Tek 2400 series digital scope usability question

I spent this morning trying to track down a clock problem only to find out that it was the scope and not the clock. Wasted morning.

I was looking at a 24MHz clock. The good scope (a TDS520B) was being used so I figured for only 24MHz I could use the Tek 2430A (a 150Mhz,

100M/s scope. The clock looked like a sine wave (not square) had bad - visible, jitter and seemed to be amplitude modulating at some lowish frequency. Three issues. Looking at it with my trusty 2430A (also a 150Mhz but analog scope) and TDS520B it looked as it should.

I'm wondering if the issue was solely to low a sample rate (only 4 samples per clock). I had hoped to upgrade the TDS scope and get rid of the 2430A but I can't afford what I want now. So I was thinking of getting a 2440 (300Mhz, 500m/s ) but if I can't look at a 50MHz square wave accurately with it then it won't do me much good. That would be 10 samples per clock at 50Mhz. Since there were three issues with looking at it with the 2430A I wonder if all three can be explained and will not be issues with the 2440 at 2x the frequency and 5x the sample rate.

Is this going to work or will I be disappointed?

Thanx Hawker

Reply to
Hawker
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To gauge the quality of a 50MHZ clock you'll need 1GSPS, at least. Better 2GSPS. Repetitive scans at any lower rate plus piecing together isn't going to work when you suspect a noisy clock. About a month ago I had to look at laser diode noise, became frustrated with their digital scopes and convinced that client to buy a used Tek 2465. One week and about $350 later that allowed us to get the job done ;-)

Another downside of digital scopes is their rather limited vertical resolution or effective number of bits.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

IMHO, a 100 M/s scope won't give you a very good indication of a 25 Mhz waveform, especially a square wave. In fact, I'd expect it to look almost exactly like the way you described it!

Remember, a square wave consists of two frequencies, the slopes (leading edge, and trailing edge are very high frequency) while the middle part between the slopes constitutes a low frequency.

Reply to
PeterD

Digital scopes with low sample rates like this have to use repetitive sampling techniques and not real-time sampling for high frequencies. This can be very troublesome and cause lots of issues, and is why almost all new DSO's are "real-time" sampling that have a sample rate at least 10 times the analog bandwidth.

If your trigger point is stable then your 100MS/s 150MHz DSO *should* be able to display a stable *repetitive* signal at up to the 150MHz bandwidth. But once you single shot capture it, it is using real-time mode and you only get your 4 samples per cyle for a 25MHz signal.

A 500MS/s scope will be *just* ok with a 50MHz signal, as 10 times minimum is the general rule of thumb. But you will most likley be a bit dissappointed with the 500MS/s, I would be aiming at a 1GS/s scope fro this kind of work. Even the cheapest sub $1000 entry level scope can do 1GS/s, albeit with limited sample memory size (a few KB) and analog bandwidth.

Dave.

Reply to
David L. Jones

Not if the trigger point is stable and you aren't using single shot capture, it will work fine in repetitive mode. That is why manufacturers actually make 150MHz bandwidth scopes with 100MS/s sampling, if it didn't work they would make the bandwith only 50MHz or so at best. They also make 20GHz bandwidth scopes with 100MS/s sampling etc.

Dave.

Reply to
David L. Jones

It consists of a whole lot more frequencies. If you can't acquire up to the 7th-9th harmonic you won't get much of a display to write home about.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

"Troublesome" and "stable repetitive" are the right words. If there was indeed a clock problem that can easily turn the displayed trace of a repetitive scan into pulp.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Wouldn't trust 'em as even a door stop. Last week I used a customer's expensive 100's of MHz TDS to look at a slow synching pulse alongside a 50Hz 5V sawtooth. At 100ms/div noticed bad distortion on the sawtooth. Did not waste time investigating as previous experience said "suspect the digital scope first". At 5secs/div my sawtooth magically reappeared in all it's glory but now apparently .05Hz. Damned waveform had been aliasing all the way down and was worthless. (I won't mention the hit and miss display of the synching pulse) Nice coloured traces. Nice printouts for those important management presentations and emails. Nice FFT display. Small and light.

Tiny, lo-res, pixelly display. May as well be looking at a ZX81 screen. Kind of "is that my waveform or a display artefact"?. Unable to trace low duty cycles. Continual "am I seeing the real waveform or some alias"? and am I really generating all that noise?. As a technology demonstrator marvellous, as a 'scope f****** worthless.

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Reply to
john jardine

For sure. You get lots of nice random dots on the screen though, and it's kinda pretty to look at :->

Dave.

Reply to
David L. Jones

Younger users may like to push the obfuscation button which collects lots of those random dots and makes it all look like graffiti :-)))

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

On 3/15/2007 5:40 PM, The digits of David L. Jones's hands composed the following:

Thanx. That is the information I thought, but didn't want to hear. The reality is my 500Mhz 1 G/S TDS520B (I think) is starting to show it's age. There are signals I need to see but it can't show very well (esp outputs from CCDs in one sweep). I had hoped to make it my second scope, get rid of the 2430A and get a new scope (TDS 600 or TDS700 series?) but looks like I need to shell out $4k-$10k for one of those and I don't have it now - probably not till the end of the year. I may get a 2440 just to get me by for now. I assume it will hold it's value a bit. I looked at TDS300 and TDS400 stuff but other than active FET probes they actually look like a step down from a 2440.

Anyone have a suggestion for a basic feature, active FET capable scope with 500Mhz Bandwidth and 2 G/S (perhaps 4-5GS?) resolution? No fancy features, although FFT would be handy. Step up from a TDS520B. I assume I'm looking at used but if the price is right I could consider new.

Hawker

Reply to
Hawker

Quite frankly I have never found the FFT feature on "modern" scopes that useful. If it was me I'd go for a used 2465 plus a nice 1GHz+ spectrum analyzer for any spectral investigation. Should be able to obtain that for under $2k total if you don't mind boat anchor size. That, plus a simple digital scope if you need to be able to see pre-trigger.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

For this performance you won't get new scope for under around $4K or so. But there are plenty of used Lecroy's on Ebay with 500-1GHz bandwidth,

2GS/s+, and massively deep memories. They are a dog to drive, but the performance is there, and the price is right at under $2K This is but one example:
formatting link

Dave.

Reply to
David L. Jones

Hawker wrote in news: snipped-for-privacy@mid.individual.net:

you can use a FET probe with a 2400 scope;you just have to power the probe externally.TEK used to make an external supply expressly for this.

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Jim Yanik
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Reply to
Jim Yanik

I just got my first DSO. Honestly so far I'm really impressed compared to my old Hitach V650-F. It's certainly allot easier to see what happened before the trigger event. ;-)

I also got my first taste of aliasing too, kinda scarey. I was watching a PWM that updates ~80 times/second on channel 1 and put the CLKOUT from the PIC onto channel 2. CLKOUT was 2MHz. Instead of seeing the nice thick "bar" across the screen as I would expect on an analog scope, I could see the clock signal and it looked just like the clock signal, only running wayyyyyyyyyy tooooo slowwww. That's the part that really surprised me: It looked EXACTLY like the CLKOUT signal including the little voltage spikes. :-O

OTOH, it's the most amazing thing I've ever used. :-)

Reply to
Anthony Fremont

I have a 2445 and access to a 2465b but honestly don't use it much. When I need high speed analog I need FET probes which the 2400 series stuff does not have. But more often then not I need to do fancy triggering and holding (trigger on an event and hold the capture) that can only be done with a DSO. The analog stuff gets some usage but not much anymore. I hate to admit it because for what it can do it usually is better than any digital scope.

Reply to
Hawker

Ok, depending on your typical design a DSO might indeed be the better choice. Definitely when chasing digital state changes where you don't want to crack out the logic analzyer.

As Jim said a FET probe can be powered externally. I use a Philips FET probe on a Tek scope and they never even growled at each other.

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Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

It

Yes. Other than good sex, there's nothing finer than a new toy turning up in the mail :) My main scope is a straightforward 100MHz Hitachi V-1065A but look forward to the day I can dump it in favour of some digital model. Probably be a wait as they'll need to offer something realistic such as a 1THz sample rate at

10bits and a display of 1024x768 with screen update at 70-100HZ john
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Reply to
john jardine

Three samples per cycle is inevitably going to give you a horribly distorted waveform. Ten samples per cycle won't be pretty, but should be usable. If you're pushing the sampling speed that way, an analog scope is probably more useful.

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Lionel

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