Frequency Division with Amplitude Restore: Design

A traditional Frequency Division bat detector has the following stages:

  • A microphone to pick up the sounds;
  • A preamplifier to make the signal from the microphone larger;
  • A comparator to convert the signal to a form suitable for the digital circuit;
  • A digital frequency divider which reduces the pitch;
  • An audio output which produces an audible sound.

Our improved design adds

  • A "detector" stage which measures the loudness of the bat call, and
  • An "amplitude restore" stage that recreates this loudness profile.

 

Below you will find an explanation of how each of these stages works.

This version of the FDAR detector was developed by K Horton and the information here is provided by him with all rights reserved.

Information on other pages

  • The "documents" page provides all the information you need to build this detector - schematic, parts list, etc.
  • Details of construction are provided on the "construction" page.
  • The microphone can be a bit tricky to mount, and the "microphone" page gives more detail.
  • Finally you will find photgraphs of the completed detector.

Microphone and preamplifier

Here the sound from the bat is picked up by the microphone, and converted into an electrical signal. The signal is very small, - around 1mV - so it needs to be amplified before we can change its frequency. This first stage makes it 20 times bigger - but thats nowhere near enough. We need two more stages of amplification.

 

Amplifier

After passing through all three stages the signal is now around five thousand times bigger - about 5 volts.
The amplifier also provides bandwidth control, using high and low pass filters, to prevent unwanted high or low frequency signals from being detected.


Comparator and counter

This is the stage that does the work of changing the pitch of the sound so that we will be able to hear it. The "Schmitt trigger" circuit changes the shape of the wave to produce a square wave suitable for the counter. The counter responds by producing a wave that is 16 times lower in frequency. (As shown on previous page.)
However the shape and size of this signal are not representative of the bats' call.


Audio output stage

The audio output stage takes the signal from the frequency divider stage (comparator and counter) and generates sounds - now at a pitch we can hear.


Now let's examine the new stages we included to measure and restore the amplitude of the bats' call.

Detector

Simply, the detector measures the size of the signal and produces a corresponding voltage at the output.

The detector originally used a pair of Germanium diodes to achieve a wide dynamic range. Modern shottky diodes, with a forward voltage of only around 0.2V are even better. The signal is then buffered by a junction FET.
C15 with R16 provides a low pass filter which separates the bat call amplitude changes from the original signal.


Amplitude restoration

The amplitude restore stage uses a CA3080 configured as a voltage controlled amplifier. The voltage from the detector controls the output signal amplitude so that a quiet call will be output as a quiet sound, while a loud call creates a loud sound.