A Versatile Decibel Meter

A decibel meter is an instrument I to measure levels of input signal(s) in decibel units. The main areas of application of decibel meters are audio level indicators, record level indicators, spectrum analysers and the like.

The circuit shown here provides a resolution of 20 steps. Although the steps are linear, they are calibrated with corresponding decibel value so that it suits lost of the requirements.

The circuit is shown in Fig. 2. It is based on IC LM3914 integrated circuit intended for bar graph generator circuits. The IC has 11 op-amps, out of which one is used for buffering the input and the rest as comparators. The inverting inputs of all the comparators are tied together and linked to output of he buffer op-amp, inside the chip. The non-inverting inputs are each given a different reference voltage generated by a resistor ladder. Both the ends of these ladders are available externally at pins 4 and 6, The IC incorporates a stable reference voltage at pin 7. This reference output can be elevated by raising the voltage at pin 8, which is the reference preset input. The outputs are active low, and the current at the outputs can be determined by a load resistor connected to pin 7 and ground. IC I .M3914 has two modes of operation— bar mode and dot mode—which can be selected by applying appropriate logic at its pin 9.

The circuit incorporates two IJVB914 ICs for obtaining 20 steps. One more IC may be cascaded to obtain 30 steps. But 20 steps were found adequate for most of the practical purposes.

The divider’s lower end of IC1 (pin 4) is grounded so that the lowest levels of input signals arc detected linearly. The divider’s higher end (pin 6) is given a reference voltage of 1.25V obtained from pin 7. So each comparator switches at steps of 0.125V of the input signal. The load resistors R2 and R3 each of 1.5k limit the current through each LED to 12 mA.

The reference voltage of IC2 is made 2.5 V by raising pin 8 to 1.25 V obtained from pin 7 of IC1. The divider’s lower end is also raised to 1.25V so that signals above 1.25V are handled by IC2. The voltage at pin 7 of IC2 is now 2.5 V and is given to pin 6 of IC2 to fix the divider’s higher end of IC2 to 2.5V.

The circuit as a whole has 20 comparators, each with a reference voltage of 0.125 V. The output can be selected to bar or dot mode by applying high or low logic respectively to pin 9.

The circuit can be used for any range. The range can be obtained by attenuating the input signal accordingly. Otherwise, a multi-way switch can be used to select the range which switches over to different potentiometers, that were preset for different ranges.

The unit Bel or Decibel is only a ratio of two quantities such as voltage or power and is not an absolute value. Whenever a quantity is expressed in decibels, it should also provide a reference level to which the quantity is referred to.

Decibel values related to some parameters are given by

Power in dB = 10 log (P2/P1) where F2 is the power in question and PI is the power to which P2 is referred where V2 is the voltage in question and V1 is the voltage to which V2 is referred to.

In most cases, a power of 1 mW dissipated in 600 ohms resistor is taken as reference. That corresponds to a voltage level of 0.7745 volt. This means that 0.7745 volts is taken as 0 dB. As such, the sixth LED corresponds to

0 dB in this circuit. The decibel values for other LEDs can be calculated by the formula dB = 20 log (n x 0.125/0.77459) where n is the number of the particular LED.

To change the above values to suit someone’s requirement, divide the incoming voltage through a potentiometer as shown in Fig. 3. The choice of VR1 depends upon the input impedance required.

For using the circuit in tape decks, a well calibrated equipment should be available for pre calibration of the circuit.

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