Nuts and Volts - February 2009

phase waveforms at once, where we can see the 60 degree phase shifts. To get it to work, I had to raise the voltage gain slightly (my R5 is now 233K). If you build one, add a 50K preset trimmer in series with R5. Mine oscillated at 2.928 kHz against a theoretical 2.768 kHz. The errors are probably due to random component selection and the construction method, and the scope traces for the 'middle stages' are a bit distorted.

In summary, the phase shift oscillator produces nice sine waves where the frequency is set by the time constant of the RC sections. The output amplitude is regulated by the non-linear gain of the amplifiers which occurs with all amplifiers when the signal drives their output near to either ground or the supply.

Many analog oscillator circuits use a variable gain element (FETs are popular) to regulate the gain and maintain a low distortion sine wave output. Traces are: Phase 1, Phase2, and Output; Y = 500 mV/div, X = 50 μs/div.

The LTspice and other files for this session are zipped together and available for download from the Nuts & Volts website www.nutsvolts.com).

Digital Oscillators

Digital circuits require the "marking of time" with a pulse rather than low distortion sine waves; simple oscillators constructed from analog circuits can produce pulses compatible with digital gates.

For two interesting analog oscillator circuits that produce pulses,

■ FIGURE 5. Phase shift oscillator breadboard.

check out the examples bundled with the LTspice download. Astable.asc is a two transistor astable oscillator, and LM555.asc is a nice transistor level Macromodel of the popular 555 timer IC. Both are included in the download for this session, and I encourage you to have some fun with them in LTspice!

Using Digital Logic Gates as Oscillators

One other interesting approach is to make digital logic components operate in the analog realm. Just a few components make an oscillator and these can include a crystal for high accuracy and stable frequency control.

It's easy to get a basic CMOS inverter logic gate to act as an amplifier by placing a high value resistor of several megohms from output to input. This biases the gate as an inverting amplifier, but usually causes the gate to oscillate due to stray capacitance and the inherent delay through the gate. Adding additional capacitors and a second gate as a buffer makes this circuit both practical and very handy.

This is a good place to jump back into LTspice, which includes Macromodels for simple logic gates in the download bundle.

Bad Macromodels

My first attempts didn't work at all, and to make a long story short, the LTC supplied digital-gate Macromodels are defective for this task. Luckily, better models are readily available

■ FIGURE 6. Phase shift oscillator breadboard frequency.

■ FIGURE 4. Phase shift oscillator simulation.

and these are included in the download for this article. If you haven't added Macromodels to your LTspice library yet, refer back to the sidebar, “Adding New Macromodels to LTspice”, in last month's installment.

Testing Logic Gates

A good way to see if a logic gate Macromodel is "only digital" (and therefore not useful for oscillators) is to drive it with a ramp waveform (as shown in Figure 8) and the downloaded file NV_SPICE_32.asc. Digital circuits usually don't like slow rise pulses applied to the inputs with the exception of the Schmitt-Trigger types which have better defined upper and lower logic level thresholds than ordinary gates, and internal positive feedback to speed up the output's edges.