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Oscilloscope bandwidth question.

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Ash Small

Wed Aug 15 2012, 02:34PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've read in several posts here that,for accurate readings, a 'scope should only be relied upon for up to half of it's maximum bandwidth.

I picked up a 50Mhz 'scope with a 30Mhz module at a hamfest for Â£20 at the weekend (Telequipment D83).

My question is as follows:

Assuming it works ok (I've not plugged it in yet), is the maximum reliable frequency half the frequency of the 'scope (50Mhz/2=25Mhz) or half the maximum frequency of the plugin module (30Mhz/2=15Mhz)?

(I hope this question makes sense)

HV Enthusiast

Wed Aug 15 2012, 02:57PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Here is a good article on bandwidth

Steve Conner

Wed Aug 15 2012, 03:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The system will presumably be 3dB down at 30MHz, as that is the definition of bandwidth.

As to "reliable frequency" it very much depends on what waveform you're looking at. To get a faithful picture of an arbitrary waveform, you want a bandwidth about 10 times the fundamental frequency of the waveform. For looking at fast digital signals, 3x is an absolute minimum as it lets you see the 3rd harmonic of a square wave. If you're just looking at a sine wave, you can go right up to 30MHz or even a little over.

If you're looking at pulses, there is a relation between bandwidth and rise time. If the pulses are too fast for the scope, the scope's own rise time will dominate.

Ash Small

Wed Aug 15 2012, 03:20PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

If you're looking at pulses, there is a relation between bandwidth and rise time. If the pulses are too fast for the scope, the scope's own rise time will dominate.

But is this determned by the 'scope itself, or by the plug in module?

(maybe I didn't phrase the inital question very well)

...

Wed Aug 15 2012, 03:39PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

It is whichever is slower, so it will be that of the module. If they are both about the same (ex, a 50MHz module in the 50MHz slope) you enter a grey zone where you might end up with a scope that only has a bandwidth of slightly slower than each component (would be about 45MHz in this example), unless the scope manufacture thought of that and included that in their bandwidth calculations.

Ash Small

Wed Aug 15 2012, 05:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks for the replies.

The article that EVR linked to is quite informative.

It seems I just need to input a square wave from a sig. gen., and increase the frequency until the waveform on the 'scope starts to show a non-linear response to determine the accuracy of this particular 'scope.

I hope to get it fired up later, but I'll also need to invest in some higher frequency probes (I assume I should be looking for 100Mhz probes) and get my 'other' sig. gen. fired up.

I'll post again later.

Sulaiman

Wed Aug 15 2012, 11:23PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I use these probes at home and at work

They are fairly robust, have more than enough bandwidth for your 'scope/plugin, and are fairly cheap. (maybe even cheaper on eBay)

Ash Small

Thu Aug 16 2012, 09:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, I fired it up tonight, but couldn't get a trace.

I've downloaded the service maunal, and I'll use my other 'scope (a Telequipment D61a) to try and diagnose the problem(s).

Is this the best forum to ask for pointers, or can anyone recommend other forums?

(It's a Telequipment D83, circa 1976)

Ash Small

Fri Aug 17 2012, 04:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, after reading a few threads on 'vintage-radio.net' I've managed to get it going. (It still needs a bit more 'tweaking', though, but it's not bad for Â£20

)

The first image is at 10kHz,

the second at 1MHz.

The second clearly shows I need some faster probes (I'll probably get a couple of the ones Sulaiman linked to, unless I find a better deal on Ebay).

I've only tested to 1MHz so far, as that is the limit of my Farnell LFM2 sig. gen., and far beyond the limits of my Maplin probes.

I'll have to get my Taylor sig. gen. going but that will be the subject of another thread, as I can't even understand the operating manual.

Steve Conner

Fri Aug 17 2012, 04:41PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The waveform at 1MHz looks pretty bad, but I think it could be your signal generator to blame. The LF series were audio oscillators, not really the fastest thing around. Try looking at the scope's own calibration output. It is only 1kHz but if you wind down the timebase, you should see a nice fast rise time and flat top.

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