A passive RF limiting circuit device providing protection for receiver front ends and the like by utilizing PIN diodes acting as switching devices which transform a low-loss T-network attenuator circuit into a high-loss .pi.-network attenuator circuit under high power conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the protection of Radio Frequency (RF) 
receiving and/or transmitting equipment from undesired RF signals which 
are of sufficient energy-levels to upset and/or damage them. The invention 
limits the undesired signal levels to sufficiently low levels to prevent 
damage and/or upset of the system being protected. The invention does not 
reflect a significant amount of the undesired signal and so is applicable 
to low-observable systems. The term limiter includes receiver protection 
device and terminal protective device. 
2. Description of the Prior Art 
There is a well established need to protect front-door microwave components 
from intense RF energy. Front-door microwave components are those 
components located along intentional RF receiving and transmitting paths 
connected to an antenna in an RF system. Conventional limiter technology 
takes advantage of a conditional short to ground which serves to reflect 
the unwanted energy back towards the source. This situation is undesirable 
for low-observable platforms, because the energy reflected from the 
limiter makes the platform observable when illuminated by an intense 
signal (such as an intense radar signal or high power microwave signal). 
This is also an undesirable situation for diagnostic equipment such as 
oscilloscopes, spectrum analyzers, etc. There is a need for this equipment 
to be protected from incidental transients, however thse transients must 
not be reflected towards the source. Present limiter technology does not 
have the ability to solve both of these problems. 
Accordingly, it is an object of this invention to provide a limiter that 
distributes power to the linear and non-linear resistive elements within 
it, thus providing a limiter with a high burnout threshold. 
It is also another object of this invention to provide a limiter with low 
insertion loss, that is small in size, and that requires little or no 
power from the system. 
SUMMARY OF THE INVENTION 
Briefly, the foregoing and other objects are achieved by a passive limiting 
device that provides as much as 25 dB of protection and 30 dB of return 
loss. PIN diodes in the limiting device act as switching devices which 
cause a low-loss T network attenuator circuit to switch into a high-loss 
.pi.-network attenuator circuit.

DETAILED DESCRIPTIONS OF SPECIFIC EMBODIMENTS 
Referring to the drawings, FIG. 1 is a top view of a microstrip version of 
a passive non-reflective limiter prototype (PNRL) 10. This prototype is 
fabricated on a 1.5 millimeter thick substrate 11 which has a relative 
dielectric constant (.epsilon..sub.r) of 2.54. The signal line plane 12 
and the ground plane 13 are made of copper 34 micrometers thick. Both the 
input port 14 and the output port 15 are matched to 50 ohms when the 
limiter 10 is in both the off and on states. Diodes D1 through D8 are type 
MA4L022-137 silicon PIN diodes. Each diode needs to see about 0.7 volts to 
begin the transition from a high impedance (1 megohm) to a low impedance 
(1 ohm). 
FIG. 2 is a schematic diagram of the passive version of the non-reflective 
limiter. The switching devices cause the circuit to transition from an 
attenuator of low value in the off state to an attenuator of high value in 
the on state. It is seen that in the off state the circuit becomes a 
T-network attenuator with the inductor L.sub.1 17 providing a finite and 
relatively low impedance at 1 Gigahertz. In the on state the circuit 
becomes a .pi.-network attenuator as the L.sub.1 C.sub.1 resonant tank 
circuit provides a high impedance at 1 Gigahertz. 
FIG. 3 shows a state 0 or diodes off frequency response plot generated by a 
computer simulation of the microstrip layout of FIG. 1. The circuit 10 
yields less than 1 dB of insertion loss from DC up to 1.2 Gigahertz. FIG. 
4 is a state 1 or diodes on frequency response plot generated by the same 
computer program for the same circuit. The circuit provides better than 25 
dB of isolation and 29 dB of return loss at 1 Gigahertz. This circuit will 
operate over an 80 Megahertz bandwidth with better than 25 dB of isolation 
and 29 dB of return loss at 1 Gigahertz. 
FIG. 5 shows the power in/power out characteristics of the passive 
non-reflective limiter 10 as simulated by a computer. The computer took 
into account the non-linear I-V characteristics of the PIN diodes D1 
through D8. The plot shows the initial insertion loss of approximately 0.8 
dB when the input power is still below the turn-on threshold. The 1 dB 
compression point is a figure of merit which is used to determine the 
onset of non-linearity in a circuit. The device begins to turn on at 
approximately 12 dBm (398 mW) and 20 dB of isolation is provided at an 
input power of 44 dBm (25.1 W). By varying circuit element values and the 
types of switches, the turn-on threshold can be adjusted. 
FIG. 6 is the simulation result for the output voltage-time waveform of the 
passive non-reflective limiter for a 1 Gigahertz, 40 dBm (10 W) input 
signal. The spectral content of this output waveform is showm in FIG. 7. 
The fundamental frequency (1 Gigahertz) component is significantly higher 
than any of the upper harmonics. The major portion of the output power is 
concentrated in the fundamental frequency. 
An active switching device with external controls adds additional 
flexibility to the design. This control can set the threshold for the 
on-state and can activate the on-state prior to the arrival of the strong 
signal. Typical active switching devices are metal semiconductor 
field-effect transistors, heterojunction bipolar transistors, etc. A 
circuit configuration with such devices is shown in FIG. 8.