LISSAJOUS PATTERN GENERATOR
This is a dual, sine wave oscillator that generates audio signals of 4500Hz and 1500Hz.
The 3:1 ratio'd signals can be fed to the X and Y inputs of an oscilloscope to display what is called a "lissajous figure"
The ABC (Australian Broadcasting Commission) has used this figure as their logo since the 1970's
This circuit design is known as a "wien bridge" oscillator.
When properly adjusted it produces the best purity sinewave.
Some sources quote only 0.0025% distortion.
It uses the LM741 type op-amp.
I had some 4741 I.C's (4 op-amps in 14 pin package).
The purpose of the lamp in the circuit is to maintain a constant amplitude O/P.
The lamp does not visibly glow.
I tried a number of 12 volt minature lamps; choosing one with the most resistance (lowest wattage).
Highest resistance lamp was 12 ohm.
The lamp filament has a positive temperature-coefficient of resistance.
i.e. as it heats up its electrical resistance increases.
If the O/P voltage tended to increase; then more current flows thru the lamp, it heats up, increases its resistance and thus reduces stage gain; hence o/p remains constant.
The Hewlett-Packard company's first product in 1938 was an adjustable RC audio oscillator using the wien bridge design with incandescent lamp stabilization. (Model 200A)
The alternative to the lamp is an active zener/jfet combination, as per diagram.
A lamp seems much easier.
Feedback resistor is a 200 ohm preset pot. This is adjusted to give best purity of sine wave when viewed on the CRO.
Working of Wien bridge Oscillator
The feedback signal in this oscillator circuit is connected to the non-inverting input terminal so that the op-amp works as a non-inverting amplifier.
The condition of zero phase shift around the circuit is achieved by balancing the bridge, zero phase shift is essential for sustained oscillations.
The frequency of oscillation is the resonant frequency of the balanced bridge and is given by the expression fo = 1/2 π RC
At resonant frequency ( ƒo), the inverting and non-inverting input voltages will be equal and “in-phase” so that the negative feedback signal will be cancelled out by the positive feedback causing the circuit to oscillate.
From the analysis of the circuit, it can be seen that the feedback factor ß= 1/3 at the frequency of oscillation.
Therefore for sustained oscillation, the amplifier must have a gain of 3 so that the loop gain becomes unity.
For an inverting amplifier the gain is set by the feedback resistor network Rf and Ri and is given as the ratio -Rf/Ri.