Lecroy 9450 opportunity

[Jwillis]

As the subject states.i have an opportunity to acquire a tested fully functional Lecroy 9450 fairly cheap.The question I have is the sample rates.According to the data sheet it says  sampling rates of up to 400 MS/sec for transients and 10 GS/sec for repetitive waveforms .Is that good and why the difference in sample rates. Granted I'm inexperienced with oscilloscopes but this is a good deal and don't want to pass it up.Any help would be appreciated.

[Stray Electron]

  Price?  I gave $350 for a very clean 9450 with cover and PCMCIA memory card about 7 years ago.  I don't know what it is worth today. Prices are also generally higher in Ca.  I don't know what you're requirements are but I highly recommend the LeCroy scopes in general.

[Jwillis]

Its about 300 dollars CA.New ones go for several thousands of dollars . But I'm mainly interested in the sample rates.Most of the new affordable scopes have a 500Ms/sec to 1 Gs/sec .10Gs/sec seems like a lot .But the 400 MS/sec for transients is what has me confused.

[digsys]

... sampling rates of up to 400 MS/sec for transients and 10 GS/sec for repetitive waveforms .Is that good and why the difference in sample rates ....

Pretty much easy answer. It can digitize at a max of 400MS/s, so any single shot / pulse, that's all it can do .. BUT if it is a repetitive pulse it will simply still digitize at that rate but start creating "averages". It's a standard maths - take 4 readings / divide by 4 = 2bits more accurate etc. Theoretically, you can go up to the bandwidth limit, because after that, the signal is degraded and there is no point. So, unless you are dealing with >400MHz single shot pulses, it's all good.
I've had a few LeCroys, and they are superb.

[edpalmer42]

I have a LeCroy 9384L which also has a 10 Gsps for repetitive signals.  LeCroy's specs always seem a bit odd to me, but I think I understand what they're saying.

Your 400 Msps means that at best, you can look at a 200 MHz signal.  But that only gives you the bare minimum requirement of 2 samples per cycle that Nyquist demands.  Realistically, you're probably limited to more like 100 MHz and 4 samples per cycle - and many would say that's still not enough.

So what's the meaning of the 300 MHz spec?  The analog front end of the scope can pass up to 300 MHz.  But that's only going to be useful if you use their RIS (Random Interleave Sampling) triggering mode.  This is a sub-sampling idea that takes multiple cycles to build up a picture of the signal.  As such, it only works where every cycle is the same as every other cycle, i.e. repetitive signals.  I think every digital scope has something like this.  They all have different names for it.

Ed

[tggzzz]

The single key spec that determines the high frequency performance is the bandwidth, measured in Hz. The samples per second is a secondary specification, which can be and is fudged.

As extreme example, my fastest scope can see 140ps edges (~4GHz), but has a 40kS/s sampling rate. The key point is that it requires the waveform is repetitive, and it measures the complete waveform gradually, just fast enough to avoid flicker being visible.

So, when capturing a single shot one off event, the 400MS/s spec is important. When characterising a repetitive waveform, e.g. when massessing signal integrity, the 10GS/s is important.

Probing technique at these speeds is critical. Have a look at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Summary: you have a good scope, and will need to learn how to use it.

[tggzzz]

Your 400 Msps means that at best, you can look at a 200 MHz signal.  But that only gives you the bare minimum requirement of 2 samples per cycle that Nyquist demands.  Realistically, you're probably limited to more like 100 MHz and 4 samples per cycle - and many would say that's still not enough.

Sorry, that's very wrong. See my previous reply to begin to get an understanding.

[edpalmer42]

Your 400 Msps means that at best, you can look at a 200 MHz signal.  But that only gives you the bare minimum requirement of 2 samples per cycle that Nyquist demands.  Realistically, you're probably limited to more like 100 MHz and 4 samples per cycle - and many would say that's still not enough.

Sorry, that's very wrong. See my previous reply to begin to get an understanding.

Sorry, but that's very right.  In fact, it's 'Digital Scopes 101'.

To be really picky, Nyquist says that you have to sample at twice the frequency of the highest frequency component in your signal.  So, if you have a 200 MHz square wave, sampling at 400 Msps will only recover the fundamental and you'll have a 200 MHz sine wave rather than a square wave.  This is for the typical 'real-time' sampling scenario and assumes that the scope front end has enough bandwidth to pass the harmonics in the first place.

Realistically, a 300 MHz front end will give you mushy-looking square waves at 50 MHz because you'll only get the third, fifth, and maybe a bit of the seventh harmonics.  But the 400 Msps limit won't pass more than 200 MHz components so you'd have to switch to the 10 Gsps RIS sampling to even see that much.

The 40 Ksps scope you mentioned doesn't violate this rule because it takes multiple cycles to build up an image of the signal. 

Building a generator that puts out a good square wave at 200 MHz (or even 50 MHz) is left as an exercise for the student. 

Ed

[tggzzz]

Your 400 Msps means that at best, you can look at a 200 MHz signal.  But that only gives you the bare minimum requirement of 2 samples per cycle that Nyquist demands.  Realistically, you're probably limited to more like 100 MHz and 4 samples per cycle - and many would say that's still not enough.

Sorry, that's very wrong. See my previous reply to begin to get an understanding.

Sorry, but that's very right.  In fact, it's 'Digital Scopes 101'.

To be really picky, Nyquist says that you have to sample at twice the frequency of the highest frequency component in your signal. 

You need to understand the definition of "signal" in the sampling theorem. Many people don't.

In particular, if you have a repetitive waveform, the signal bandwidth approaches zero.

That is of key importance to sub-sampling, which is a common technique in digital radios and DSP.

So, if you have a 200 MHz square wave, sampling at 400 Msps will only recover the fundamental and you'll have a 200 MHz sine wave rather than a square wave.  This is for the typical 'real-time' sampling scenario and assumes that the scope front end has enough bandwidth to pass the harmonics in the first place.

Partially correct.

Firstly note that I carefully made the distinction between capturing one-off single-shot signals and capturing repetitive signals. You seem to have missed that.

Secondly, you would benefit from understanding an important use of oscilloscopes with non-repetitive high speed signals: eye diagrams.

Finally, in general mixing MS/s and MHz willy-nilly is likely to lead to very misleading statements and confusion.

Realistically, a 300 MHz front end will give you mushy-looking square waves at 50 MHz because you'll only get the third, fifth, and maybe a bit of the seventh harmonics.  But the 400 Msps limit won't pass more than 200 MHz components so you'd have to switch to the 10 Gsps RIS sampling to even see that much.

The latter is the key point, and why RIS and the many similar variants are useful.

For digital signals in particular, you would do well to concentrate on a waveform's risetime rather than its irrelevant fundamental frequency. See a brief intro to the theory and some practical measurements at https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ There's nothing new there, but people repeatedly misunderstand the point.

The 40 Ksps scope you mentioned doesn't violate this rule because it takes multiple cycles to build up an image of the signal. 

Correct, but you are repeating what I wrote!

Building a generator that puts out a good square wave at 200 MHz (or even 50 MHz) is left as an exercise for the student. 

It is trivial, even with jellybean CMOS digital logic, to get 250ps risetimes. See, for example, https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

[shakalnokturn]

If it is going cheap the 9450 is a nice DSO to start with, the vector display is unbeatable for a monochrome scope.
The downsides are the weight and noise. A common problem are the 1M input amplifiers so make sure it's working correctly on DC 1M coupling. (There's a spare amplifier on the Ext. Trig. input if you never use that.)
The 9450A are not affected by this problem.

[egonotto]

Hello,

@tggzzz: Sorry, I dont understand the outcome of the 1GHz Agilent MSOS104A in your risetime measurement.

In "edgeGen02.JPG" the risetime is 280ps. Your 4GHz LeCroy HDO9404 shows 260ps.
Than the risetime of the 1GHz Agilent MSOS104A must be about 100ps. Little for a 1GHz scope.

Best regards
egonotto

[tggzzz]

Hello,

@tggzzz: Sorry, I dont understand the outcome of the 1GHz Agilent MSOS104A in your risetime measurement.

In "edgeGen02.JPG" the risetime is 280ps. Your 4GHz LeCroy HDO9404 shows 260ps.
Than the risetime of the 1GHz Agilent MSOS104A must be about 100ps. Little for a 1GHz scope.

I'm not quite sure of your reasoning.

If you take the traditional rule of thumb, the risetime t_r=^0.35/_BW, so a 1GHz scope "ought" to have a risetime of ~350ps. However, there are many factors that affect the validity of the rule of thumb, especially:

• it presumes a Gaussian input filter
• different digitising scope manufacturers have different input filters, almost certainly not pure Gaussian, and aimed at slightly different optimisations
• the bandwidth ought to be specified conservatively, and ought to be higher than the specified minimum
• those measurements were very hurried and not well controlled, especially the 1m of coax

Summary: I would not attempt to read too much into the detailed values, other than to note that the risetime is around 250-300ps.

The major point is that risetimes <<1ns are easily available from standard jelly-bean CMOS logic gates.

If you want to use off-the-shelf PECL/LVDS logic, then I expect you can get down to ~50ps using some of the Analog Devices ADCMP comparators.

[egonotto]

Hello tggzzz,

thanks for your answer.

You wrote:
"Summary: I would not attempt to read too much into the detailed values, other than to note that the risetime is around 250-300ps"

Keysight say 430ps for the DSOS104A.

But you measured 280ps.
If the DSOS104A has risetime 430ps and your puls has risetime 250ps you should measure 500ps
Only if your scope has 120ps you can measure 280ps risetime for your pulse.

So my question has the DSOS104A perhaps 4GHz?

Best regards
egonotto

 

[tggzzz]

Hello tggzzz,

thanks for your answer.

You wrote:
"Summary: I would not attempt to read too much into the detailed values, other than to note that the risetime is around 250-300ps"

Keysight say 430ps for the DSOS104A.

But you measured 280ps.
If the DSOS104A has risetime 430ps and your puls has risetime 250ps you should measure 500ps
Only if your scope has 120ps you can measure 280ps risetime for your pulse.

So my question has the DSOS104A perhaps 4GHz?

I cannot provide a useful comment, other than those pictures.

The scope was not mine, I was only in the room when someone else made the measurements, and I touched the scope for a maximum of 15s. (Unfortunately.)