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Requirement needed...somewhat odd.

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Requirement needed...somewhat odd.

Postby jom » Tue Apr 10, 2007 1:17 am

Hello

I have two requirements for a scope. One is pretty straight forward, the other is odd and I'm not sure I can do it.

The first is that the scope would need to have at least 100 to 200 MHz input. This puts me in the 3204-3206 class or more. Simple.

However, my other requirement would be to use the scopes external trigger to be driven by an RF spectrum analyzer (SA). The analyzers "Y" output would be go into the normal scope input at audio frequencies and the SA's tracking generator would drive the trigger ("X" input). In this case all of the signal processing is done by the SA so the Picoscope itself doesn't need to be fast at all. Normally, you'd use any old analog oscilloscope which has a trigger for the SA to show it's output. I guess my question is whether a digital scope can perform this function and how many bits would be needed...which I think is a bit difficult to answer.

thanks for any help you can give.

jom
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Postby ziko » Wed Apr 11, 2007 8:53 am

Hi and thank you for your post.

In response to your first requirement, bear in mind that digital storage scopes (DSOs) are specified differently to older analog scopes. As well as ensuring that you have adequate input bandwidth for the measurement you want to make, you must also consider the sample rate of the oscilloscope. We would recommend a sample rate at least five times higher than the fastest signal you need to view so, for a 100MHz input signal, you should consider a scope with a sample rate of at least 500MS/s.

There is more advice on choosing an oscilloscope on our website at:
http://www.picotech.com/applications/os ... orial.html

Your second requirement is actually much easier to answer: a digital scope will have no trouble at all displaying the signal from an external RF spectrum analyser. If your analyser has a trigger output which you normally connect to the external trigger of your benchtop scope, simply connect it to the external trigger of a PC scope and select an appropriate timebase to get the whole spectrum displayed on screen. Of course, a PicoScope PC oscilloscope can itself be used as a spectrum analyser - meaning that for signals within its bandwidth you don't even need the external analyser!

Kind regards
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Postby jom » Wed Apr 11, 2007 9:05 am

ziko wrote:Of course, a PicoScope PC oscilloscope can itself be used as a spectrum analyser - meaning that for signals within its bandwidth you don't even need the external analyser!


That's true however many signals intended for the SA in my work are well above the specs for your scopes. Which is fine since that is what the analyzer if for. :D

Would there be any requirement for the number of bits needed to display this type of signal from an SA? The "noise floor", as used with your FFT function, is not applicable here since all that is being done with the SA. Maybe it's possible one of the lower BW higher bit scopes would be more appropriate?

Thanks for the info!

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Postby ziko » Thu Apr 12, 2007 10:16 am

Hi please read below what our hardware engineer says:

"You are absolutely correct that the noise floor in the FFT is not what you need to consider for your application. If you are interested in looking at signals in the audio frequency range (such as the output of your spectrum analyser) then a higher-resolution lower-bandwidth scope will give you better DC accuracy (our 12-bit scopes are accurate to 1%, the 8-bit ones to 3%) and a less noisy-looking trace on the screen compared to a faster, lower-resolution scope.

A suitable high-resolution scope would be the PicoScope 3224. It has two channels, but no external trigger (so you'd need to use the second channel for the trigger signal from the spectrum analyser). A possible reason for choosing a lower resolution scope might be if you need the extra bandwidth for general scope use, or if you wanted some of the extra features not available on the 3224 such as the external trigger or signal generator."

Hope this helps.
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Postby jom » Thu Apr 12, 2007 3:54 pm

That's the info I need to decide. I'm probably leaning toward the higher BW rather than the higher resolution but the info helps me out. Thanks!

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Postby ziko » Tue Apr 17, 2007 9:20 am

No problem.
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