[Test-Equipment] Oscilloscopes

[J. Forster]

Apples and oranges.

Digital scopes sample the waveform at intervals. The important spec is the
sampling rate (samples/second). The sampling theorem applies in that 1 GHz
sampling rate cannot reproduce a sine wave of more than 500 MHz. In fact, you
likely cannot tell the difference between a sine and a square 500 MHz waveform.
There are also problems with 'aliasing' or the folding over of frequency space
(because sampling is like mixing which causes upper and lower side bands to fold
into the lower side band and makes a hash of things. Tek introduced 'random'
sampling to lessen this problem.

Also, sampling often requires a repetitive waveform (a Tek 7S11  and S6 cannot
see a 35 ps 1 shot waveform, despite the spec)

Analog scopes have a 3 dB BW spec and degrade gracefully.

As to RF, scopes are not all that useful above 100 MHz or lower. They really
don't tell you much, except modulation depth on AM (AM is less common above 30
MHz). You really want a spectrum analyzer, or if testing components, a scalar or
vector Network Analyzer.

-John



Edward J White wrote:

> Hi Gang:
> Wile on the subject of oscilloscopes what is the difference between the
> digital and analog scopes? I read  speed or should we say bandwidth of 1GHz.
> will this work at 1 GHz RF or are we talking speed of a digital signal of 18
> or less v P/P.
> What is some recommendations for a good RF scope?
> Ed
> WA3BZT