Simple Variable Notch Filter Circuit

This circuit features a special filter that can be tuned to eliminate particular frequencies of sound.

One can change it to suit its needs which makes it helpful.

This filter is also known as a band reject filter or band stop filter.

These filters primarily eliminate or reduce specific noises within a given range.

Both higher and lower sounds are permitted to go through.

A particular kind of filter that can both block out any sound frequency and be tuned to target different frequencies is required in electronic music.

The all pass notch filter with a changeable notch frequency is used for this purpose.

Circuit Working:

Parts List:

Throughout the whole range of sound frequencies the rejected frequency is shifted by the stated variable notch filter circuit over a 10kHz range.

The voltage provided to the control input controls where the notch is placed.

Above the acoustic range IC1 and IC2 both operate as all pass filters with a flat frequency response.

However they cause the input and output signals to phase shift by 0.5 CR.

Eventually this phase shift nears 180 degrees assuring that signal cancelation happens when the input and output are combined.

The variable notch filter circuit in other words functions as an all pass notch filter allowing all frequencies with the exception of 0.5 CR.

By acting as voltage controlled resistors the two transistor circuits (Q1, Q2 and Q3, Q4) allow the notch frequency to be changed in response to changes in the control voltage.

Formulas:

The IC LM324 is powered by external components connected to its input and output pins.

The following formulas are used to operate op-amp circuits:

Amplifier in reverse:

The input voltage Vin and output voltage Vout of an IC LM324 op-amp set up as an inverting amplifier which is connected using the formula below:

V_out = – (V_in * R2) / R1

where,

• The inverting input terminal is connected to ground by the resistor R1.
• Between the output terminal and the inverted input terminal lies the resistor R2.

Non Inverting Amplifier:

When a non-inverting amplifier configuration is used the formula changes to:

V_out = V_in (1 + R2 / R1)

• The inverting input terminal is connected to ground by the resistor R1.
• Between the non-inverting input terminal and the output terminal lies the resistor R2.

Amplifier with different:

For differential amplifier designs the calculation is complex and depends on the specific circuit design.

Important Information:

These calculations assume perfect op-amp features such as unlimited gain and zero input offset voltage.

In actual circuits these components could somewhat affect the output voltage.

Because the IC LM324 has four different op-amps one can use these formulas to build separate amplifier circuits for each op-amp inside the IC.

For particular functions (filtering, oscillation etc.) which goes beyond basic amplification users will need to refer to circuit schematics and design standards.

These will specify which configurations and component values are needed to achieve the desired result.

How to Build:

To build a Simple Variable Notch Filter Circuit follow the below steps for connections:

Gather the Parts:

• Using the schematic design as a guide gather all necessary components.
• Check the resistor, capacitor and other component values shown in the schematic.

A diagram Structure:

• Get a circuit diagram for the variable notch filter.
• Electronics reference books and the internet both provide appropriate schematics.
• The relationships between components and their values will be specified in the schematic.

PCB Configuration:

• For temporary circuit construction prepare a PCB.
• Using jumper wires to connect the components arrange the parts on the breadboard as shown in the diagram.

Configurations for Op Amps:

• Use the diagram to arrange the operational amplifiers IC1 and IC2.
• Make sure the power source is properly grounded and connect the positive and negative pins.

Circuits for transistors:

• As shown in the schematic diagram arrange transistor circuits Q1, Q2, Q3 and Q4.
• Use voltage controlled resistors for the variable notch frequency and make sure all connections are correct.

Initial Connection:

• To change the notch frequency connect a preset to the control input.
• The voltage used to change the rejection frequency will be managed by this setting.

Testing:

• Turn on the circuit and check the output using an oscilloscope or a signal generator that has an audio amplifier.
• Watch how the notch frequency varies to make adjustments to the preset.

Adjusting:

• To get the required notch frequency range fine tune the circuit by modifying the values of the resistor, capacitor or preset settings.

The enclosure:

• After the circuit functions properly on the PCB one might want to move it to a more permanent configuration and cover it with an enclosure.

Records:

• For future reference note any changes and the values of the component adjustments performed.

Note:

• Remember it is important to carefully follow the circuit diagram, check connections twice and take proper safety measures while working with electrical components.
• If somebody is unfamiliar with electronics it is best to get help from someone with more experience.

Conclusion:

In audio processing the Simple Variable Notch Filter Circuit is a flexible circuit that may lower and focus on different frequencies based on the specific requirements of the application.

References:

High-speed notch filters

Datasheet 2N3904