This Design Idea describes a circuit that uses a PC's serial port to control a sine-wave generator that covers a frequency range of 2 Hz to 20 kHz in 1-Hz steps (Figure 1). The circuit's output voltage of approximately 2.2V p-p remains constant over the entire frequency range. The circuit's signal source, a Linear Technology LTC6904, IC₁, consists of a digitally programmable square-wave oscillator that, without using a clock crystal, covers a frequency of approximately 1 kHz to 68 MHz at 0.1% resolution and a few percentage points of accuracy. The LTC6904 features an I²C serial-communications interface that controls the output frequency according to: OSC_CLK=2a×2078 Hz/[2–(b/1024)]. The variables "a" and "b" represent 4- and 10-bit digital codes, respectively, and the equation's frequency unit, OSC_CLK, is in hertz.

The OSC_CLK output from IC₁ at Pin 6 drives IC₂, a 14-stage 74HC4020 binary counter whose outputs at Q4 and Q10 serve as the clock and input signals for IC₃, a Maxim eighth-order, switched-capacitor MAX291 lowpass filter.

The –3-dB corner of the filter's frequency response occurs at one-one-hundredth of IC₃'s clock frequency. Fixed at one-sixty-fourth of the clock-signal frequency, only the input square wave's fundamental frequency can pass the filter without undergoing considerable attenuation. The filter's eighth-order response removes higher order harmonics, and the filter's output thus consists of a sine wave at the input's fundamental frequency. The filter's clock and input always occur in a 64-to-1 frequency ratio, and the sine wave's output amplitude thus remains constant over the entire frequency range.

To generate a 1-kHz sine wave, the circuit requires a filter clock at a frequency of 64 kHz, which sets the OSC_CLK frequency 64 times higher, or 1.024 MHz. To satisfy the equation, the LTC6904 requires programming constants of a=09_H and b=3d8_H to generate an OSC_CLK frequency of 1023.94 kHz and the nearest output frequency of 999.9 Hz.

Written for IBM-compatible PCs, the C-language program accompanying this Design Idea (Listing 1) accepts an output-frequency request, calculates the nearest values of programming codes "a" and "b," transmits the codes to IC₁, and shows the calculated frequency on the PC's display. Although a PC's serial port delivers RS-232 signals, diodes D₁ through D₄ limit the voltages available at Pin 4, the data-terminal-ready pin, and Pin 7, the ready-to-send pin, to levels compatible with the I²C bus's SDI and SCK signals, respectively.