Many applications require a low-distortion sine wave which tracks the clock from a microprocessor. For most designs, this involves a digital/analog converter (DAC), sine table in read-only memory (ROM), and a timer. The maximum sampling rate of DACs which are included in many microcontrollers limit the maximum frequency of the sine wave to approximately 30 kHz.
However, you can use the switched-capacitor band-pass filter MSHFS5 IC to generate a low-distortion (0.5%) 1 MHz sine wave. The circuit uses two high-performance Si-Gate CMOS counters for the clock dividers. The MSHFS5 is a switched-capacitor, selectable low-pass/band-pass filter with six settings. In this application, 1/6th -octave band-pass filter setting provides the lowest-distortion sine wave, for a constant sine output.
For amplitude-shift keying (ASK) or AM applications, the Butterworth low-pass setting provides better impulse response. The clock is 12.5 times the input frequency to center the band pass. The MSHFS6 samples at twice the clock rate, reducing the sampling error when compared to other filters.
The 74HC390A and 74HC393A are high-performance CMOS ripple counters, capable of dividing clocks up to 50 MHz. The 74HC390A is a dual, divide-by-2/divide-by-5 counter, while the 74HC393A is a dual binary counter. For 1-MHz sine-wave output, the master clock of the system is set to 50 MHz.
The 74HC390A divides the clock by 50 to obtain the 1-MHz square-wave input. The 10-kΩ resistors divide the CMOS level clock to one-half its amplitude. The ac-coupling allows the MSHFS5 to bias the input at mid-supply (2.5VDC). The 74HC393A provides a divide-by-4 clock for the master clock (12.5 MHz). To reduce clock noise, apply the clock to the MSHFS5 through a 1-kΩ resistor. Figure 1 shows the schematic for this circuit.
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Figure 1: Sine-wave source circuit schematic.
Figure 2 is a photograph of the waveform on an oscilloscope.
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Figure 2: Resultant waveform shows low distortion achieved.
Total harmonic distortion (THD) is measured at 0.26% using a Krohn-Hite 6900B distortion analyzer. The sine-wave output tracks the clock input down to a 5-Hz clock (0.l Hz sine wave). Newer PIC microcontrollers, such as the 24HC family, will operate with an external clock as high as 64 MHz, and with timers to provide the clock control needed for a swept-sine application.
About the author
John Ambrose is the applications and systems engineering manager at Mixed Signal Integration Corp., San Jose, CA.