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.
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