Detectors

Updated January 22, 2006

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Here's a clickable index for this page:

Applications of detectors

Basic detector circuit and detector terminology

Level detector

Types of detector diodes

Tunnel diodes (separate page)

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.

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".

More definitions are coming soon:

RF bandwidth and video bandwidth

Flatness versus frequency

Rise/fall time

Maximum input power

Video bandwidth

Tangential sensitivity

VSWR

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.

Coming soon: a schematic of a level detector! We've got a good one, but there never seems to be enough time to create figures for this stinking web site!

Types of detector diodes

Diodes used in detectors can be Schottky or Esaki tunnel diodes. The two ports of a detector are the RF port and the video port. As a piece of lab equipment, a coaxial detector might have an SMA connector on the RF port and a BNC connector on its video port. BNC cables work great for video signals, provided the bandwidth is only hundreds of megahertz. "Video" is an arcane RF term that comes from television, where signals are broadcast at UHF frequencies. Note that the video port of a detector may or may not contain RF frequencies depending on whether the RF signal is AM-modulated. It will contain a DC term since a detector works by rectifying RF (one side of the sine wave is eliminated). An excellent source for microwave detectors for all-around lab use? Try Krytar of beautiful Sunnyvale, California.

Types of detector diodes

Tunnel diodes

Schottky diode

Planar-doped barrier diode