  # Electronic Warfare and Radar Systems Engineering Handbook- Detectors -

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. Figure 1. Typical Diode Detector Circuit 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=V /R, Pi is proportional to Vo

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 approximately  equal 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 point on the oscilloscope becomes the measurement 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 WhenInput Power is reducedby Half (3 dB) 0.5 Vin 0.707 Vin A very small value.- 0.15 V for for typicalby 5 stage log amplifier Sensitivity &Dynamic Range Good sensitivitySmall dynamic range Less sensitivityGreater dynamic range Poorest sensitivityGreatest dynamic range(to 80 dB)

Dynamic Range Small dynamic range Greater dynamic range Greatest dynamic range (to 80 dB)
Also see Section 6-10, Microwave / RF Testing, subsection entitled "Half Power or 3 dB Measurement Point".    About RF Cafe Copyright: 1996 - 2024Webmaster:    Kirt Blattenberger,    BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website:  AirplanesAndRockets.com 