Patent No. 4752730 Radiation monitor diode detector with constant efficiency for both CW and pulsed signals

 

Patent No. 4752730  Radiation monitor diode detector with constant efficiency for both CW and pulsed signals (Aslan, Jun 21, 1988)

Abstract

An electric field sensitive probe capable of measuring both CW and pulsed signals with consistent efficiency using a diode detector connected to a dipole antenna in which a shunt capacitor is connected in parallel with the dipole and one or more resistive elements are connected in series between the diode and the dipole.

Notes: 

SUMMARY OF THE INVENTION

The present invention is directed to radiation monitors utilizing diodes as sensors for making measurements on pulsed RF sources such as radar and also variable CW sources with the same degree of accuracy as measurements are now made on CW sources operating at more or less even power levels, such for example as microwave ovens. The invention is based on the recognition that diodes such as Shotky diodes used in electromagnetic radiation monitors differ in their response to low and high levels of power or power density. Low level power results in a square law detection characteristic and high power in a linear detection characteristic. This change of mode is a defect for measurement purposes and it arises from the change in video resistance of the diode with the level of RF current through the diode. The video resistance is drastically reduced at high levels of power and the resultant time constant is short, resulting in a rising wave form that tracks the pulse. When the signal being tested for radiation switches off at the end of the pulse, the diode video resistance reverts to its characteristic high value with the resultant long time constant. Thus the measurement signal now decays at a very slow rate as the capacitance of the measuring loop discharges thus maintaining the output at something close to peak value for an extended period. Low level pulsed or variable CW signals on the other hand see a uniformly high video resistance in the diode resulting in a uniformly long time constant during both the on and off periods of the pulse. In accordance with the invention a high resistance is inserted in series with the diode which is of the same order of magnitude as the low level video resistance of the diode thus limiting the level to which the circuit resistance can drop when the diode reverts to its peak detection mode.

The insertion of a high resistance in the loop increases the area of the loop because microwave chip resistors achieve high resistive values by means of long meandering resistive lines to obtain long length in a small space. This geometry results in a relatively large shunt capacitance which mitigates against its ability to reduce the induced loop current generated by the magnetic field at higher RF. This offsetting effect can be eliminated in accordance with the invention by inserting an additional resistor of much lower value in series with diode to act as a substantial resistance even at high RF and thus prevent coupling of the circuit loop into the magnetic field being measured.

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Radiation monitor diode detector with constant efficiency for both CW and pulsed signals. Filed October 1985, granted June 1988. Funded by Dept. of Navy. A device and/or method to make radio frequency detection apparatuses more reliable for detection of pulsed and/or variable continuous electromagnetic waves. Coupled with the modified version (or version as stated) in patent 5512823 should be able to detect field strength of said pulsed or variable continuous
waves and the frequency of them.