Oscillators and mixers are used in heterodyne bat detectors.
Oscillators generate a signal of a certain frequency.
Mixers can multiply two signals (bat signal with a local oscillator signal) to produce an audible difference tone.


Multiplying two signals

The multiplication operation that is required to generate a signal of a frequency equal to the difference of the input frequencies, can be implemented in several ways.
The intuitively easiest way is to use a special mixer IC, like the NE612 (or NE602) (pdf) or the MC1496 (pdf).
Actually a multiplied signal generally also appears when two signals are added and subsequently put through a non-linear device, like a pn-junction in a diode or transistor. Examples of this type of mixer are a transistor biased into non-linear operation, a simple two-diode mixer and more advanced four-diode balanced mixers.

switching multiplier circuit When using a square wave oscillator, the multiplication operation simply comes down to reversing the polarity of the bat signal at the frequency of the local oscillator. This can be implemented by using an analog switch to switch between the bat signal and an inverted version of the bat signal.
In the picture on the right a basic implementation of this idea is shown. The leftmost opamp is configured as an inverting amplifier to give the signal some extra boost. The rightmost opamp is an inverting amplifier with gain -1. A 4053 is used to switch the output to either the output of opamp 1A or opamp 1B. The positive inputs of the opamps are biased at half the supply voltage.


Types of oscillators

Several types of oscillator exist, each with their own advantages and disadvantages.
I know of the following types suitable for bat detectors:

Stable triangle wave generator circuit

triangle wave generator A very nice circuit is shown on the right (source: National Semiconductor, Application Note 31, February 1978, Opamp circuit collection). It outputs a triangle wave with a frequency essentially independent of supply voltage. The triangle wave is already quite close to the ideal sine wave, with the 3rd harmonic down by 19 dB.
The opamp on the left is configured as a schmitt-trigger, whose treshold determines the triangle's amplitude. The opamp on the right is configured as an integrator, converting the square wave from the other opamp into a triangle.
To use this circuit with a single supply voltage, the grounded inputs on the opamps should be connected to half the supply voltage (by means of a resistive divider). For the opamps, a single TL072 dual-opamp IC can be used.
This page was last updated Sunday, June 25, 2000