Hardware Reference
In-Depth Information
spectrum of a radio-frequency signal at 1184 kHz (1.184 MHz). Numerous other
signal components are present including the second, third, fourth, and fifth
harmonics. The cursors have been set to the fundamental (1.184 MHz) and
third harmonics (3.553 MHz). The level and frequency of each harmonic has
been displayed. Note that the fundamental has an amplitude of 17.46 dB whilst
the third harmonic is at
−
4.08 dB. Needless to say, this type of measurement
would be impossible with a conventional oscilloscope!
Sound card
oscilloscopes
Any PC with a standard sound card (or equivalent interface integrated with the
motherboard) can be used to record and playback analogue signals. In order
to do this, the sound card interface incorporates 16- or 32-bit analogue-to-
digital and digital-to-analogue converters (ADC and DAC, respectively). Sound
cards can usually be configured for stereo or mono operation with sampling
rates of 11 025 Hz ('voice quality'), 22 050 Hz ('tape quality'), and 44 100 Hz
('CD quality'). The stereo capability makes it possible to have two independent
channels (Y1 and Y2) as with a conventional oscilloscope.
Provided that you are willing to accept the bandwidth limitation (i.e. 20 Hz
to 20 kHz) typical of audio signals, the ability to convert an analogue signal
into digital stored data makes, it possible to use a sound card as the hardware
component of a digital storage oscilloscope (DSO).
With the aid of appropriate software (available at minimal cost) the stored data
produced by a sound card can be made to display on a screen in much the same
way as it appears on the screen of a conventional oscilloscope. Furthermore,
the use of Fast Fourier Transformation (FFT) makes it possible to produce a
frequency domain display as well as the more conventional time domain display.
Whilst it is possible to use a sound card without any external interface (most
sound cards have ample sensitivity so additional gain from a pre-amplifier stage
Photo 11.3
A typical low-cost sound card supplied in PCI format
