Three-phase sinusoidal-waveform generator uses PLD

Using the circuit in this Design Idea, you can develop and implement a lightweight, noiseless, inexpensive, three-phase, 60-Hz sinusoidal-waveform voltage generator. Although targeting use as a circuit for testing power controllers, it can serve other applications that require three sine waves with a 120° relative phase difference. A 22V10 PLD (programmable-logic device) at IC₁ generates three three-phase, 60-Hz, square-wave voltages. Internal register IC₁ and Q₀, Q₁, and Q₂ bits set the Q₃ bit to lead the Q₄ bit by 120° and set the Q₅ bit to lag behind the Q₃ bit by 240° (Figure 1). Setting IC₁'s clock frequency to 748 Hz produces 60-Hz outputs at Q₃, Q₄, and Q₅.

IC₁'s three square-wave output voltages—Q₃, Q₄, and Q₅—drive IC₂, IC₃, and IC₄, three Maxim MAX294 eighth-order, lowpass, switched-capacitor filters to produce three 2V sinusoidal waveforms (Figure 2). When you connect IC₅, a common 555 timer as an astable oscillator, it produces a 6-kHz, TTL-level source that clocks all three filters at 100 times the desired 60-Hz output frequency. A 100-nF dc-blocking capacitor at each filter's output ensures that the three-phase outputs swing from +2 to –2V with respect to ground. Note that each filter inverts its output and introduces a 180° phase shift with respect to its input square wave.

Figure 3 depicts the phase relationships among IC₁'s outputs and yields Boolean equations (Table 1). The equations translate into set/reset signals that produce 64 logic states when you apply them to a 6-bit sequencer block in IC₁. Outputs Q₅, Q₄, and Q₃ represent the three most-significant bits, and Q₂, Q₁, and Q₀ represent the three least-significant bits. After translation, an emulated Basic program (Listing 1), produces fuse-programming code for IC₁'s sequencer and logic states. Although only 16 logic states define the sequencer's functions, its remaining 48 states also require definition to avoid anomalous operation.