A detector is used in receiver circuits to recognize the presence of signals. Typically a diode or similar device is used as a detector. Since this type of detector is unable to distinguish frequency, they may be preceded by a narrow band-pass filter.
A typical simplistic circuit is shown in Figure 1.
The power/voltage characteristics for a typical diode detector is shown in Figure 4.
Square Law Detector
In the square law region, the output voltage Vo is proportional to the square of the input voltage Vi, thus Vo is proportional to the input power.
Vo = nVi2 = nPi or Pi is proportional to Vo
Where n is the constant of proportionality
In the linear detection region, the output voltage is given by:
Vo = mVi and since P = V2 / R , Pi is proportional to Vo2
Where m is the constant of proportionality
Log Detector Amplifier
Another type of detector arrangement is the Log detector amplifier circuit shown in Figure 5. It is formed by using a series of amplifiers and diode detectors. Due to the nature of the amplifier/diode characteristics, the output voltage is related to the power by:
Pi is proportional to 10pVo + q
Where p and q are constants of proportionality
The Log detector has good range, but is hampered by large size when compared to a single diode detector.
Pulse Width Measurements
If the pulse width of a signal was specified at the one-half power point, the measurements of the detected signal on an oscilloscope would vary according to the region of diode operation. If the region of operation is unknown, a 3 dB attenuator should be inserted in the measurement line. This will cause the power to decrease by one-half. That peak amplitude on the oscilloscope becomes the reference point for the pulse width when the external 3 dB attenuator is removed.
These voltage levels for half power using the three types of detectors are shown in Table 1.
Also see Microwave / RF Testing, subsection entitled "Half Power or 3 dB Measurement Point".