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Chapter: RF and Microwave Engineering - Microwave Tubes and Measurements

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Measurement of Power

Power is defined as the quantity of energy dissipated or stored per unit time.

MEASUREMENT OF POWER

 

Power Measurement

 

ü Power is defined as the quantity of energy dissipated or stored per unit time.

 

ü Microwave power is divided into three categories – low power (less than 10mW), medium power (from 10mW to 10W) and high power (greater than 10w).

 

ü The general measurement technique for average power is to attach a properly calibrated sensor to the transmission line port at which the unknown power is to be measured.

 

ü The output from the sensor is connected to an appropriate power meter. The RF power to the sensor is turned off and the power meter zeroed. This operation is often referred to as “zero setting” or “zeroing.” Power is then turned on.

 

ü The sensor, reacting to the new input level, sends a signal to the power meter and the new meter reading is observed.

 

ü There are three popular devices for sensing and measuring average power at RF and microwave frequencies. Each of the methods uses a different kind of device to convert the RF power to a measurable DC or low frequency signal. The devices are the diode detector, the bolometer and the thermocouple.

 

Diode Detector

 

The low-barrier Schottky (LBS) diode technology which made it possible to construct diodes with metal-semiconductor junctions for microwave frequencies that was very rugged and consistent from diode to diode. These diodes, introduced as power sensors in 1974, were able to detect and measure power as low as −70 dBm (100 pW) at frequencies up to 18 GHz.

 

Bolometer Sensor:

 

Bolometers are power sensors that operate by changing resistance due to a change in temperature. The change in temperature results from converting RF or microwave energy into heat within the bolometric element. There are two principle types of bolometers, barretters and thermistors. A barretter is a thin wire that has a positive temperature coefficient of resistance. Thermistors are semiconductors with a negative temperature coefficient.

 

Thermistor elements are mounted in either coaxial or waveguide structures so they are compatible with common transmission line systems used at microwave and RF frequencies.

 

Power meters are constructed from balanced bridge circuits. The principal parts of the power meter are two self-balancing bridges, the meter-logic section, and the auto-zero circuit. The RF Bridge, which contains the detecting thermistor, is kept in balance by automatically varying the DC voltage Vrf, which drives that bridge. The compensating bridge, which contains the compensating thermistor, is kept in balance by automatically varying the DC voltage Vc, which drives that bridge. The power meter is initially zero-set (by pushing the zero-set button) with no applied RF power by making Vc equal to Vrfo (Vrfo means Vrf with zero RF power). After zero-setting, if ambient temperature variations change thermistor resistance, both bridge circuits respond by applying the same new voltage to maintain balance.

 

If RF power is applied to the detecting thermistor, Vrf decreases so that


Where Prf is the RF power applied and R is the value of the thermistor resistance at balance. But from zero-setting, Vrfo= Vc so that



Thermocouple Sensors

 

Thermocouple sensors have been the detection technology of choice for sensing RF and microwave power since their introduction in 1974. The two main reasons for this evolution are: 1) they exhibit higher sensitivity than previous thermistor technology, and 2) they feature inherent square-law detection characteristic (input RF power is proportional to DC voltage out).

 

Since thermocouples are heat-based sensors, they are true “averaging detectors.” Thermocouples are based on the fact that dissimilar metals generate a voltage due to temperature differences at a hot and a cold junction of the two metals. The power sensor contains two identical thermocouples on one chip, electrically connected as in Figure.


The thermocouples are connected in series as far as the DC voltmeter is concerned. For the RF input frequencies, the two thermocouples are in parallel, being driven through coupling capacitor Cc. Half the RF current flows through each thermocouple.

 

Each thin-film resistor and the silicon in series with it have a total resistance of 100 Ω. The two thermocouples in parallel form a 50 Ω termination to the RF transmission line. The lower node of the left thermocouple is directly connected to ground and the lower node of the right thermocouple is at RF ground through bypass capacitor Cb. The DC voltages generated by the separate thermocouples add in series to form a higher DC output voltage. The principal advantage, however, of the two thermocouple scheme is that both leads to the voltmeter are at RF ground; there is no need for an RF choke in the upper lead. If a choke were needed it would limit the frequency range of the sensor. For a square wave modulated signal the peak power can be calculated from the average power measured as where T is the time period and Շ is the pulse width. Y P peak av P.



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