Fundamentals and Instruments 873conjunction with a high-speed double trace cathode-ray oscilloscope. I have tried, in the
following pages, to show how these instruments are used in audio testing, how the results
are interpreted, and how they are made. Since some of the circuits that can be used are
fairly simple, I have given details of the layouts needed so that they could be built if
required by the interested user.
30.2 Signal Generators ..................................................................................................
30.2.1 Sinewave Oscillators
Variable frequency sinusoidal input test waveforms are used for determining the voltage
gain, the system bandwidth, the internal phase shift or group delay, the maximum output
signal swing, and the amount of waveform distortion introduced by the system under
test. For audio purposes, a frequency range of 20 Hz to 20 kHz will normally be adequate,
although practical instruments will usually cover a somewhat wider bandwidth than
this. Except for harmonic distortion measurements, a high degree of waveform purity is
probably unnecessary, and stability of output as a function “ of time and frequency ” is
probably the most important characteristic for such equipment.
It is desirable to be able to measure the output signal swing and voltage gain of the
equipment under specifi ed load conditions. In, for example, an audio power amplifi er, this
would be done to determine the input drive requirements and output power that can be
delivered by the amplifi er. For precise measurements, a properly specifi ed load system, a
known frequency source, and an accurately calibrated RMS reading AC voltmeter would
be necessary, together with an oscilloscope, to monitor the output waveform to ensure
that the output waveform is not distorted by overloading.
Some knowledge of the phase errors (the relative time delay introduced at any one
frequency in relation to another) can be essential for certain uses—for example, in
long-distance cable transmission systems—but in normal audio usage such relative
phase errors are not noticeable unless they are very large. This is because the ear is
generally able to accept without diffi culty the relative delays in the arrival times of sound
pressure waves due to differing path lengths caused by refl ections in the route from the
speaker to the ear.
Oscillators designed for use with audio equipment will typically cover the frequency
range 10 Hz to 100 kHz, with a maximum output voltage of, perhaps, 10 V rms. For