This circuit converts a square wave into a sinewave, which is at 1/100th of the original frequency. The output frequency ranges
from 0.1Hz to 30kHz using the popular MF10 switched-capacitor filter device, or 0.1Hz to 50kHz using the LMF100 chip.

This circuit converts a square wave into a sinewave, which is at 1/100th of the original frequency. The output frequency ranges
from 0.1Hz to 30kHz using the popular MF10 switched-capacitor filter device, or 0.1Hz to 50kHz using the LMF100 chip.

A square wave can be converted into a sinewave by a low pass filter tuned to the same frequency. Unfortunately, if the square
wave frequency is changed then the filter must be retuned as well. Using a traditional active filter this is prohibitively complicated
because the values of a large number of resistors and capacitors need to be tuned simultaneously.

A neat solution is a switchedcapacitor filter device such as National Semiconductor’s MF10 integrated circuit (IC). In this chip,
traditional op amp integrators are replaced by on-chip capacitors and clocked switches as shown in Fig.2.

When the analogue switch is in position ‘a’ the input capacitor charges to the input voltage. When the switch changes to position
‘b’ the charge is transferred to the op amp capacitor. The result is an integrator whose speed is determined by the clock frequency.
This allows several integrators to be tuned by the same variable clock rather than by changing many resistors and capacitors.

The MF10 contains two separate second-order filters. In Fig.3 the two filters are connected in series in the datasheet’s Mode 1 to
provide a 4th order filter with a centre frequency 1/50th of the clock frequency on pins 10 and 11. ICI, a 74HC390 dual decade
counter, divides the MF10 clock by 50 to give a square wave at the centre frequency. This is fed into the MF10 via resistor R1,
which has a higher value than the other resistors in order to avoid over-driving the filter, leading to clipping.

A spare section of ICI divides the input clock by two to give a 50% mark-space ratio clock for the MF10, giving a divide-by-100
overall. The sine wave is available at pins 19 and 20 of IC2. Here pin 19, the Bandpass Output, is used because this filters out
the DC offset from the input clock.

An oscilloscope will show the sinewave to have discrete switching steps caused by the clock. Resistor R7 and capacitor C3 filter
out the clock frequency to give a smooth waveform. However, the switching noise will be more apparent at lower frequencies, so
it may be desirable to increase the value of C3 in low frequency applications.

Switched-Capacitor Sinewave Generator – Sine of the times Fig.2. Traditional op amp integrator (left) and its capacitor/clocked
switched filter (right) Fig.3. Complete circuit diagram for the Switched Capacitor Sinewave Generator Several variations on the
theme are possible. Tying IC2 pin 12 to 0V changes the centre frequency to 1/100th of the clock.

Adjusting the IC1 divider likewise would give a finer quality sinewave, but a lower maximum frequency. Also, the MF10 can
operate from a single 10V supply instead of split 5V supplies. Readers are referred to the chip’s datasheet for details, obtainable
via National Semiconductor’s site at www.national.com.

The CCN suffix in the type number for IC2 refers to the dual-in-line package normally preferred by hobbyists.

Article reproduced by permission of Wimborne Publishing. www.epemag.com