETERS

Recovery Time:

The time period from the end of a high power pulse to the point where the insertion loss value has returned to within 3 dB of the quiescent loss state.

Spike Leakage:

After pulsed high power is applied, the limiter will momentarily pass significantly more power than when it is totally saturated. This power rise is seen as a spike on the leading edge of the leakage pulse. The rise time of the high power pulse and the turn-on time of the diode determine the spike’s amplitude. The spike is defined by its energy content, i.e., in ergs. The formula for calculating the spike leakage is as follows:

          SPIKE LEAKAGE (ERGS) = ts x Psx 107

where ts equals spike width at the half-power point in seconds, and Ps equals maximum spike amplitude in watts.

Power Handling:

There are two important things to consider when defining the power handling required of a limiter. Two important considerations for defining the required power handling of a limiter are:

• Peak Pulsed Power: for narrow pulses, equated to an equivalent CW power by multiplying the Peak Power  by the Duty Cycle. For pulses exceeding 10 microseconds, Peak Power is considered CW
• Source VSWR: When is it fully turned on, the Limiter short circuits across the transmission line, and 90% incident power is reflected back towards the source.

Any mismatch at the source reflects power back toward the limiter, resulting in standing waves.  In a correct limiter-source phase relationship, the maximum current point occurs at the input diode, causing the diode to dissipate a greater level of power than incident power.  For a source VSWR of up to 2.0:1, an approximate maximum effective power can be achieved by multiplying the source VSWR by the incident power.

      CONSIDERATIONS IN USING LIMITERS

• The difference between the flat leakage and the 0.1 dB compression point is typically between 10 and 13 dBm, but may vary according to limiter type
• Noise of 10 dBm may be generated following the start of limiter compression. However, limiters can and usually do exhibit signs of limiter compression at 0 dBm
• Limiters dissipate approximately 8% of incident power as heat. Therefore, all limiters should be attached to a heatsink whose temperature does not exceed the maximum rated ambient temperature
• Limiters are inherently broadband components. Band limitation results from DC return are required by some limiter designs. Limiters with bandwidths of up to 10:1 are relatively simple, while those with bandwidths exceeding 10: 1 are progressively more complex and costly.

CAUTION! Limiters are NOT bilateral components! They have a defined input and output.  Backwards installation will damage the component.