Oscilloscopes

Archiver

Hi,

Keeping in mind that I'm an electronics hobbyist, can anybody recommend a
"good" oscilliscope? I'm really just getting back into this hoby after a
many year abscence. However, 25+ years ago I recall things like "storage
'scopes" being very expensive etc.... but is that "standard equipment" now?
25 years is a long time for 'scope technology to come down in price.

The "cheapest" 'scope I've seen is about $300 but I don't want to exceed
$800... I think that would be my limit. What sort of "features" are
important? I'd be using this mostly for stamp type projects... looking at
analog signals (i.e. to see that a particular signal is actually a sine wave
etc, or measuring the width of a pulse... that sort of thing.

I THINK I'd want a dual-channel 'scope if I understand the concept
correctly... It would allow me to see two diffent signals at the same time,
and thus check relative timing. i.e. if a signal had to change from hi to
low within 100 uSec of something else happening, I could see that on the
'scope?

I presume the "speed" (20MHz/100Mhz etc) of a 'scope refers to the shortest
pulse-width it can detect.

Thanks very much,
Don Russell

Archiver

Hi Don,

You can find a little article on buying a scope at
http://www.givemetalk.com/asp/DisplaySeries.asp?SeriesID=372

Ebay is a hotbed of activity for used scopes. I'd recommend 2 trace 100Mhz
(50Mhz if you are on a budget). Digital storage is still fairly expensive,
but there are a few PC-based scopes that are nice (I did a review of the
Velleman unit for Nuts & Volts a while back). I don't care for the PC-based
scope as a main scope but it makes a great storage scope and also makes
pretty screen shots.

Look for delayed sweep.

The speed is not quite that simple. A 100MHz scope has an analog bandwidth
to pass a 100Mhz sine wave with a certain (small) amount of attenuation and
distortion. However, if you look at the composition of a square wave, it is
a sine wave plus all the odd harmonics. So a 10MHz square wave is really a
10Mhz sine wave, a 30Mhz sine wave, a 50Mhz sine wave, etc. The more sine
waves you add the better square you get (this is basic Fourier analysis). So
a 15Mhz scope, for example, will not show a very good square wave at
10MHz... it will look pretty much like a sine wave.

That's not to say an analog scope will shut down at the rated frequency. You
can usually see something at much higher frequencies although you may not be
able to trigger on it.

Digital scopes are different. The sampling rate also contributes. So a
digital scope with a 50MHz front end and a 100Mhz sample rate will really
conk out at 50Mhz because of the nyquist limit. In this case, the scope
really does go nuts after 50Mhz because of the way sampling happens. Some
scopes can oversample, but that requires a repeating wave. You basically
take X samples on one trigger sweep and another X samples on the next and
that gives you 2X samples. The PC-based scopes can also act as strip
recorders and spectrum analyzers.

Be careful. Many of the PC-based scopes have appalling sample rates or
analog bandwidths.

Hope that helps!


Al Williams
AWC
* 8 channels of PWM: http://www.al-williams.com/awce/pak5.htm

Archiver

Hi Don,
Suggest you consider and look for a Tektronix 465 and/or it's variants. The
465 has been a main stay of the electronics industry for many, many years and
has many useful features including dual channel, delayed sweep, XY plotting,
lightweight (all things considered), easily carried, etc. I believe it has a
100 Mhz bandwidth so that it will useful for most hobby type activities
involving analog and digital. Be sure you get two probes as well with ground
clips also. Make sure the display tube phospor is not burnt. Check to
determine when it was last calibrated. Look before you buy. Good Hunting!
Burt