User's Manual

Appendix B Digitizer Basics

• Source impedance—Most digitizers and digital storage oscilloscopes

(DSOs)havea1MΩ input resistance in the passband. If the source

impedance is large, the signal will be attenuated at the amplifier input

and the measurement will be inaccurate. If the source impedance is

unknown but suspected to be high, change the attenuation ratio on

your probe and acquire data. In addition to the input resistance, all

digitizers, DSOs, and probes present some input capacitance in parallel

with the resistance. This capacitance can interfere with your

measurement in much the same way as the resistance does.

• Input frequency—If your sample rate is less than twice the highest

frequency component at the input, the frequency components above

half your sample rate will alias in the passband at lower frequencies,

indistinguishable from other frequencies in the passband. If the

signal’s highest frequency is unknown, you should start with the

digitizer’s maximum sample rate to prevent aliasing and reduce the

digitizer’s sample rate until the display shows either enough cycles

of the waveform or the information you need.

• General signal shape—Some signals are easy to capture by ordinary

triggering methods. A few iterations on the trigger level finally render

a steady display. This method works for sinusoidal, triangular, square,

and saw tooth waves. Some of the more elusive waveforms, such as

irregular pulse trains, runt pulses, and transients, may be more difficult

to capture. Figure B-8 shows an example of a difficult pulse-train

trigger.