New Scope

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mfinkelstein3
 
Posts: 3
Joined: Wed Dec 30, 2009 10:11 am

New Scope

Post by mfinkelstein3 »

Hi,

I'd like your recommendation on a good first scope. I build beginner, intermediate projects from kits and am thinking of expanding into ugly construction of radio receiver/xmitter circuits. I am an amateur radio operator and would like to build an amp along the way.

I have an old analogy scope, but would like to upgrade to a digital one. Which do you think would work best for me, a 100MHz 2 channel, or a 50MHz 4 channel scope. Both are the same price.

Thank you in advance,
Mark

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adafruit_support_mike
 
Posts: 67446
Joined: Thu Feb 11, 2010 2:51 pm

Re: New Scope

Post by adafruit_support_mike »

Hmm.. good question.

In general, bandwidth beats everything else. Having more channels is convenient, but not absolutely necessary. A 2-channel scope can measure anything a 4-channel scope can. A 100MHz scope can measure things a 50MHz scope can't though.

Having said that, doubling the bandwidth may or may not be all that big an improvement.

In practical terms, a scope has three operating regions:

- below 20% of its top speed, you can examine waveforms (time domain)
- above 20% of its top speed, you can measure sine waves (frequency domain)

That's more a rule of thumb than a hard limit.. every trace on a scope's screen is a combination of the actual input and distortion produced by the scope's limits. Below 10-20% of a scope's top speed, the effects of distortion are so much smaller than the effects of the input that you can ignore them.

Above 20%, the distortion becomes too big to ignore. Square waves start rounding off and looking like sine waves, fast spikes start losing height and gaining width, and the delay between "when the input changed" and "when the scope started to respond" becomes a noticable fraction of the overall signal response time.

To make matters worse, you generally look at waveforms on a scope to see distortion within the circuit you're testing.. does a rising edge rise fast enough? Does the leading edge of a square wave ring? Is there a lag between a signal being generated in one part of the circuit and the response in some other part? The larger the distortion from the scope, the harder it is to tell whether you're seeing a problem in the circuit or an artifact of the scope.

Those limits are well known, and there's a whole field of circuitry where the basic signals are reduced to sine waves and information is carried in the parameters of those waves.. frequency, phase, and amplitude. For those circuits, you can use the top half of a scope's bandwidth to good effect. Most radio is done in that realm, though even 100MHz is well below most transmission frequencies. You'll be able to use either scope for intermediate-frequency work and audio frequency work though.

So.. the specific question to ask is "do you need to work in the 50MHz to 100MHz frequency domain?" If so, you need the 100MHz scope. If not, a 4-channel scope will make testing circuits a bit easier.

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