A circuit generates three-phase sine waves with excellent amplitude and phase symmetry.
A variable-frequency, three-phase, sine-wave generator circuit has been designed for use as a source of polyphase excitation in studies of the propagation of traveling waves in plasmas. This circuit, combined with three power-amplifier channels and three high-voltage transformers, is used to power the plasma apparatus that is used in the studies.
The circuit (see figure) internally generates three symmetrical square-wave voltages with precisely 120° phase difference, each square wave containing only odd harmonics. Three switched-capacitor, six-pole Butterworth low-pass filters (U10, U11, and U12) remove the harmonics but pass the fundamental sine-wave component.
The operating-frequency range of the circuit, 10 Hz to 10 kHz, is covered in three decade ranges. A Zener-stabilized voltage-controlled oscillator (VCO) functions as a variable-frequency oscillator and covers just over one decade, while a switch-selectable frequency-divider chain (U2, U3, and U4) provides frequency-range selection by decades.
The operating frequency is 1/120th of the switched-capacitor filter clocking frequency. This frequency-division ratio is set by a fixed frequency-divider chain (U5, U6, U7, and U8). Inasmuch as this ratio is constant, the low-pass-filter cutoff frequency automatically tracks the operating frequency.
Decade counter/decoder U8 generates set and reset pulses in the proper sequence for flip-flops U9A, U9B, and U9C. In turn, these flip-flops generate symmetrical square-wave voltages. Once each cycle, AND gate U7 resets counter/decoder U8 to count zero after U8 reaches a count of six.
The amplitude of the sine-wave voltages generated by this circuit is proportional to the amplitude of the square wave at the filter input. The square-wave voltages are symmetrically clamped by diodes D1 through D6. The peak-to-peak voltage swing is limited by the total dc voltage developed at the outputs of operational amplifiers U13A and U13B. Amplitude control A2 sets the bias on the operational amplifiers, thereby setting the square-wave amplitude and ultimately the sine-wave amplitude. Controlling the amplitude in this way ensures that the amplitudes of the three sine waves track each other accurately over a wide range of amplitude settings.
Because dc control of amplitude and frequency is used in this design, the frequency- and amplitude-control components (A1, S1, and A2) can be located remotely without affecting the quality of the three-phase sine-wave signals.
This work was done by R. Ziemke of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components and Systems category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16696.