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Microwave Detector
Detector is a
two-terminal device that rectifies an RF signal (like a diode rectifies an ac
signal for a power supply). Detectors are used as the receiving element for
amplitude modulated signals.
Detectors are
nonlinear semiconductor
diodes that generate, mix, detect, and switch microwave signals. Detector was
used for the first time as receiver of crystal radio to rectify amplitude
modulated signal. It had a galena (PbS)
crystal and a metal pin called "cat's
whisker" that exhibited Schottky effect on this
metal/semiconductor junction.
Marconi used the
detector “Coherer” in 1902 to receive Morse coded electrical signal across the Atlantic. The detected weak RF signal did something like micro-welding
on metal filings in Coehrer, which became electrically conductive.
Most diodes used in
the microwave industry are made on Silicon, but for some applications gallium
arsenide is a better choice.
Applications: Detectors convert amplitude-modulated μwave signals to baseband and so they are
used for μwave power measurements. They are also used for scalar network analyzer to
evaluate circuit gain as well as port impedance match. Detecor diodes are used as amplifiers, oscillators at µwave freq,
for devices like airborne collision avoidance radar, anti-lock brakes, motion and traffic detectors, traffic
signal controllers, car radar detectors, distance traveled recorders, slow-speed (22m/sec)
sensors, automatic door openers, process control equipment to monitor
throughput, burglar alarms, sensors to avoid derailment of trains, remote
vibration detectors, rotational speed tachometers, moisture content monitors.
In general, diodes
will conduct when the anode voltage is higher (more positive) than the cathode
voltage. An exhaustive list of the important diodes is:
Varactor diodes
Step recovery diodes
Noise diodes
Schematic of a detector circuit: The heart of the circuit is the
detector diode, whose non-linear characteristics facilitate the process of
detection.
Operation:
By rectifying the
incident power, the diode produces a signal of single polarity whose amplitude
is proportional to the input power level (square-law) and which gets applied to the
bypass capacitor. This detector circuit gives a +ve voltage and if a -ve
voltage is desired, the diode has to be reversed.
To obtain a dc voltage
from the detector, a dc return path is created by placing an RF choke across
the detector diode. This inductor offers a low-impedance path to ground at lower
frequencies but at μwave frequencies it behaves like an open circuit.
The bypass
capacitor grounds μwave frequencies and determines the upper limit of the
signal bandwidth. It provides video capacitance to detector circuit that works even
at 0 GHz, i.e. when input is a continuous wave. The video bandwidth is linked with
minimum rise and fall time of the detector circuit, and the minimum width of
detectable RF pulse.
The
input impedance of a diode when it is switched on, is <50 Ω, so an impedance
transformer that raises its impedance, precedes it.
For
a certain range of power levels, the output voltage of a detector is
proportional to its incident power. In linear operation, as per Ohm's law the
voltage is proportional to the square-root of power. Thus, in the square-law
region, power is proportional to the square of voltage. The ratio of
output voltage to incident power is a constant in the square-law region for
detector diode, typically value is 500 mV/mW.
Types of detector diodes: Schottky or Esaki tunnel diodes are
used as detectors. The two ports of a detector are the RF port and the video
port. A coaxial detector might have an SMA connector on the RF port and a BNC
connector on its video port. The video port may not contain RF frequencies if
its RF signal is rectified AM-modulated.
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