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DETECTORS

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.
To integrate a pulse radar signal, we can add capacitance to the circuit in parallel with the output load RL to store energy and decrease the bleed rate. Figure 2 shows a typical input/output waveform which detects the envelope of the pulse radar signal. From this information pulse width and PRF characteristics can be determined for the RWR UDF comparison.
When the diode is reverse biased, very little current passes through unless the reverse breakdown voltage is exceeded. When forward biased and after exceeding the cut-in voltage, the diode begins to conduct as shown in Figure 3. At low voltages, it first operates in a square law region. Detectors operating in this region are known as small signal type. If the voltage is higher, the detector operates in a linear region, and is known as the large signal type.

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

Linear Detector

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.

Table 1. Detector Characteristics

 
Square Law
Linear
Log
Output voltage when
input power is reduced
by half (3 dB)
0.5 Vin
0.707 Vin
A very small value.
Approx 0.15 Vin for typical
5 stage log amplifier
Sensitivity and
dynamic range
Good sensitivity
Small dynamic range
Less sensitivity
Greater dynamic range
Poorest sensitivity
Greatest dynamic range
(to 80 dB)

Also see Microwave / RF Testing, subsection entitled "Half Power or 3 dB Measurement Point".