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.

This Article Continues...

Frequency Multiplier using LM331 Chip
12 V to 5 V Digital Signal Converter
LM331 Frequency to Voltage Converter Circuit
Op Amp Circuit with Gain 16
LM331 Voltage to Frequency Converter Circuit
5 V TTL Signal to 12 V Level Conversion
12 V Regulated Power Supply
Tach Pulse Multiplier Circuit Documentation