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Friday, 10 May 2019

Audio Frequency Power Meter

AF Power Meter

Measurement of power at audio and radio frequencies is largely different from power measurement at power frequencies. The frequency and matching of the load impedance play important role at frequencies other than power frequencies.

For audio- frequency power measurement a rectifier type voltmeter will be used choosing proper diodes for the rectifier that offer minimum capacitance. A standard resistance of 600 ohm will be taken and the rectifier type voltmeter will be connected across it.

When 1 W of power is dissipated across this resistance the voltmeter reads 0.775 V. This can be taken as 1 mW over the scale of the voltmeter. This principle is extended to design a Power Output Meter. Such an arrangement is useful only in the frequency range from 20 kHz to 20 kHz. If accuracy is not important its range can be extended up to 100 kHz.

The rectifier type voltmeter cannot be used for Radio Frequency Power Measurement. The reason is the effect of capacitance of the rectifier, the inductance offered by the moving coil and the power consumed by resistor at Radio Frequencies.

Replacing the rectifier instrument with the thermocouple instrument can improve the situation. Even the thermocouple instruments cannot offer error free performance beyond 100 kHz. Special measurement techniques are used for high frequency power measurements.
In this blog the following topics are covered :

1. Measurement of audio frequency power
2. The A.F. Power Output meter
3. R.F. Power measurement using thermocouple instrument
4. Bolometer method of R.F. power measurement 

The Audio Frequency Power Output Meter

The Audio Frequency Power Meter is one that measures directly the absolute power in mW or in dB, with reference to a chosen signal level, derived by an audio frequency system, in to a variable external load. The load will be incorporated in the power output meter itself. 

(a) Description of the Block Diagram of the Power Output Meter :

The block diagram of the audio frequency power output meter is shown in Figure. It consists of a variable ratio transformer, a loss adjusting network, meter multiplier and the indicating meter. The indicating meter is nothing but a rectifier type of voltmeter calibrated in terms of dB, with reference to the power allowed in to a load of known impedance contained in the instrument itself and is substituted in place of the original load impedance where the power is developed.

The rectifier type of voltmeter is calibrated from 1-50 mW with an auxiliary scale, reading from 0-17 dB, above the reference level of 1 mW. The input impedance of the meter ranges from 2.5 ohm to 20 k ohm, consisting of forty steps, each step providing 25% of increase.

The internal arrangement of the instrument is such that the variable ratio transformer and the loss adjusting network put together will match the known value of standard resistance across which the voltmeter is connected. Therefore, for the different input impedance settings the meter is matched and will give the power output directly in watts.

The range selector switch changes the voltmeter's multipliers to get the higher range of power. 

(b) Measurement Method :

To measure the output power of an audio frequency source the regular load to the source will be disconnected. Knowing the load value the power output meter's impedance switch will be adjusted to the load value. The range selector switch will be adjusted such that the required multiplier is connected to the meter. Now the indication given by the meter is the power output given by the source. 

(c) The dB Scale :

The power output meter's scale will also be calibrated in decibels. One milli watt (1mW) is taken as the reference power and this is taken as '0' for the dB scale as the power output meter is frequently used for measurement of the power output of radio receivers during the standard tests etc., The 50 mW power is taken at the lowest useful power output from a receiver or the like to be of practical value of the listener. 50 mW is calibrated as 17 dB and powers less than 1 mW are given the negative reading in dB.

The decibel scale can be used for direct measurement of the gain in dB provided that the impedance of the power output meter is properly adjusted for the load impedance of the source giving the signal. The dB scale is graduated to read from -10 dB to +17 dB with a reference of '0' dB at 0.775 V which is the voltage that is developed across the standard resistance of 600 ohm when 1 mW of power is dissipated across it. To read dB over the scale the range selector switch will have to be kept in '0' dB position. For all other range switch settings algebraic sum of the scale reading and the range switch indication must be taken. 

(d) Design Variations :

Transistorized power output meters have been designed with better accuracy over a better frequency range. These use a high power wire wound resistor of the same value of the load of the source in place of the load. Different values of such resistances are made available which could be selected by a rotary switch and multiplier switch.

The voltage across the resistor is given as input to a 'transistor amplifier which drives the meter. An attenuator will also be ganged with the resistance selector switches to provide a direct reading of output power on the meter. The frequency range of such output meters can be extended up to 100 kHz with a little reduction in the accuracy.

The current trend is to use a digital power output meter that displays numerically the power output over a display. 

(e) Applications of A.F. Power Output Meter :

It can be used for the measurement of audio power in the output of radio receivers, audio stages of T.V. receivers and Public address amplifiers.
It can be used as the output indicator in conducting standard tests on communication receivers and radio receivers.
It can be used as a null indicator in A.C. bridges working at audio frequencies.
It can be used as an output indicator in transmission measuring set in telephone lines.
It can be used to determine the source impedance of the equipment under test by finding the load in to which maximum power is delivered. 

(f) Specifications of Audio Power Output Meter :

Power range: 0.1 mW to 10 W in four ranges
Impedance range: 2.5 ohm to 20 k ohm in 40 steps
Impedance Multiplier: xl, x10, x100 and x1000
Frequency range: 20 Hz to 20 kHz useful with reduced accuracy up to 100 kHz
Power supply: 230 V 50 Hz A.C.

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