874 Chapter 30
general purpose use, harmonic distortion levels in the range of 0.5–0.05% will probably
be adequate, although equipment intended for performance assessments on high-quality
audio amplifi ers will usually demand waveform purity (harmonic distortion) levels at
1 kHz in the range from 0.02% down to 0.005 %, or lower. In practice, with simpler
instruments, the distortion levels will deteriorate somewhat at the high- and low-
frequency ends of the output frequency band.
A variety of electronic circuit layouts have been proposed for use as sinewave signal
generators, of which by far the most popular is the “ Wien bridge ” circuit shown in
Figure 30.1. It is a requirement for continuous oscillation in any system that the feedback
from output to input shall have zero (or some multiple of 360°) phase shift at a frequency
where the feedback loop gain is very slightly greater than unity. Although to avoid
waveform distortion, it is necessary that the gain should fall to unity at some value of
output voltage within its linear voltage range.
In the Wien bridge, if R 1 R 2 R andC 1 C 2 C, the condition for zero phase shift
in the network is met when the output frequency f 0 1/(2 π RC). At this frequency the
attenuation of theRC network, from Y toX, in Figure 30.1 , is 1/3. The circuit shown
will therefore oscillate atf 0 if the gain of the amplifi er, Am , is initially slightly greater
than three times. The required gain level can be obtained automatically by the use of a
thermistor (TH 1 ) in the negative feedback path and the correct choice of the value of R 3.
Since C 1 R 1 and C 2 R 2 are the frequency-determining elements, the output frequency of
the oscillator can be made variable by using a twin-gang variable resistor as R 1 / R 2 or a
twin gang capacitor asC 1 / C 2. If a modern, very low distortion, operational amplifi er,
such as the LM833, the NE5534, or the OP27, is used as the amplifi er gain block ( A 1 )
in this circuit, the principal source of distortion will be that caused by the action of the
thermistor (TH 1 ) used to stabilize the output signal voltage, where, at lower frequencies,
the waveform peaks will tend to be fl attened by its gain–reduction action. With an
RS Components “ RA53 ” type thermistor as TH 1 , the output voltage will be held at
approximately 1 V rms, and the THD at 1 kHz will be typically of the order of 0.008%.
The output of almost any sinewave oscillator can be converted into square-wave form
by the addition of an amplifi er that is driven into clipping. This could be an opamp. A
string of CMOS inverters, or, preferably, a fast voltage comparator IC, such as that also
shown in Figure 30.1 , where RV 1 is used to set an equal mark to space ratio in the output
waveform. An alternative approach used in some commercial instruments is simply to