Author Topic: Yet another fast edge pulse generator  (Read 58622 times)

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Offline KE5FX

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Re: Yet another fast edge pulse generator
« Reply #375 on: May 23, 2018, 09:36:19 am »
I'm not at *all* unhappy with the instrument.  It's *really* nice.   I'm unhappy with marketing.  The rise time is specified as "calculated"  in the datasheet and the conventional 0.35/BW rise time is listed.  But rather clearly the MSOX3104 rise time specification is measured.  Were it calculated, it would be <350 pS.

The 0.35/BW rise time equation is a loose approximation that's based on a specific filter response, which itself is a tradeoff between edge fidelity and raw 3-dB bandwidth.  Not all scopes are purely Gaussian. 



(from http://literature.cdn.keysight.com/litweb/pdf/5988-8008EN.pdf)

You're fine.  Nobody is trying to put one over on you.   :)
 

Offline rhb

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Re: Yet another fast edge pulse generator
« Reply #376 on: May 23, 2018, 10:47:27 am »
I'm not at *all* unhappy with the instrument.  It's *really* nice.   I'm unhappy with marketing.  The rise time is specified as "calculated"  in the datasheet and the conventional 0.35/BW rise time is listed.  But rather clearly the MSOX3104 rise time specification is measured.  Were it calculated, it would be <350 pS.

The 0.35/BW rise time equation is a loose approximation that's based on a specific filter response, which itself is a tradeoff between edge fidelity and raw 3-dB bandwidth.  Not all scopes are purely Gaussian. 



(from http://literature.cdn.keysight.com/litweb/pdf/5988-8008EN.pdf)

You're fine.  Nobody is trying to put one over on you.   :)

I'd prefer the Gaussian step response.  But I've not observed that to be an option except for R&S.  I can't see any reason that you could not let the user choose the step response.

My complaint is changing the method of calculation of rise time between models in the same line of instruments. It's a matter of integrity, not the performance of the instrument that bothers me.  The closest analogy I can think of is finding out your wife of 20 years is cheating on you.
 

Offline KE5FX

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Re: Yet another fast edge pulse generator
« Reply #377 on: May 23, 2018, 11:12:38 am »
My complaint is changing the method of calculation of rise time between models in the same line of instruments. It's a matter of integrity, not the performance of the instrument that bothers me.  The closest analogy I can think of is finding out your wife of 20 years is cheating on you.

I imagine most people would prefer the familiar Gaussian response, given a choice.  The decision probably comes down to the Nyquist frequency margin associated with the achievable sampling rate.  If you're building the fastest possible scope with a given technology, you are probably not oversampling by the 5x or more that we'd all prefer.  You may be cutting things as close as 3x.  The Gaussian filter falls off so slowly that aliasing is objectionable at 3x, so the faster scopes in a given family may be more likely to incorporate flat-response filters than the slower ones.

Figure 1 in the Keysight app note is a good illustration of this problem, although of course the marketing folks don't spin it that way:



They are assuming a 4 GHz clock in that figure.  If they could sample at 5 GHz or higher, they would presumably have used an industry-standard Gaussian filter.  The usual 0.35/BW rule of thumb would have applied, and nobody would have complained about questionable risetime specs or overshoot on fast pulses.  But marketing tells engineering, "Yeah, I'm gonna need you to come in on Sunday and push this chipset to 1 GHz," and this is the resulting compromise.

Quote
I can't see any reason that you could not let the user choose the step response.

Keep in mind that we're talking about the antialiasing filter in front of the ADC, not a set of coefficients.  Allowing the user to switch between two different antialiasing filters would be quite expensive, and it would impress maybe 1% of the customer base.
 

Offline rhb

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Re: Yet another fast edge pulse generator
« Reply #378 on: May 24, 2018, 08:34:50 am »
I think that's a fair rendition.  My complaint really is changing the calculation for the last scope in the datasheet.

The MSOX3000T has only a single reduced bandwidth option, 20 MHz.   My MSO2204EA will let me choose 20, 100 or 200 MHz.  There's no reason you couldn't do a digital filter to give the user a Gaussian step response.  It would cost some bandwidth, but so what?

I don't like the "flat as far as we can push it" model.  I was quite stunned to see 7% overshoot.  God only knows how you'd diagnose overshoot in a DUT.

My major project is to implement FOSS FW for Zynq based DSOs.  I was all set to start serious work on that when watching my brother in-law rapidly deteriorate from Parkinson's convinced me I should spend some money on test gear I have always wanted.  That led to a major reorganization project.

I was actually browsing eBay for older gear and seriously considering an 8104A, but the 6 fans on the side and the physical dimensions scared me off.  Then I found a 1/2 price deal from Keysight that included probes, fresh cal with data and a 3 year warranty.  I'm afraid I succumbed to bargain fever.
 

Offline KE5FX

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Re: Yet another fast edge pulse generator
« Reply #379 on: May 24, 2018, 10:18:42 am »
I think that's a fair rendition.  My complaint really is changing the calculation for the last scope in the datasheet.

The MSOX3000T has only a single reduced bandwidth option, 20 MHz.   My MSO2204EA will let me choose 20, 100 or 200 MHz.  There's no reason you couldn't do a digital filter to give the user a Gaussian step response.  It would cost some bandwidth, but so what?

The marketing department will -- justifiably -- scream bloody murder at that.  Why?  Because for better or worse -- usually worse -- bandwidth is what people buy.  If they didn't use the flat-response filter, they (presumably) couldn't have offered a 1 GHz scope in the MSOX3000 lineup.  They would have lost sales to competitors who had no such compunctions.  Most users will be fine with this decision, but as you note, it is something you need to be aware of when looking at rise time and overshoot. 

A post-acquisition filter option might cost more than just bandwidth.  It would have to run at the fastest sample rate supported by the acquisition hardware, so it might not be feasible to do it in realtime, especially given their ASIC-oriented block diagram.  And it would require some further hand-waving to explain on the spec sheet. 

That's not to say this sort of thing can't be done -- some scopes rely on frequency-domain correction to deliver their bandwidth specs, in fact, even to the extent of performing the necessary filtering in 'screenspace,' which is an especially ugly hack.   I wouldn't criticize the Keysight guys for not doing stuff like that in this particular model line.

Besides, now that you know what the impulse response looks like, you can deconvolve it by eye. :)

Quote
I don't like the "flat as far as we can push it" model.  I was quite stunned to see 7% overshoot.  God only knows how you'd diagnose overshoot in a DUT.  My major project is to implement FOSS FW for Zynq based DSOs.  I was all set to start serious work on that when watching my brother in-law rapidly deteriorate from Parkinson's convinced me I should spend some money on test gear I have always wanted.  That led to a major reorganization project.  I was actually browsing eBay for older gear and seriously considering an 8104A, but the 6 fans on the side and the physical dimensions scared me off.  Then I found a 1/2 price deal from Keysight that included probes, fresh cal with data and a 3 year warranty.  I'm afraid I succumbed to bargain fever.

We've all been there, that's for sure.  Nothing wrong with a brand-new MSOX3000T at half price!
« Last Edit: May 24, 2018, 10:26:52 am by KE5FX »
 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #380 on: May 24, 2018, 03:26:51 pm »
I think that's a fair rendition.  My complaint really is changing the calculation for the last scope in the datasheet.

The MSOX3000T has only a single reduced bandwidth option, 20 MHz.   My MSO2204EA will let me choose 20, 100 or 200 MHz.  There's no reason you couldn't do a digital filter to give the user a Gaussian step response.  It would cost some bandwidth, but so what?

I don't like the "flat as far as we can push it" model.  I was quite stunned to see 7% overshoot.  God only knows how you'd diagnose overshoot in a DUT.

My major project is to implement FOSS FW for Zynq based DSOs.  I was all set to start serious work on that when watching my brother in-law rapidly deteriorate from Parkinson's convinced me I should spend some money on test gear I have always wanted.  That led to a major reorganization project.

I was actually browsing eBay for older gear and seriously considering an 8104A, but the 6 fans on the side and the physical dimensions scared me off.  Then I found a 1/2 price deal from Keysight that included probes, fresh cal with data and a 3 year warranty.  I'm afraid I succumbed to bargain fever.

The 20MHz bandwidth is usually a analog feature in the front end. But i have seen some scopes offer adjustable digital filtering options where you can set any frequency you want within reason. Often this seams to be done afterwards in software tho.

When doing a important measurement one should avoid working close to the scopes bandwidth specs anyway. You might see the signal but you don't know whats hiding beyond what you can see and what might be attenuated without your knowledge. This is the main reason why its nice to have a scope faster than 100 MHz, but it unfortunately costs a pretty penny to get lots of bandwidth.

You did the right thing going for the MSOX3000T. For day to day use these MegaZoom ASIC scopes are a great choice. I have a MSO9000 Infiniium scope and while packed with a ton of features and performance, but its not a good daily driver. Its huge, its loud, its slow, you have to dig trough menus to set stuff up. This is a scope you use when you need to make more serious measurements, rather than just checking if your clock is moving on a board. You use such a scope when you want to make a eye diagram of a high speed line and you spend time to carefully tap into the circuit with a active probe, calibrate the probes for skew, set up the scope to do clock recovery in the waveform etc.
 

Offline Howardlong

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Re: Yet another fast edge pulse generator
« Reply #381 on: May 24, 2018, 05:09:31 pm »
Berni’s right regarding the 3000T vs the 8104A as a daily driver.

As well as a 3000A, I have the forerunner of the 8104A, the 54832D (which, if you upgrade the motherboard, thinks it’s an 8104A). The 3000A is far easier to use for 99% of tasks.

My only slight difference of opinion to Berni is that I’ve found I really like the big screen format of the 7104B, and so that tends to be the goto scope for me. In my setup, none of the scopes take up any bench space, they are raised above it by various means. The UI of the 5/6/7000 series is the forerunner to the 3000 series, and despite being Vxworks rather than Windows CE, in practice it’s almost identical. That’s unlike the 54832D or 8104A which has many of the functions accessible only from menus in the Windows UI.

You can never have enough scopes, it’s not at all uncommon to be running two scopes when you’re probing two different trigger domains, and even three sometimes, particularly if you’re monitoring some real time serial bus diagnostic information.

« Last Edit: May 24, 2018, 05:13:59 pm by Howardlong »
 

Offline texaspyro

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Re: Yet another fast edge pulse generator
« Reply #382 on: May 24, 2018, 05:19:38 pm »

You can never have enough scopes


Is 50 scopes too many?   :-//   I could prune a few if it is...
 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #383 on: May 24, 2018, 06:27:53 pm »
That's a nice digital scope collection you got there.

My daily driver scope is a Agilent MSO6034A and i really appreciate its UI speed. I always wanted a MSO7000 series scope for its large screen, but i could never find one for a good price.

From all the scopes on the market i still think the MSO7000 is the best one for a quick probe around. Its got the features and speed (okay not quite 1M waveforms but who cares) of the MSOX3000 but has faster boot times, higher resolution and larger display, more sample memory etc.

 

Offline HighVoltage

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Re: Yet another fast edge pulse generator
« Reply #384 on: May 24, 2018, 07:12:25 pm »
Sometimes I do think, I have too many scopes but somehow they are all used.

I would not use my 1GHz MSOX3104A if I need to measure anything close to the upper BW limit.
For that I use the 6 GHz 6000X series scope.

But the one scope that I find most convenient to use is the 7000 series one. I have two of them and they are just very pleasing, not just for the large screen and fast response to any key input, they are in my opinion almost perfect scopes. And once in a while they are offered at really low cost on ebay.



There are 3 kinds of people in this world, those who can count and those who can not.
 

Offline Howardlong

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Re: Yet another fast edge pulse generator
« Reply #385 on: May 24, 2018, 09:05:28 pm »
Sometimes I do think, I have too many scopes but somehow they are all used.

I would not use my 1GHz MSOX3104A if I need to measure anything close to the upper BW limit.
For that I use the 6 GHz 6000X series scope.

But the one scope that I find most convenient to use is the 7000 series one. I have two of them and they are just very pleasing, not just for the large screen and fast response to any key input, they are in my opinion almost perfect scopes. And once in a while they are offered at really low cost on ebay.

I agree. I find the 7000 UI to be slightly more responsive compared to the 3000A, that might be another reason why I use it more than the 3000A. The 7000 boots more quickly. Both of these are probably due to the use of VxWorks in the 7000 compared to Windows CE in the 3000.

One further thing, the 7000B has a few more buttons and knobs compared to the 5000, 6000 and 7000A, it’s more similar to the 3000 in that respect, but the control layout between the 3000A and the 7000B is frustratingly slightly different which is a minor irritation.

There are a few extra bits of functionality on the 3000A that I like, but equally the 7000B has one or two things that the 3000A doesn’t such as optional equivalent time sampling which can be useful on high speed stuff.

The 1Mwfm/s of the 3000 frankly for me is very rarely of any benefit compared to the 100kwfm/s of the 7000. I’d rather have the deeper memory that the 7000 offers. The 3000 offers runt triggering but not the 7000, but then it’s incredibly rare I ever use that functionality. Nice to have, sure, but not at the top of my requirements.

My main criticism of both in my use cases is that neither decode faster than about 30MHz SPI.

Overall though it’s the Megazoom together with the super responsive UI that makes these scopes so useable on a day to day basis, and I can live with the relatively limited memory depth that it dictates.
 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #386 on: May 24, 2018, 10:40:35 pm »
Yeah the 1M waveforms per second is pretty much the same as 100K. As long as its not 0.5K like some of the old scopes.

The button layout diference from the 5000/6000/7000 to the X2000/X3000 does annoy me as well. At work we have a MSOX3000 and i keep on hunting around the wrong corner of the scope for the button, hovering my finger around the front panel while feeling like im blind before realizing its not in that area.

The MSO9000 front panel also has some neat buttons i wish i had on my MSO6000. The trigger channel, slope, auto/manual are all dedicated buttons and i use them all the time, they even include dedicated LEDs next to them to show the current state of that setting, love it. But then its missing some useful buttons for other stuff i use all the time (Like aquisition mode and display settings).
« Last Edit: May 24, 2018, 10:43:23 pm by Berni »
 

Offline zitt

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Re: Yet another fast edge pulse generator
« Reply #387 on: May 31, 2018, 08:08:57 am »
BNC: http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=295

How does one get on the Waitlist for new BNC options?
The website says "sold out"; but no link for a Waitlist.
 

Offline rhb

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Re: Yet another fast edge pulse generator
« Reply #388 on: May 31, 2018, 10:02:18 am »
Putting an optional Gaussian filter on the waveform via an ASIC is *not* hard. Marketing gets their "1 GHz" spec and the user gets a sensible step response when they need one.  I'm hoping that user defined digital filters are an option.

I'm comfortable with my choice.  I've been rewiring my bench so I haven't given it a full work out, but so far it has been very pleasant to use just playing around.  I'm a retired research geophysicist, so this was a stretch to justify. But after buying a bunch of 90's HP gear, I got *really* tired of Chinese stuff.  So I bought a 33622A from the Keysight eBay store to replace my F***Tech FY6600 which borked itself the day after i fixed the 176 Vpp AC on the BNC grounds.  I had the misfortune to get one with the V3.0 FW.  Lots of promises of a fix from F***Tech, but nothing actually done.

I certainly can deconvolve it, but I wouldn't try doing it by eye.

As for BNC pulsers, Leo is making a new batch.  Email him.  He's *very* responsive.  But not website obsessed.
 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #389 on: May 31, 2018, 03:50:14 pm »
Putting an optional Gaussian filter on the waveform via an ASIC is *not* hard. Marketing gets their "1 GHz" spec and the user gets a sensible step response when they need one.  I'm hoping that user defined digital filters are an option.

I'm comfortable with my choice.  I've been rewiring my bench so I haven't given it a full work out, but so far it has been very pleasant to use just playing around.  I'm a retired research geophysicist, so this was a stretch to justify. But after buying a bunch of 90's HP gear, I got *really* tired of Chinese stuff.  So I bought a 33622A from the Keysight eBay store to replace my F***Tech FY6600 which borked itself the day after i fixed the 176 Vpp AC on the BNC grounds.  I had the misfortune to get one with the V3.0 FW.  Lots of promises of a fix from F***Tech, but nothing actually done.

I certainly can deconvolve it, but I wouldn't try doing it by eye.

As for BNC pulsers, Leo is making a new batch.  Email him.  He's *very* responsive.  But not website obsessed.

Well the issue is that scopes do this in the analog domain. Historically it was simply how fast the analog front end could go. It was difficult and expensive to make a 1GHz scope AFE so lower end models had cheaper ones that didn't go as fast. These days its not nearly as hard to do anymore but the scope manufacturers still want to sell 100MHz scopes to the ones who cant afford to pay more for a scope while still getting the money out of the big costumers who need the 1GHz. Same reason why the iPhone comes with various amounts of flash and becomes very expensive towards the higher end. Filtering the waveform digitally is noticeable by the user because the ADC starts to look better than it actually is. So as a result the new Keysight S series scopes have a AFE ASIC that has built in switchable filters to make the scope be a 500MHz or a 8GHz scope. It sucks being sold a license for something you already have but i do understand Keysight needs to make money on this so i can't really blame them for it.

That being said when they do provide digital filtering in the acquisition ASIC its actually very useful. I have a Agilent DSO9000 scope and they have decided to implement this as "High resolution mode", unlike the MegaZoom ASIC scopes here this simply puts ADC samples trough a digital low pass filter. Depending if you select 9bit 10bit 11bit or 12bit mode you get different low pass cutoffs (For a while they tried to sell this as a 10bit scope as a knee jerk reaction to LeCroys 10bit stuff). Because the ADC is a speedy 20GS/s means it gives it a lot of data points to work with and this brings the so called "12bit mode" with 500MHz of bandwidth. That's still plenty of bandwidth for most work, however the low pass filtering really cleans up the ADC noise a lot. Even on 10mV/div you get a nice sharp line on the screen rather than a big fuzzy band, but you don't get any of the funky effects that High Res mode does on Agilents MegaZoom scopes. It simply makes the scope act like its 500MHz but with a really low noise ADC.

So id love to see this done in more scopes. But there is a catch. The penny pinching 100MHz costumers get superbly low noise scope, but when the big fish 1GHz costumer comes along he will get the usual noise on his scope as there has to be no filtering to be able to reach 1GHz. This makes the 1GHz model seam like its not as good as the rest of the series might have you expect. Some of the specs are also going to look worse in the datasheet for the 1GHz model. So i don't think scope manufacturers are all that motivated to do this.
 

Offline rhb

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Re: Yet another fast edge pulse generator
« Reply #390 on: May 31, 2018, 10:57:45 pm »
Why do you think that the BW filtering has to be implemented in the AFE?  You have to anti-alias in the analog domain.  But it doesn't make economic sense to implement a separate analog filter for each sample rate. The initial sampling is done at the maximum rate and then subsampled with digital anti-alias filters for lower sample rates.

I don't know what's in the Keysight MSOX ASIC, but I do know what's in the Instek MSO.  That's a Zynq 7010.  And there are definitely not a large number of analog filters in the front end of the Instek 2000E line.  That's just a set of attenuators and buffering for the ADC input.  My MSO2204EA offers 200 MHz, 100 MHz and 20 MHz BWs.


 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #391 on: June 01, 2018, 03:20:54 am »
I looked into it again and sorry it seams i was wrong about the S series.

The S series does make use of all digital filtering to limit bandwidth. They talk about easy bandwidth upgrades when talking about the fancy new analog front end ASIC, but looking at the specs in detail the ASIC only has 20MHz and 200MHz analog filters inside it.

As a result the lower bandwidth units have significantly lower noise floors. At 1V/div the 500MHz model gets 5 mVrms of noise while the 8GHz model gets 24.1 mVrms. So yes the lower bandwidth models are simply locked into using that "DSP high resolution mode" permanently, only the 8GHz model allowing you to turn it off completely.

The X2000 and X3000 are not up gradable in the same way to take the lowest bandwidth model and turn it into the highest bendwith one so the analog front ends are different between some models.

I went and had a try at what this high resolution filter does on a signal from this picosecond pulse generator and here is what i got in "12 bit" mode where the bandwith is supposed to be 500MHz. There is a little bit of overshoot visible but its not nearly as drastic as in the example from KE5FX
 

Offline Howardlong

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Re: Yet another fast edge pulse generator
« Reply #392 on: June 01, 2018, 05:43:57 am »
auto/manual are all dedicated buttons and i use them all the time

Yes, auto/manual trigger really should have its own dedicated button. The 3000 you can assign trigger mode to the Quick Action key, but that’s not an available option on the 7000. Even the 54832D has a dedicated trigger mode button.
 

Offline rhb

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Re: Yet another fast edge pulse generator
« Reply #393 on: June 01, 2018, 06:53:45 am »
That's a pretty respectable step response.  Attached is the MSOX3104T response to my pulser which Leo measured at 36 pS.   I'm not concerned about the actual rise time, but I'm not happy about the overshoot.  How would you ever use it to check for ringing on a DUT line?

As for SW upgrading the X3000T BW you certainly can.

https://www.keysight.com/en/pdx-x202185-pn-MSOX3034T/mixed-signal-oscilloscope-350-mhz-4-analog-plus-16-digital-channels?pm=upg&nid=-32541.1150362&cc=US&lc=eng

So why they don't allow the user  to select  100, 200, 350 or 500 MHz BW on a scope licensed for 1 GHz is beyond me.

Edit: I just spoke with Keysight support.  The math functions give me a low pass filter option.  It doesn't work at all at 500 pS/Div, but does at 1 nS/Div.  Interestingly if I apply a 1 GHz low pass filter I get ~1% overshoot and a 520 pS rise time.
« Last Edit: June 01, 2018, 08:04:41 am by rhb »
 

Offline Berni

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Re: Yet another fast edge pulse generator
« Reply #394 on: June 01, 2018, 03:15:31 pm »
Yep the auto/manual trigger mode button is very useful. Then again at least these scopes have very little in the way of completely useless buttons, some could be buried in menus to make room for more useful ones tho.

And yes you can upgrade the bandwidth on the X3000 series but not all the way. It appears they make 3 flavors of the scope mainboard:
-Low: 100Mhz and 200MHz
-Mid : 350MHz and 500MHz
-High: 1GHz

So the analog front end might have two bandwidth modes to select between much like the ever popular Rigol 1000Z series (I think they use a varicap or PIN diode or something to switch it). But yes they surely could expose this trough software I could see it useful being able to switch a scope between 100 and 200 MHz.
 

Offline Converter

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Re: Yet another fast edge pulse generator
« Reply #395 on: July 03, 2018, 10:52:04 pm »
That's what I got with the LeCroy SDA 6000. I used a homemade adapter for the Leo BNC-connector, so it could have a little effect on the result. Nevertheless, this result is in line with the LeCroy specification (<75 ps), if you take into account your own Rise time of the signal source - about 30 ps.
 

Offline Zenwizard

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Re: Yet another fast edge pulse generator
« Reply #396 on: August 01, 2018, 10:21:54 am »
Is this still available? I would like to get one so I can do a high frequency response cal on a Tek 485 my self.
 

Online tautech

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Re: Yet another fast edge pulse generator
« Reply #397 on: August 01, 2018, 10:24:18 am »
Is this still available? I would like to get one so I can do a high frequency response cal on a Tek 485 my self.
http://www.leobodnar.com/shop/index.php?main_page=product_info&cPath=124&products_id=295
Avid Rabid Hobbyist
 

Offline Leo Bodnar

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Re: Yet another fast edge pulse generator
« Reply #398 on: August 01, 2018, 06:34:52 pm »
Is this still available? I would like to get one so I can do a high frequency response cal on a Tek 485 my self.
We are making a new batch of BNC ones, they should be back in the shop in one-two weeks (mid-August.)
Thanks
Leo

 
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Offline Hydron

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Re: Yet another fast edge pulse generator
« Reply #399 on: August 01, 2018, 11:32:43 pm »
So why they don't allow the user  to select  100, 200, 350 or 500 MHz BW on a scope licensed for 1 GHz is beyond me.

100% agree - especially on units with many bandwidth steps available in the same hardware (e.g. my R&S unit has 70, 100 and 200 MHz options below the 300 mine is licensed for) the system is calibrated for the other steps and the analogue or digital filter is already waiting to be used. If I have a 50MHz bandwidth signal I'd love to be able to see it without all the other 250MHz worth of noise I don't need (and sometimes aberrations - e.g. a probe specced at 50MHz may not simply roll off afterwards, it could have some nasty peaks above the rated frequency).
 


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