RF Detector or Microwave Detector - Design, Characterization, Applications, Parts

by Sanjay Mishra

An RF detector or microwave detector is not only the simplest but also the most useful of RF and microwave elements. It can be constructed using discrete Schottky diodes or using one of the many integrated chips available.

Design, Characteristic Parameters, Applications, Parts

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Design

Here is the block circuit diagram of a rf or microwave detector

RF-->Matching Network----> Diode/IC-->Low pass filter--> Voltage out

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 Printed transmission line of length 

 equal to quarter wavelength of RF signal of interest

                     OR

          RF inductor acting as choke

             to provide DC return

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                      |

                     GND

The matching network ensures that the maximum possible power reaches the diode or the Detector IC. The matching network can be constructed of transmission lines of varying widths and lengths or of inductor and capacitor circuits.

Once the RF signal reaches the diode/or integrated chip it is rectified. The rectification may be half wave rectification in which case only the positive or negative half of the Rf signal is used or it may be a full wave rectifier.

The low pass filter takes out ripples and produces a voltage proportional to the incident RF wave. Often a single capacitor acts as the low pass filter.

The interesting thing is if the pass band of the low pass filter allows only DC or very close to DC pass through, we have a RF peak detector that is used to measure incident RF power. If the pass band of the low pass filter is above the voice band, we have an envelope detector that can be used to demodulate simple AM transmissions. A crystal radio works on this principle.

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Characteristic Parameters

Sensitivity is the most important parameter. It is defined as the output voltage produced for a given incident RF power level and for a given load resistance.

Tangential sensitivity (TSS) is the lowest incident RF power for which the device produces an output voltage above the noise floor of the device. RF detectors with TSS ranging from -60 dbm to -30 dbm are commonly available.

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Applications

Testing and debugging RF and microwave circuits

It is much easier to test using a voltage display coupled to a RF detector than to lug a RF power meter or a spectrum analyzer around.

All the simple spy bug or cell phone detectors :-)

They all use some form of a RF detector to produce a voltage. Of course, below the noise floor spy bugs can be constructed which would be undetectable by the simple spy bug detectors that you can buy on the market.

If you have a legitimate reason to detect a below the noise floor bug or to construct a below the noise floor bug contact us.

To control the output power of a transmitter power amplifier

A RF detector connected at the output of the transmitter power amplifier provides feedback to microcontroller, baseband processor or DSP which enables the controller to regulate the transmitted output power. There are many situations where this is especially important:

Many power amplifiers, especially in mobile radios, are directly connected to the battery to save power. As the battery voltage goes down, so does the transmitted power.

A mobile radio usually transmits at minimum power required to establish a reliable link. Transmitting at minimum power not only conserves the battery but also reduces interference with nearby cells.

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Parts to construct

RF Detectors can be constructed out of discrete diodes. One issue with discrete diodes is that their output is dependent on their temperature. Integrated chips that have temperature compensated diodes, internal band gap voltage references and ability to provide voltage output based on the Root Mean Square of the incident RF waveform are available. To learn more, contact us.

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