Notes:
Built around a single 8038 waveform generator IC, this
circuit produces sine, square or triangle waves from 20Hz to
200kHz in four switched ranges. There are both high and low
level outputs which may be adjusted with the level control.
This project makes a useful addition to any hobbyists
workbench as well.
Allof the
waveform generation is produced by IC1. This versatile IC
even has a sweep input, but is not used in this circuit. The
IC contains an internal squarewave oscillator, the frequency
of which is controlled by timing capacitors C1 - C4 and the
10k potentiometer. The tolerance of the capacitors should be
10% or better for stability. The squarewave is
differentiated to produce a triangular wave, which in turn
is shaped to produce a sine wave. All this is done
internally, with a minimum of external components. The
purity of the sine wave is adjusted by the two 100k preset
resistors.
The wave
shape switch is a single pole 3 way rotary switch, the wiper
arm selects the wave shape and is connected to a 10k
potentiometer which controls the amplitude of all waveforms.
IC2 is an LF351 op-amp wired as a standard direct coupled
non-inverting buffer, providing isolation between the
waveform generator, and also increasing output current. The
2.2k and 47 ohm resistors form the output attenuator. At the
high output, the maximum amplitude is about 8V pk-pk with
the square wave. The maximum for the triangle and sine waves
is around 6V and 4V respectively. The low amplitude controls
is useful for testing amplifiers, as amplitudes of 20mV and
50mV are easily achievable.
Setting
Up:
The two 100k preset resistors adjust the purity of the sine
wave. If adjusted correctly, then the distortion amounts to
less than 1%. The output waveform ideally needs to be
monitored with an oscilloscope, but most people reading this
will not have access to one. There is however, an easy
alternative:- Winscope. This piece of software uses your
soundcard and turns your computer into an oscilloscope. It
even has storage facility and a spectrum analyser, however
it will only work up to around 20KHz or so. Needless to say,
this is more than adequate for this circuit, as alignment on
any range automatically aligns other ranges as well.
Winscope is available at my download page click
here. Winscope is freeware and designed by Konstantin
Zeldovich. After downloading, read the manual supplied with
winscope and make up a lead to your soundcard. My soundcard
is a SoundBlaster with a stereo line input, i made up a lead
with both left and right inputs connected together. Connect
the lead to the high output of the function generator, set
the output level to high, shape to sine, and use the 1k to
10k range, (22nF capacitor). A waveform should be displayed,
see the Figure 1 below:-
Figure 1.
Here an
undistorted sine wave is being displayed. The display on
winscope may flicker, this is normal as it uses your
soundcard to take samples of the input waveform. The "hold"
button on winscope will display a steady waveform.
Alignment:
First adjust the 100k preset connected to Pin 1 of the 8038.
An incorrect setting will look similar to the waveform
below:-
Adjust the
preset so that the top of the sine wave has a nicely rounded
peak. Then adjust the other preset, again an incorrectly
adjusted waveform is shown below:
The two
presets work together, so adjusting one affects the other. A
little is all that's needed. When your waveform is adjusted
and looks similar to Figure 1 press the FFT button on
winscope. This will perform a fast Fourier transform and the
displayed output will be a spectrogram of the input. For a
pure sine wave, only one signal is present, the fundamental
frequency, no harmonics will be present and so a spectrogram
for a pure sine should contain a single spike, see Figure 2
below:-
Figure 2.
A distorted
sine wave will contain odd and even harmonics, and although
the shape of the sine may look good, the spectrogram will
reveal spikes at the harmonics, see below:-
Once
alignment of the sine wave is complete, the other wave
shapes will also be set up correctly. Below is a picture of
the triangle waveform generated from my circuit:-
Finally the ICL8038PCD is
available from
Maplin
Electronics order code YH38R. |
