[RFI] Oscilloscopes

[Ford Peterson]

> What specs should I look for when purchasing an oscilloscope for general use including Amateur radio?

"General use" is a broad concept.  The natural response is "What do you want to do with it?"

I use an o'scope for many things.  I tune my RTTY signals using the X-Y function on the scope.  I look at signal levels of audio.  I look at RF signals at IF frequencies.  I check P-P output and convert to RMS using the scope.  I inject time delays to zero in on a waveform that is delayed from the trigger waveform.  Looking at binary signal timing is beneficial in many circumstances, so more than two inputs may be important to you.  You can check keying waveforms of CW.  Use it as a DC voltmeter if you want.  In all these functions, bandwidth is important to what you intend to do.  If it's audio, you can use some pretty simple scopes.  If you are looking at RF, obviously the bandwidth has to be sufficient to have a calibrated output on the screen.  

If you want to just look at waveforms, and not absolute timing of same, a pretty simple indicator may be sufficient.  Likewise, if you need to look at logic levels and determine accurate time periods, you will need a calibrated scope with sufficient accuracy to do what you want to do.  By calibrated, the gradients on the screen are calibrated to the settings of the horizontal and vertical amplifiers in the scope.  Looking at wave form edges down to 1 ns or 2 ns is no real trick with a decent scope.  Doing it accurately takes proper probes and proper setup of the test.  

A 100mHz scope can be usable beyond 100mhz, but the gradients will no longer be calibrated.  In fact, without proper probes, terminations, and test configuration, the results will be wildly out-of-line with reality.  One can be had for a couple hundred bucks on the used market. 

If you want to automate your tests, and convert time to frequency, P-P to RMS, and digitally display the results, you spend more money.  If you want to connect your scope to a computer, you will spend more yet.  Screen capture, displaying the setup parameters and such, all are extra money and extra nice.  Bang for the buck as they say.  

My 4 bay Tech 7704A scope frame actually has a handle on the top of it, but I would hardly call it a portable.  Plug-ins are pretty reasonable at maybe $50 to $150 each, depending on function.  Plug-ins can include spectrum analyzers too.  So one frame can serve multi-purposes.  (time domain vs frequency domain)

Digital vs analog is another option that can prove very valuable.  Wave form filtering may be important to you.  Selectable bandwidths can be handy.  

Form a budget for what it's worth to you, and go buy one.  There are hundreds of styles and functions.  But they all display in the time domain.  So it really depends on what you intend to do with it.  If you want one that does everything, plan on $10K or more, and even then there will always be better available.

Ford-N0FP
ford at cmgate.com