Generating a simple sine wave

Posted on Feb 6, 2014

I needed a sine wave carrier for a magnetic transponder system I am working on. Various types of crystal based oscillator are available such as the Pierce, but these can be tricky to design and ensure they work correctly. Rich suggested using a square wave oscillator with a filter to turn it into a sine wave. The filter attenuates all frequencies

Generating a simple sine wave
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except the fundamental frequency, which should remove much of the square wave. Over a couple of nights I tested this in ltspice then built it on a breadboard and checked the real life results. The filter used is a simple LC low pass, called a pi filter due to the shape. ltspice can easily be used to test a filter and produce an FFT to visualise the response. The circuit used for testing is shown below and uses nothing more than a pulsed voltage source (to generate a square wave) and the filter. The value of 3. 6864Mhz was chosen as it is a commonly available oscillator that divides very nicely into serial baud rates, making it excellent for serial communication. Simulating a few cycles of this provides the waveforms shown below, which verify the square wave and the resulting "sine", which looks a little triangular at a first glance. The FFT shows that the first harmonic is around -30dB, others are far below -40dB. The sine wave is not perfect in the simulation but looks quite reasonable, so it is time to build a real circuit to test. Whilst the simulator is perfect, real life is not. The circuit was built on a breadboard with an AEL crystal, Murata inductor and ordinary ceramic capacitors. Using my Rigol DS1052E I captured data from the "post filter" node at 250Msa/sec for 1Meg worth of samples, using the "long memory" option. The graphic below shows the slightly rough sine wave, the frequency counter at the top right...

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