Applications of microwave detector circuits

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					Applications of microwave detector circuits
Detectors are used to convert amplitude-modulated microwave signals to
baseband (or video) signals. A crystal radio is the original example of a detector;
here a crude point-contact diode was used to rectify a AM modulated signal back
to baseband so the listener could take the A-train to Harlem with Count Basie
over headphones if the room was quiet enough. Early point contact diodes were
made from a galena crystal (lead sulfide) and a metallic pin known as a "cat's
whisker".

Two applications of detectors that are important in your laboratory are power
heads (the business end of a power meter, check out our page on power meter
measurements) and scalar network analyzers. Using a swept frequency source, a
dual-directional detector and a three detectors, and a computer operating a Lab
View A/D interface, you can construct a poor-man's scalar network analyzer, and
evaluate circuit gain (or loss), as well as port impedance match. You too can
open your own Microwave Monster Garage!

As you may have guessed, at the heart of the radar detector you use in your car
to avoid speeding tickets uses a microwave detector circuit.

Basic detector circuit and terminology
Here is a schematic of a simple detector circuit. The heart of the circuit is the
detector diode, whose non-linear behavior is what causes the "detectitation" .
Various types of detector diodes will be described below.




                                 Basic detector circuit

Detector diode
The diode rectifies the incident power, providing a signal that is of one polarity
(either all positive or all negative) to the bypass capacitor, with an amplitude
proportional to the input power level (square-law). For the detector circuit shown
in our figure, a positive voltage will be developed. Typical detectors provide a
negative voltage, which would occur by reversing the diode in the schematic.

DC return
In order for a detector to generate a DC voltage, a DC return must be supplied.
This is typically done by placing an RF choke (shunt inductor) across the detector
diode; at RF frequencies the inductor looks like an open circuit and has no effect,
at video frequencies it provides a low-impedance path to ground.

Video capacitor
A bypass capacitor forms an RF ground for diode. It also is provides what is
known as the video capacitance (CV) of the detector circuit. This capacitor
determines the upper frequency limit of the video signal's bandwidth (the
detector will work down to a video frequency of 0 GHz (DC),which is what
happens when your input signal is a continuous wave (CW). The video
bandwidth is related to the minimum rise and fall time of the detector circuit,
and how short an RF pulse you can detect. At the video frequency, you want the
video capacitor to look ideally close to an open circuit. To calculate capacitive
reactance on our calculator, click here!

Input matching network
The diode equivalent circuit is never a good match to fifty ohms, so some
overpaid microwave engineer like you had to synthesize a network that would
transform it to something close. Usually a diode that is "turned on" will behave
like less than 50 ohms, so an impedance transformer is used to step up its
impedance.

Below some other terms are defined that you will need to know when you
specify a detector.

Square-law range
For a certain range of power levels, a detector's output voltage is proportional to
its incident power measure in watts. Why is this called "square law"? In "linear"
operation, Ohm's Law says that voltage should be proportional to the square-
root of power. Thus, in the square-law region, power's relationship to voltage
has been squared.

Open circuit voltage sensitivity (K)
The ratio of output voltage to incident power is a constant in the square-law
region for detector diode. Units for K are millivolts per milliwatt; a typical
detector might provide 500 millivolts per milliwatt.

Negative versus positive detectors
Depending on which way a detector diode is grounded, the video signal is either
positive or negative voltage. Most detectors you will find in your lab are negative
detectors. If looking at negative voltages on your oscilloscope is making you
seasick, push the "invert display" button on the scope!

Video resistance
Video resistance is real part of the "dynamic" output impedance of a detector (at
its video port). You can't measure this with an ohmmeter, but you can with a
voltmeter and a resistor. With an incident CW signal incident on the the detector,
find a series resistance that decreases the output DC voltage by half. The video
resistance will be equal to this value.

Bias voltage
The sensitivity of a diode to detecting weak AM signals can be improved by
adding just a wee bit of DC voltage to move the operating point slightly closer to
forward conduction. Most detectors are not biased; they are referred to as "zero-
bias detectors".

Level detectors
The previous discussion was on analog detectors, that is, a device that outputs a signal
that is somewhat proportional to a power level (depending on whether it is in square-law
range). A level detector uses a conventional detector along with a comparator circuit, to
create a binary output signal that indicates when a threshold power is exceeded. In order
to avoid fluctuating output when the detected signal is near the threshold, hysteresis is
often added to the comparator circuit, typically on the order of 0.2 dB referenced to the
input power level. Another term used for a level detector is a high power indicate (HPI)
circuit.

				
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Description: Detectors are used to convert amplitude-modulated microwave signals to baseband (or video) signals. A crystal radio is the original example of a detector; here a crude point-contact diode was used to rectify a AM modulated signal back to baseband so the listener could take the A-train to Harlem with Count Basie over headphones if the room was quiet enough. Early point contact diodes were made from a galena crystal (lead sulfide) and a metallic pin known as a "cat's whisker".