Hardware Reference
In-Depth Information
is unlikely to be required) there are several good reasons for using an input
interface:
•
The 50 k
input impedance of a sound card (which is appropriate for use
with audio equipment) is too low for general-measurement applications.
•
The 50 k
input impedance is inappropriate for use with standard 'scope
probes which are designed to work with standard 1 M
oscilloscope inputs.
•
The sensitivity of most sound cards varies according to software gain settings
and the size/resolution of PC displays also tends to vary widely hence some
method of calibration is essential if accurate measurements are to be made.
These limitations can be overcome by means of a simple interface of the
type shown in Figure 11.17. This circuit incorporates two identical channels
for the Y1 and Y2 inputs. The input signal is fed to a switched potential divider
attenuator (R10 to R15 and R20 to R265) which has a constant input impedance
of 1 M
. Junction gate field-effect transistors (TR10 and TR20) are connected
as unity gain source followers with the output voltage developed across the two
variable gain controls (VR10 and VR20). Capacitors, C10 and C20, are used
to provide DC isolation at the input whilst C11 and C21 provide DC isolation
at the output.
The sound card input interface incorporates its own AC mains supply which
also provides a 1 V peak-peak calibration signal at 50 Hz. Diodes, D1 and D2,
provide full-wave bi-phase rectification with an output of approximately 8.5 V
appearing across the reservoir capacitor, C1. Secondary current from T1 is also
fed to the anti-parallel diode clamp, D3 and D4. A potential divider, R2 and
R3, provides the 1 V peak-peak calibration signal which is applied to the input
attenuator when switch, S2, is set to the 'calibrate' position. Light emitting
diodes, D5 and D6, indicate whether S2 the instrument is set to the 'calibrate'
or the 'operate' position.
As mentioned earlier, the upper signal frequency limit of a DSO is deter-
mined primarily by the rate at which it can sample an incoming signal. Using a
maximum sound card sampling rate of 44.1 kHz a sinusoidal signal at 20 kHz
can be displayed with acceptable accuracy (the Nyquist criterion).
However, in order to display non-sinusoidal signals faithfully, we need to
sample at an even faster rate if we are to accurately display the signal. In
practice we would need a minimum of about five points within a single cycle
of a sampled waveform in order to reproduce it with approximate fidelity.
Hence the sampling rate should be at least five times that of highest sig-
nal frequency for a DSO to be able to display a waveform with reasonable
accuracy.
With a fixed 44.1 kHz sampling rate this would suggest that audio-frequency
signals of up to 4 kHz will be faithfully displayed using a sound card; however,
it is also necessary to take into account the fact that a sound card interface is
AC coupled and therefore is unable to respond to DC levels!
Windows Oscilloscope 2.51
Windows Oscilloscope 2.51 was written by Konstantin Zeldovich who is a
research associate in the Physics Department at Moscow State University.
