# Frequency Multiplier using LM331 Chip

A frequency multiplier is a circuit that takes a signal of any frequency and multiplies it by a preset amount. A circuit such as this is also useful for multiplying a tach signal from a car tachometer.

I designed this circuit as an alternative circuit to solve an issue of multiplying a tach signal from a car engine. The other circuit is in the Tach Pulse Multiplier article, which might serve as a useful reference point.

These types of issues often occur in avionics where a new jet engine replaces an older version; however, the tach signal produced by the new engine may not be compatible with the computer system of the older planes.

This circuit intercepts a tach signal and multiplies its frequency by a factor of 16 before sending it to the engine management system such as an ECU. In this solution, the method of pulse multiplication utilises a Frequency to Voltage (FTV) converter IC, and an op-amp with a gain of 16 performs the multiplication of the voltage. Once this voltage is amplified (or multiplied), it feeds a Voltage to Frequency (VTF) converter which converts it back into a proportional frequency.

## Tach Signal Frequency Range

The input frequency f IN varies between approximately 500 Hz to 33 Hz, determined in the Tach Pulse Multiplier article.

## Frequency to Voltage

The first stage, which is the FTV, converts this frequency to voltage thereby providing 0.5 V and 0.0331 V respectively.

## Amplification

The operational amplifier with a gain of 16 performs the multiplication process; hence, this will multiply the input voltages by 16 to provide outputs of 8 V and 0.53 V respectively.

## Voltage to Frequency

These amplified voltages feed the final stage VTF converter, therefore the frequency output is 7974 Hz and 528 Hz respectively.

## Scaling

All of this is of course possible because of the linear response of all the stages involved.

I decided to scale the voltages this way, because the primary limiting factor here is the op-amp powered by a 12 V rail, which dictates that any amplified output signal cannot be greater than 12 V. I chose the maximum voltage output of 8 V keeping a 4 V margin from Vcc and keeping well within the linear region of the op-amp.

In order to have an 8 V output as the maximum the input signal had to be at 0.5 V maximum which is determined by the values of capacitor Ct and resistor Rt. Once you have these figured, everything else falls into place.