Updated May 20,
to go to our main page on diodes
A detector is a two-terminal device that is used to rectify an RF signal (like a diode rectifies an alternating current signal in a power supply). Detectors are used as the receiving element in amplitude modulation schemes among other uses.
Here's a clickable index for our material on detectors:
History of detectors
circuit and detector terminology
Types of detector diodes
diodes (separate page)
History of detectors
Detectors have been around since the earliest attempts at the parlor trick of wireless transmission of electrical energy. Marconi's incredible feat in 1902 of receiving a Morse code signal across the Atlantic used what was known as a coherer as the receiver. Coherers are not exactly detectors but perform the same function. The coherer is filled with sharp metal filings, and starts to conduct when a weak RF signal is passed through it, in a process akin to micro-welding. Unfortunately it keeps on conducting after the RF is absent, hence the "decoherer" which repeatedly slaps against it and loosens the filings back to their non-conductive state. The DC current is passed to a pair of headphones, and in a sound-proofed room the operator waits for a ridiculously weak audio signal and writes it down, then solves the message like a puzzle for all of the letters he missed. The coherer below is displayed at the Twin Lights Museum in Highlands New Jersey, site of a 19th century Marconi wireless station.
Coherer and decoherer
Not long into the history of radio the "cat's whisker" detector was discovered and enabled a million hobbyists to to build a radio receiver, the beginning of the long distance social interaction that eveyone seems to think is so new and exciting in the Facebook age. Soon Reginald Aubrey Fessenden figured out how to broadcast voice and music, and an industry was born.
radio is the original example of a true 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". Detection by a cat's whisker can be attributed to the Schottky effect. Walter Schottky figured out the physics behind this type of metal/semiconductor junction, and now an annual prize is offered in his name for the very best works in physics.
John Ambrose Fleming, a British electrical engineer (if such a title existed back then), invented the two-terminal diode tube (or valve) in 1904, which could be used as a detector, providing little direct improvement over the crystal detector for radio and with the added complication that it consumed power to heat the filament, of course as the first electronic diode the implications were enormous for power supplies. Edison had "discovered" vacuum tubes years earlier in his quest to build a marketable light bulb but had not recognized the implications nor the physics behind the phenomenon. However, the term "Edison Effect" took hold for this discovery. Fleming's outstanding contribution is that he was on the right track and his work was soon fully leveraged by others....
It wasn't long before vacuum tubes took over for crystal detector. Soon radios were produced that had "loud speakers" such that everyone in the room could enjoy a broadcast. Lee De Forest's three-terminal "audion" (patented 1906) became the go-to detector for signal corps efforts during the Great War, at least for the Allied side. The name suggests that De Forest was interested in audio reception for telephony and had no idea about the RF properties of his enormous contribution to mankind (it has to be in the top ten inventions of all time, up there with the wheel and arguably above the transistor as it came first and changed society far more). By 1914, Edwin Armstrong recognized that the audion could be used as an radio-frequency amplifier as well and patented the regenerative receiver (the world's first RF amplifier!), which created one of the greatest patent battles in history and no doubt contributed to Armstrong's 1954 suicide.
Here's some new perspective on the audion, by someone who is old enough to remember why it was an innovation (thanks to OAH!)
...don't know whether you ever contemplated it or not but the reason that telegraphy was king for so many years was that by use of a relay they in effect had an in-line amplifier... which audio (the telephone) did not. So they had communications, coast-to-coast, and inter-continental until that famous "audion" was invented. And those early ones were notoriously unstable....they would break into self-oscillation at the drop of the hat. It was only when some smart guy thought to insert a second or third grid (tetrodes and pentodes) between the signal grid and the plate that stable audio and RF amplifiers were finally born. And still, in those early days, if you wanted to make a coast-to-coast telephone call you would so inform the operator, who would tell you to stand by, during which time she phoned from relay station to relay station, setting the individual gains just right (both ways) after which she would phone you and tell you that the circuits were ready. My Gawd!.....now you can use your cell-phone for the same thing.....almost instantaneously.
Another interesting feature....right up through WWII the big intercontinental high-powered radio transmitters used power triodes.....all my radio transmitters in Nanking after WWII used these, but we had a tune-up process which we would use, sending a small signal thru these power triodes, and then tweaking a oscillation-preventing feedback capacitor just right, before applying the full power....it worked every time!
And except for the output audio amplifier and the all-purpose rectifier tubes my family cabinet radio (RCA) uses triodes (27's) exclusively.
Another little item of the good old days....up thru the mid-20-s cabinet radios operated on battery power. There were three batteries available: The "A" battery which powered all of the filaments (usually 6 volts); the "B" battery which delivered 90 volts...you'd use two of these if your set required 180 volts; and the "C" battery which produces whatever bias your control grids required. This last battery had a number of outlets, ranging from 1/2 volt to 4-1/2 volts, each voltage available via those miserable Fahnstock binding clips. And every cabinet radio had a separate shelf for the placement of these batteries.
Something else you might be interested in....some smart guy invented a very sensitive RF receiver using a single tube......the Regenerative Receiver. The device had what was called a "squelch" circuit which would periodically turn the single tube on and off, at a rate sufficiently higher than the audio range to prevent interference with the output audio. When the tube was turned on it would build up from noise, heading for self-oscillation, but turned off before it ever got to that level. If the tuner was tuned to a radio station signal this signal would be amplified, since it was well above the noise level. And can you believe it....when Eye was in Boston attending the Harvard and MIT radar schools, Eye saw two of these old one-tube sets for sale in a radio store in Harvard Square....there were no other sets available, what with the war on.
of microwave detector circuits
Detectors are used to convert
amplitude-modulated microwave signals to baseband (or video) signals.
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.
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". Yes we made up that word and no don't use it at a design review. Various types of detector diodes will be described
Basic detector circuit
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.
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.
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
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.
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
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
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 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.
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
Maximum input power
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)
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!
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).
Planar-doped barrier diode