RF power is not always easy to measure. There are several methods of measuring RF power, each one having its own advantages and disadvantages. Accordingly the type of RF power sensor used will depend upon the type of signal to be measured. Some types of RF power sensor technology will be more applicable to low powers, others to modulation techniques where the envelope varies and so forth.
Typically an RF or microwave power meter will comprise a unit where all the control and processing circuitry is contained, but the power itself will be detected in what is normally termed a sensor or "head". Thus it may be possible for a power meter to utilise one of a number of power heads according to the exact requirements, particularly with respect to power.
It is important to note that power meters act as a load for the RF power which is absorbed by the head. This high power meters have large loads that can dissipate the required level of power. Alternatively a small portion of the power can be extracted by means of a coupler, or by using a high power attenuator so that the power rating of the RF power meter head is not exceeded.
RF power sensor technologies
The RF power sensors are the key element of any RF power meter, and the choice of the type of sensor will depend of the likely applications that are envisaged. The RF power meter technologies fall into one of two basic categories:
- Diode detector based
- Heat based
Although both varieties of meter have been available for many years, both technologies have been greatly refined over the years and are able to meet very high levels of performance. In view of their different attributes, they are also used in different types of application.
Diode detector based RF power meter sensors
The most straighforward form of RF power sensor used in RF power meters, employs diode rectifiers to produce an output. RF power sensors using diodes are designed so that the sensor dissipated the RF power in a load. A diode detector then rectifies the voltage signal appearing across the load, and this can then be used to determine the power level entering the load.
It is this form of power sensor that is used in many basic analogue power meters, although the technology is also used in high performance power meters.
Diode based RF power sensors have two major advantages:
- The first is that they are able to measure signals down to very low levels of power. Some of these diode based RF power sensors are able to measure power levels as low as -70 dBm. This is much lower than is possible when using heat based RF power sensors.
- The other advantage of diode based RF power meter sensors, is the fact that they are able to respond more quickly than the heat based varieties. In some older power meters, the output from the diode RF power sensor will be processed in a simple way, but far more sophisticated processing of the readings can be made using digital signal processing techniques. In this way the readings can be processed to give the results in the required format, integrating over time is necessary, or having faster, more instantaneous readings if needed.
Although the basic principles of operation of diodes as detectors are well known, the design of diode detectors presents some challenges when designing accurate test instruments. The first is that the stored charge effects of ordinary diodes limit the operating range of the diodes. As a result, metal semiconductor diodes - Schottky barrier diodes - are used in RF power sensors as these diodes have a much smaller level of stored charge and they also have a low forward conduction turn on point.
Despite the low turn on voltage of the Schottky diode (0.3 volts for silicon), this still limits the lowest signal levels that can be detected - a signal of around -20dBm is required to overcome this voltage. One approach is to AC couple the diode and apply a 0.3 volt bias, but this only increases the sensitivity by around 10 dB as a result of conduction noise and drift introduced by the bias current.
Today, Gallium-Arsenide (GaAs) semiconductor diodes are now often used because they provide superior performance when compared to silicon diodes. The gallium arsenide diodes used in RF power sensors are typically fabricated using planar doped barrier technology, and although the diodes are more costly, they provide significant advantages for power sensors at microwave frequencies.
RF and microwave diode power sensors are often the sensor of choice. The output from the diode can be processed using advanced digital signalling processing. This means that a single sensor is able to provide a large variety of capabilities that would not be possible with heat based sensors. With diodes detecting the envelope, a variety of waveforms can be measured.
Heat based RF power meter sensors
As the name suggests, heat based sensors dissipate the power from a source in a load and then measured the resulting temperature rise.
The heat based RF power sensors have the advantage that they are able to measure the true average power as the heat dissipated is the integral of the power input over a period of time. As a result these RF power sensors measure the RF power level independent of the waveform. Thus the measurement is true regardless of whether the waveform is CW, AM, FM, PM, pulsed, has a large crest factor, or consists of some other complex waveform. This is a particular advantage in many instances, especially as QAM, and other forms of phase modulation are being increasingly used and a these do not have a constant envelope.
In view of the time constant with these RF power sensors, they are not suitable for measuring instantaneous values. Where these measurements are required, other types of sensor may be more suitable.
There are two main technologies that are used:
- Thermistor RF power sensors: Thermistor RF power sensors have been widely used for many years and provide a very useful method of enabling high quality RF power measurements to be made. While thermocouple and diode technologies have become more popular in recent years, the thermistor RF power sensors are often the RF power sensor of choice as they enable DC power to be substituted to enable calibration of the system.
The thermistor RF power sensor uses the fact that temperature rise results from dissipating the RF in an RF load. There are two types of sensor that can be used to detect this temperature rise. One is known as a barretter - a thin wire that has a positive temperature coefficient. The other is a thermistor - a semiconductor with a negative temperature coefficient which may typically only be around 0.5 mm in diameter. Today only thermistors are used in RF power sensors.
A balanced bridge technique is normally used. Here the resistance of the thermistor element is maintained at a constant resistance by using a DC bias. As RF power is dissipated in the thermistor tending to lower the resistance, so in turn the bias is reduced to maintain the balance of the bridge. The decrease in bias is then an indication of the power being dissipated.
Today's thermistors sensors contain a second set of thermistors to provide compensation against changes in the ambient temperature that would otherwise offset the readings.
- Thermocouple RF power sensors: Thermocouples are widely used these days in RF and microwave power sensors They provide two main advantages:
- They exhibit a higher level of sensitivity than thermistor RF power sensors and can therefore be used for detecting lower power levels. They can easily be made to provide power measurements down to a microwatt.
- Thermocouple RF and microwave power sensors possess a square-law detection characteristic. This results in the input RF power being proportional to DC output voltage from the thermocouple sensor.
- They provide a very rugged power sensor to be made - they are more rugged than thermistors.
The principle of a thermocouple is simple - junctions of dissimilar metals give rise to a small potential when placed at different temperatures.
Modern thermocouples as used in RF and microwave power sensors are typically designed within a single silicon integrated circuit chip. They detect the heat dissipated as a result of the RF signal in the load resistor.
Many RF power meters provide the facility for a variety of radio frequency power sensors to be used dependent upon the exact nature measurements to be made. While the heat based RF power sensors are more applicable to applications where an integrated measurement is required, diode based ones are more suitable where low level or instantaneous measurements are needed. Accordingly it is necessary to choose the sensor dependent upon the foreseen applications.
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