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DIY PC Oscilloscope

Hi all,
I've started designing a 200 MSPS Oscilloscope with a 25 MHz analog bandwidth, 3Ksample/channel buffer.
The oscilloscope can be connected to a PC via USB, and eventually via Ethernet and WiFi.
The trigger is ( at moment) rising, falling... auto, normal.

I would appreciate suggestions and or comment for possible improvement and functionality that could make this oscilloscope interesting.

Thanks,
Francesco
 
F

Francesco Poderico

It will have 5 ns p-p trigger jitter, unless you do tricks, which would be

interesting.
In theory is possible to have a 5 ns p-p trigger jitter, but only when the noise floor is less then 0.5 LSB, otherwise you need to add some form of histeresys that will increase the jitter opening. but in theory is possible
Are you using an FPGA? Even low-end parts usually have more memory than that.
I'm using an FPGA for triggering and buffer. at moment a small device, as I'm quite impressed with the results (tomorrow I will post a video on my blog) next revision I'm planning a SPartan 6 with external memory IF.
The hard part is the front-end, attenuators and such.
Correct, I have spend 2 months or maybe 3 just to design the analog parts, I must admit I believe I have over designed, but i'm very happy with the results so far. My plan is to measure and publish the analog bandwidth.
You can build a scope with nasty transient response, untweaked attenuators and

such, and fix the response in the PC software.

The scope has a compensated attenuation in the front end that seems doing agood job. Therefore there is not needed for SW tricks. I personally don't like
What's the cheapest comparable thing on the market now? Your parts cost might be

in the $40 ballpark, maybe even at 50 MHz.

I think the Picoscope 200 series is the main competitor, which is around 200£ + VAT
 
T

Tim Williams

Francesco Poderico said:
I'm using an FPGA for triggering and buffer. at moment a small device,
as I'm quite impressed with the results (tomorrow I will post a video
on my blog) next revision I'm planning a SPartan 6 with external memory
IF.

Why only 25MHz? Yes it's easier, but the ADC probably has an analog
bandwidth in the GHz. Why waste it?

My TDS460 has 100MSa/s and 350MHz bandwidth. For periodic signals, it's
as good as an off-the-shelf MSO or DPO series (arguably better since it
doesn't have that retarded DPO crap you can't turn off), which usually
claim something ignorant like 5GSa/s and a mere 100MHz.

As John says, the attenuator is the hard part. Perhaps you could pilfer
them from old analog or digital scopes, and feed the output to your ADC
and trigger generator.

Tim
 
F

Francesco Poderico

Why only 25MHz? Yes it's easier, but the ADC probably has an analog

bandwidth in the GHz. Why waste it?



My TDS460 has 100MSa/s and 350MHz bandwidth. For periodic signals, it's

as good as an off-the-shelf MSO or DPO series (arguably better since it

doesn't have that retarded DPO crap you can't turn off), which usually

claim something ignorant like 5GSa/s and a mere 100MHz.



As John says, the attenuator is the hard part. Perhaps you could pilfer

them from old analog or digital scopes, and feed the output to your ADC

and trigger generator.



Tim



--

Deep Friar: a very philosophical monk.

Website: http://seventransistorlabs.com

Thanks Tim,
good point,
Actually in theory I could use the DLL inside the FPGA and changing the clock phase with a resolution of 25 ps... so in theory I have a 1 GSPS FPGA for repetitive signals!
Do you think it would make more attractive to buy?
Actually the maximum bandwidth is 70 MHz, I'm low filtering for aliasing.
 
F

Francesco Poderico

There are plenty of similar USB scopes on the market with similar

specs. Use their feature list as a checklist to see if you're missing

something. I presume that you're asking for advice on how to make the

hardware unique in order to obtain a marketing advantage. Most of the

product differentiation is in the software, so I'll comment only on

the hardware. What *I* might want in a USB scope is (whether I'm

willing to pay for these is another question):



1. USB microscope camera on the probe tip so that I can see where I'm

sticking the probe.

2. IR thermometer with LED targeting on the probe so that I can

simultaneously measure and record any localized heating.

3. Replacable front end so that when I inevitably blow it out, it can

be easily replaced.

4. Built in function generator, pattern generator, arbitrary waveform

generator, two tone generator, DDS generator, white/pink noise, sweep

generator generator. Extra points for adding AM/FM/PM/pulse

modulation.

5. DC power output for powering active probes and assorted sensors.

6. Presumably, it's dual channel. If so, make it stackable via a

common bus or preferably via ethernet, so that multiple units can be

conglomerated into a 4,6,8, etc channel scope.

7. Switchable low pass filter on input for high RF level

environments.

8. Self test and calibration check.

9. An accurate and readable schematic so I can fix it.

10. Security cable to keep it from getting um.... borrowed.

11. Built in smoke detector.



--

Jeff Liebermann [email protected]

150 Felker St #D http://www.LearnByDestroying.com

Santa Cruz CA 95060 http://802.11junk.com

Skype: JeffLiebermann AE6KS 831-336-2558

Thnanks Jeff,
I like the idea of the Cable that stop "unwanted borrowed" I will actually do this... not sure about the camera on probe :)...

I may add a block diagram ( notthe full schematic) to show people how to calibrate the scope and some test points and hits to help people to self repair the scope.

also the function generator, could be a good sales point

Thanks
 
T

Tim Williams

Francesco Poderico said:
Actually in theory I could use the DLL inside the FPGA and changing the
clock phase with a resolution of 25 ps... so in theory I have a 1 GSPS
FPGA for repetitive signals!

Nah, 25ps resolution means you can take acquisitions shifted in time every
25ps -- 40GSPS equivalent! But, do beware of the manufacture and
temperature variation of the delay. FPGAs are made for worst-case timing
constraints, and anything that's adjustable (like a DLL) is put inside a
loop (hence, DL*L*).
Do you think it would make more attractive to buy?

Heck, make it better than a Rigol, from cheaper parts. And open source it
(if it's that kind of project). Create shock waves in the marketplace!
;-)
Actually the maximum bandwidth is 70 MHz, I'm low filtering for
aliasing.

My TDS460 says "50GS/s ET" (Equivalent Time) on 1ns/div. Looks like it's
actually filling the whole screen with that resolution, not just
interpolating between samples.

The 100MHz, ~10MS/s HP54600B goes down to 1 or 2ns/div or something like
that, but it uses sinc interpolation on such fine scales. I don't know
what a Rigol does for high sampling rates.

If you skip antialiasing filters, you get sharper displays and equivalent
time sampling. If you filter, you're basically saying: "Yep, I purchased
this 200MS/s ADC, and the FPGA to run it, and I'm taking both of them over
to the grinder and shaving off... ohh about 80% of the silicon, because...
I like the smell?"

Filtering basically negates the trigger generator, too. You could sample
continuously and do the trigger digitally instead. Which...

....I think for any particular period between trigger crossings, you'd want
to use some sort of interpolation to adjust the timing -- sample N is a
bit below the threshold, and sample N+1 crossed the threshold by so-and-so
and caused a trigger, so we know the dV/dt and can estimate where it *did*
actually cross. So we can stack this sample period on top of the other,
which crossed at whatever rate, by aligning it. Shift everything sideways
by the calculated time difference, using a suitable interpolation (a
simple quadratic spline would be reasonable, but a sinc interpolation
might be more physically significant), then stack it up for display.

The "stacking" itself could be an array of buffers (so you have a memory
of many traces, and can perform statistics on them to generate the
display -- decimate (view only one), average (which would now be a FIR
"sliding average", which AFAIK, no scope manufacturer ever does for
averaging mode!), peak detect, median, RMS + std dev., etc. This display
process could, of course, be done for either acquisition method
(continuous or triggered).

But again, such hackery, though interesting, doesn't get your bandwidth
back, it's just software diddling for not being clever enough to get the
bandwidth and trigger right in the first place. :)

Tim
 
Thnanks Jeff,
I like the idea of the Cable that stop "unwanted borrowed" I will actually do this... not sure about the camera on probe :)...

I may add a block diagram ( notthe full schematic) to show people how to calibrate the scope and some test points and hits to help people to self repair the scope.

also the function generator, could be a good sales point

Thanks

the first thing I would add is USB isolation (data and power) so that
when using usb and/or
powering it from usb the scope is isolated from the pc

-Lasse
 
F

Francesco Poderico

the first thing I would add is USB isolation (data and power) so that

when using usb and/or

powering it from usb the scope is isolated from the pc



-Lasse

Good point,
I was thinking to use an ADuM5xxxx from AD.
 
F

Francesco Poderico

Been there done that. Spent many weeks putting together a box that I was

very proud of, then purchased a Rigol that does everything it does plus a

lot more and a lot better. Is this really a good use of your time and

energy?

I get the point... and your are not the first one to tell me that.
but I believe that an oscilloscope is still an instruments a lot of electronic enthusiastic would love to have it if priced correctly.
Marketing will be the biggest challenge in a market so competitive.
 
N

Nico Coesel

Francesco Poderico said:
I get the point... and your are not the first one to tell me that.
but I believe that an oscilloscope is still an instruments a lot of electronic enthusiastic would love to have it if priced correctly.
Marketing will be the biggest challenge in a market so competitive.

Did you look what a Siglent scope costs on Ebay? IIRC its about US
$180 including shipping from China. How do you expect to beat that?
BTW Lecroy sells the exact same scope for a lot more.
 
J

Jamie

Nico said:
Did you look what a Siglent scope costs on Ebay? IIRC its about US
$180 including shipping from China. How do you expect to beat that?
BTW Lecroy sells the exact same scope for a lot more.
Yeah, but it has Lecroy's name on it and that's like GOLD!!!!!!!!!!!

:)
Jamie
 
J

John Devereux

Did you look what a Siglent scope costs on Ebay? IIRC its about US
$180 including shipping from China. How do you expect to beat that?
BTW Lecroy sells the exact same scope for a lot more.

I've wanted to make something like a "dynamic signal analyser" that goes
up to 1 or 10 MHz, instead of 100kHz like they all seem to.

High dynamic range (16+ bit) low noise, with arb generator.

I've got one of these, and I like it, but it is a big old slow beast of
a thing:

<http://www.teknetelectronics.com/Search.asp?p_ID=13689&pDo=DETAIL>
 
F

Francesco Poderico

Francesco Poderico wrote:








Did you look what a Siglent scope costs on Ebay? IIRC its about US

$180 including shipping from China. How do you expect to beat that?

BTW Lecroy sells the exact same scope for a lot more.



--

Failure does not prove something is impossible, failure simply

indicates you are not using the right tools...

nico@nctdevpuntnl (punt=.)

--------------------------------------------------------------

it's challenging but not impossible.
I agree...It is very difficult to compete with China, but not impossible if we point on quality.
 
N

Nico Coesel

Francesco Poderico said:
it's challenging but not impossible.
I agree...It is very difficult to compete with China, but not impossible if we point on quality.

If you think the Chinese can't make quality stuff then you've made
your first big mistake. If its good enough for Lecroy to stick their
badge on the quality bit is in order.

So re-think your competitive edge. Remember that a good DSO is 5%
hardware and 95% software. So even if you get the hardware right you
are still nowhere near a sellable product.
 
R

Roberto Waltman

Jeff said:
6. Presumably, it's dual channel. If so, make it stackable via a
common bus or preferably via ethernet, so that multiple units can be
conglomerated into a 4,6,8, etc channel scope.

From my (rather narrow) point of view, where the main use of an
oscilloscope is in debugging embedded software, this would make it
stand out above the competition.
 
F

Francesco Poderico

Jeff said:
6. Presumably, it's dual channel. If so, make it stackable via a
common bus or preferably via ethernet, so that multiple units can be
conglomerated into a 4,6,8, etc channel scope.



From my (rather narrow) point of view, where the main use of an

oscilloscope is in debugging embedded software, this would make it

stand out above the competition.

--

Roberto Waltman



[ Please reply to the group,

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Thanks Roberto,
I'm not sure what do you mean with a stackable oscilloscope, do you mean... e.g. 4 oscilloscopes... 2 channel each that makes a 8 channel oscilloscope?
The only problem I see with that is that between each scope you can have 20 ns max. of time offset.
 
H

hamilton

Thanks Roberto,
I'm not sure what do you mean with a stackable oscilloscope, do you mean... e.g. 4 oscilloscopes... 2 channel each that makes a 8 channel oscilloscope?
The only problem I see with that is that between each scope you can have 20 ns max. of time offset.

Wouldn't it be easier to pass the trigger between modules ??

Getting one signal right would be easier then getting 4-8-12 signals right.

My $0.02
 
R

Roberto Waltman

I'm not sure what do you mean with a stackable oscilloscope, do you mean... e.g. 4 oscilloscopes... 2 channel each that makes a 8 channel oscilloscope?
That is what I understood from Jeff's post.
The only problem I see with that is that between each scope you can have 20 ns max. of time offset.
That could be unacceptable in some scenarios, and not important at all
in others. May be you can add a common clock or some other HW trick to
synchronize several units?
 
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