9 - The PLL Noise EVP Receiver

[Andres Ramos]

1. Abstract

In most of the experiments in our group we employed electronic noise sources. Generally this was semiconductor noise from different types of diodes. We always used the amplitude of the noise e.g. the randomly up and down of the signal level in the time domain to feed our setups. In a vivid discussion we once talked about the frequencies in a noise signal. We wondered if a random signal could be represented by a randomly changing frequency and what would happen if we would FM demodulate such a random signal.

I've done so many electronic designs in my life that, with every upcoming question of that kind, some part of my memory starts ringing. In this case I recalled a very interesting circuit I wanted to employ for testing our hypothesis that a demodulated random FM signal would give voices as well and maybe they would be different voices, since we now were working in the frequency domain.

 

2. Electronic schematic of the PLL Noise EVP Receiver

When configuring a new design I look for an integrated circuit that incorporates most of the function I want to implement, as is my standard behavior. In this case it was the NE567. This is a very versatile circuit originally designed for touch-tone decoding. Basically it is a PLL (Phase Locked Loop) circuit that contains an oscillator that can be configured to wiggle around freely in a specified frequency area. If an input signal is fed into the circuit and it's frequency comes into the range of that of the free running oscillator, then the oscillator locks-in on the input signal and follows it phase synchronized. The original application of this circuit was to detect certain frequencies in the early multi-frequency dialing telephone systems.

My idea was to let the oscillator run wild in an area above the audible range and let it mix with incoming noise over the input. The circuit provides an FM demodulator output where you can tap an audio signal that followed the frequency movement of the oscillator. And the oscillator more or less circled randomly around the also random input frequency. In total, a very nice chaos!

In the end I configured the oscillator to run around 15KHz. A small wire of 10cm length at the input of the circuit injected environmental noise into the PLL.

 

3. Results with the PLL Noise EVP Receiver

The results were so good that we used this device for quite a long time for streaming. However I would say the results with the psychophone were better. The sound of the voices were deep and croaky, very similar to the ones with germanium and zincite but with better intelligibility.

Signal denoised by 21dB

 

A gallery of exported audio samples can be found here.