Voltage Controlled Filter

This article shows you how to make a voltage controlled filter. You can see the circuit working in the simulations.

This circuit works on a simple principle of diode capacitance changing with the biasing current and voltage controlled by variable resistor, Rbias2.

You can check the specifications/datasheets for MV2201 varactor diode that you see in this circuit. You will find the maximum diode capacitance is very small, about 10 pF = 10 * 10 ^ -12 Farads.

I never made this circuit. Thus I cannot guarantee that this circuit will work in practise. There could be issues with stray capacitance of the resistors that I used. Higher resistors usually have higher stray capacitance.

Simulation software: PSpice student edition

Parts: components shown in the circuit, matrix board.

Optional parts: solder

Tools: wire stripper, pliers.

Optional tools: soldering iron, high bandwidth USB oscilloscope, multimeter.

Calculating the minimum high pass frequency and the maximum low pass frequency is beyond the scope of this Instructable because varactor diode models are complicated.

The minimum high pass frequency is influenced the Cin and Co capacitors and circuit resistors.

The maximum low pass frequency is influenced by the three varactor diodes and biasing current/voltage and circuit resistors.

Increasing capacitor and resistor values will reduce the minimum and maximum frequency limits.

The reactance (capacitor equivalent resistance) is equal to:

Xcin = 1 / (2*pi*f*Cin)

Xco = 1 / (2*pi*f*Co)

I connected three varactor diodes in parallel to increase the value of the total capacitance because connecting capacitors in parallel is a good way of accumulating capacitance values.

Rin and Ro are used for short circuit protection. Rbias1 is used to limit the maximum biasing current.

You can see the -3 dB frequency (frequency at which output amplitude is half the maximum). This is the output maximum frequency of the filter.

Rbias2 = 10 kohms, Total biasing resistance = Rbias1 + Rbias2 = 11 kohms:

Gain = 0.45

fh = 2.9 MHz

Graph 1 and 2

Rbias2 = 0 ohms, Total biasing resistance = Rbias1 + Rbias2 = 1 kohms:

Gain = 0.08

fh = 16 MHz

Graph 3