DIY PC Oscilloscope

On Sat, 9 Mar 2013 10:22:38 -0800 (PST), francescopoderico@googlemail.com wrote:

>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

OK, here's a patentable idea that I donate to humanity:

The scope trigger fires a delay and then makes some analog pattern, like a sine
burst or chirp or some pseudo-random mess made from delay lines or something.
Something with a nice sharp autocorrelation function.

Mix that into the scope vertical signal. The PC software looks at that, figures
out its timing, and removes the +-1 clock jitter from the displayed data.

The burst can be delayed and thereby separated in time from the displayable
waveform, or it can be superimposed, no delay, and subtracted out.

Of course, the idea works even better if you have another ADC channel to spare.


-- 

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

On Thu, 14 Mar 2013 09:29:48 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>
>
>OK, here's a patentable idea that I donate to humanity:
>
>The scope trigger fires a delay and then makes some analog pattern, like a sine
>burst or chirp or some pseudo-random mess made from delay lines or something.
>Something with a nice sharp autocorrelation function.
>
>Mix that into the scope vertical signal. The PC software looks at that, figures
>out its timing, and removes the +-1 clock jitter from the displayed data.
>
>The burst can be delayed and thereby separated in time from the displayable
>waveform, or it can be superimposed, no delay, and subtracted out.
>
>Of course, the idea works even better if you have another ADC channel to spare.

We're doing something like that to synchronize phase on several
free-running ADCs in a distributed data acquisition system on a
non-deterministic network. Works, but I'd rather have it done by
hardware. 

On 3/14/2013 12:03 PM, Nico Coesel wrote:
> rickman<gnuarm@gmail.com>  wrote:
>
>> On 3/12/2013 4:11 PM, Nico Coesel wrote:
>>> rickman<gnuarm@gmail.com>   wrote:
>>>>
>>>> I would like to hear from others about why the front end is the hard
>>>> part.  Exactly how do the attenuators work?  Does the amp remain set to
>>>> a given gain and the large signals are attenuated down to a fixed low
>>>> range?
>>>
>>> You need to use capacitive dividers which need adjustment. In my
>>> design I used one varicap (controlled by a DAC) to do all the
>>> necessary adjustments for several ranges. Nowadays you could use a 12
>>> bit ADC so you wouldn't need a variable gain amplifier. Another trick
>>> to get a programmable range is to vary the reference voltage of the
>>> ADC. I think I re-did the design of the front-end about 3 or 4 times.
>>
>> I don't get how a 12 bit ADC solves the attenuator problem.  That is
>> only 2 bits more than what I would like to see in a front end.  Once a
>
> Its a factor 16 attenuation you don't need to do in hardware. So if
> the hardware attenuator does 1:1.5, 1:10 and 1:100  (which is doable
> with 2 relays) you save quite some circuitry. 8 bits is probably more
> then enough. It will be hard to get the response so flat that more
> than 8 bits actually adds accuracy of the readout.

I've worked with 8 bit scopes and the vertical clearly shows steps which 
I find interfere with making reasonable measurements.  That's why I said 
2 spare bits out of 12.  There is also a need for zooming in on a 
portion of a captured trace.  At 8 bits all you see is the steps.  With 
a full 12 bits you have a little bit of extra resolution so you can 
actually get a bit of detail.

-- 

Rick

rickman <gnuarm@gmail.com> writes:

> On 3/14/2013 12:03 PM, Nico Coesel wrote:
>> rickman<gnuarm@gmail.com>  wrote:
>>
>>> On 3/12/2013 4:11 PM, Nico Coesel wrote:
>>>> rickman<gnuarm@gmail.com>   wrote:
>>>>>
>>>>> I would like to hear from others about why the front end is the hard
>>>>> part.  Exactly how do the attenuators work?  Does the amp remain set to
>>>>> a given gain and the large signals are attenuated down to a fixed low
>>>>> range?
>>>>
>>>> You need to use capacitive dividers which need adjustment. In my
>>>> design I used one varicap (controlled by a DAC) to do all the
>>>> necessary adjustments for several ranges. Nowadays you could use a 12
>>>> bit ADC so you wouldn't need a variable gain amplifier. Another trick
>>>> to get a programmable range is to vary the reference voltage of the
>>>> ADC. I think I re-did the design of the front-end about 3 or 4 times.
>>>
>>> I don't get how a 12 bit ADC solves the attenuator problem.  That is
>>> only 2 bits more than what I would like to see in a front end.  Once a
>>
>> Its a factor 16 attenuation you don't need to do in hardware. So if
>> the hardware attenuator does 1:1.5, 1:10 and 1:100  (which is doable
>> with 2 relays) you save quite some circuitry. 8 bits is probably more
>> then enough. It will be hard to get the response so flat that more
>> than 8 bits actually adds accuracy of the readout.
>
> I've worked with 8 bit scopes and the vertical clearly shows steps
> which I find interfere with making reasonable measurements.  That's
> why I said 2 spare bits out of 12.  There is also a need for zooming
> in on a portion of a captured trace.  At 8 bits all you see is the
> steps.  With a full 12 bits you have a little bit of extra resolution
> so you can actually get a bit of detail.

I agree a 12 bit (or 16 bit!) scope would be nice. Lecroy make one I
think but it is very expensive.

The situation with the standard 8 bits is not quite as bad as you
portray in a higher end scope. They can sample at the full maximum
digitizer rate (5 or 20 GSPS say) then do real-time averaging/DSP on it
so that each point plotted at lower sweep rates represents the average
of hundreds of samples potentially. The noise at 20GSPS smears out the
steps then the averaging smooths out the noise. Or something like that.
Anyway the result is much better than you would think from the 8 bit
input.


-- 

John Devereux

rickman <gnuarm@gmail.com> wrote:

>On 3/14/2013 12:03 PM, Nico Coesel wrote:
>> rickman<gnuarm@gmail.com>  wrote:
>>
>>> On 3/12/2013 4:11 PM, Nico Coesel wrote:
>>>> rickman<gnuarm@gmail.com>   wrote:
>>>>>
>>>>> I would like to hear from others about why the front end is the hard
>>>>> part.  Exactly how do the attenuators work?  Does the amp remain set to
>>>>> a given gain and the large signals are attenuated down to a fixed low
>>>>> range?
>>>>
>>>> You need to use capacitive dividers which need adjustment. In my
>>>> design I used one varicap (controlled by a DAC) to do all the
>>>> necessary adjustments for several ranges. Nowadays you could use a 12
>>>> bit ADC so you wouldn't need a variable gain amplifier. Another trick
>>>> to get a programmable range is to vary the reference voltage of the
>>>> ADC. I think I re-did the design of the front-end about 3 or 4 times.
>>>
>>> I don't get how a 12 bit ADC solves the attenuator problem.  That is
>>> only 2 bits more than what I would like to see in a front end.  Once a
>>
>> Its a factor 16 attenuation you don't need to do in hardware. So if
>> the hardware attenuator does 1:1.5, 1:10 and 1:100  (which is doable
>> with 2 relays) you save quite some circuitry. 8 bits is probably more
>> then enough. It will be hard to get the response so flat that more
>> than 8 bits actually adds accuracy of the readout.
>
>I've worked with 8 bit scopes and the vertical clearly shows steps which 
>I find interfere with making reasonable measurements.  That's why I said 

That has more to do with how the software shows the signal.
Interpolation can solve a lot.

-- 
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

"John Devereux" <john@devereux.me.uk> wrote in message 
news:87620tvz6t.fsf@devereux.me.uk...
> The situation with the standard 8 bits is not quite as bad as you
> portray in a higher end scope. They can sample at the full maximum
> digitizer rate (5 or 20 GSPS say) then do real-time averaging/DSP on it
> so that each point plotted at lower sweep rates represents the average
> of hundreds of samples potentially. The noise at 20GSPS smears out the
> steps then the averaging smooths out the noise. Or something like that.
> Anyway the result is much better than you would think from the 8 bit
> input.

It also reduces aliasing.  Mine has a "high res" mode which does this --  
only works below a certain range, of course.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com 

On 3/15/2013 4:34 AM, John Devereux wrote:
> rickman<gnuarm@gmail.com>  writes:
>
>> I've worked with 8 bit scopes and the vertical clearly shows steps
>> which I find interfere with making reasonable measurements.  That's
>> why I said 2 spare bits out of 12.  There is also a need for zooming
>> in on a portion of a captured trace.  At 8 bits all you see is the
>> steps.  With a full 12 bits you have a little bit of extra resolution
>> so you can actually get a bit of detail.
>
> I agree a 12 bit (or 16 bit!) scope would be nice. Lecroy make one I
> think but it is very expensive.
>
> The situation with the standard 8 bits is not quite as bad as you
> portray in a higher end scope. They can sample at the full maximum
> digitizer rate (5 or 20 GSPS say) then do real-time averaging/DSP on it
> so that each point plotted at lower sweep rates represents the average
> of hundreds of samples potentially. The noise at 20GSPS smears out the
> steps then the averaging smooths out the noise. Or something like that.
> Anyway the result is much better than you would think from the 8 bit
> input.

You say that the 16 bit converters are expensive, then talk about using 
a 20 GHz 8 bit ADC.  Is that not expensive, not to mention the clocking, 
the board for the high speed signals and the power supply to make all 
this happen?  I can't imagine this is actually a better approach to 
designing a scope with a stated goal of 20-25 MHz bandwidth.  I would 
like to see at least 300 MHz, but the OP says 25 is good enough.

-- 

Rick

On 3/15/2013 6:06 AM, Nico Coesel wrote:
> rickman<gnuarm@gmail.com>  wrote:
>
>> On 3/14/2013 12:03 PM, Nico Coesel wrote:
>>>
>>> Its a factor 16 attenuation you don't need to do in hardware. So if
>>> the hardware attenuator does 1:1.5, 1:10 and 1:100  (which is doable
>>> with 2 relays) you save quite some circuitry. 8 bits is probably more
>>> then enough. It will be hard to get the response so flat that more
>>> than 8 bits actually adds accuracy of the readout.
>>
>> I've worked with 8 bit scopes and the vertical clearly shows steps which
>> I find interfere with making reasonable measurements.  That's why I said
>
> That has more to do with how the software shows the signal.
> Interpolation can solve a lot.

That may be.  The OP was talking about debugging the sinc reconstruction 
and I've been thinking a little bit about just how useful that is.  Is 
there a downside to sinc reconstruction, other than the work required? 
I was thinking an aliased signal might interfere with this, but now that 
I give it some thought, I realize they are two separate issues.  If you 
have an aliased tone, it will just be a tone in the display whether you 
use sinc reconstruction or not.

In fact, the *very* low end scope I was using may have had a limitation 
in the display itself!  8 bits is 256 steps.  That shouldn't be too big 
a problem.

-- 

Rick

rickman <gnuarm@gmail.com> writes:

> On 3/15/2013 4:34 AM, John Devereux wrote:
>> rickman<gnuarm@gmail.com>  writes:
>>
>>> I've worked with 8 bit scopes and the vertical clearly shows steps
>>> which I find interfere with making reasonable measurements.  That's
>>> why I said 2 spare bits out of 12.  There is also a need for zooming
>>> in on a portion of a captured trace.  At 8 bits all you see is the
>>> steps.  With a full 12 bits you have a little bit of extra resolution
>>> so you can actually get a bit of detail.
>>
>> I agree a 12 bit (or 16 bit!) scope would be nice. Lecroy make one I
>> think but it is very expensive.
>>
>> The situation with the standard 8 bits is not quite as bad as you
>> portray in a higher end scope. They can sample at the full maximum
>> digitizer rate (5 or 20 GSPS say) then do real-time averaging/DSP on it
>> so that each point plotted at lower sweep rates represents the average
>> of hundreds of samples potentially. The noise at 20GSPS smears out the
>> steps then the averaging smooths out the noise. Or something like that.
>> Anyway the result is much better than you would think from the 8 bit
>> input.
>
> You say that the 16 bit converters are expensive, then talk about
> using a 20 GHz 8 bit ADC.  Is that not expensive, not to mention the
> clocking, the board for the high speed signals and the power supply to
> make all this happen?

Yes, it is very expensive. I did say "high end scope", by which I mean
"really expensive". They usually need to go to GHz anyway, so already
have the high speed digitizer. At lower bandwidths they can utilise the
excess samples to increase the apparent resolution.

> I can't imagine this is actually a better approach to designing a
> scope with a stated goal of 20-25 MHz bandwidth.  I would like to see
> at least 300 MHz, but the OP says 25 is good enough.

Absolutely, to me the only point of a 25MHz scope would be if it was
higher resolution, 16+ bit ideally. Otherwise you may as well just use
one of those cheap USB gadgets. A "dynamic signal analyser" that goes
above 100kHz seems to be missing from the market AFAIK. So it could do
good spectrum analysis, evaluate noise, servo loops, have a tracking
generator and plot filter responses, that sort of thing.


-- 

John Devereux

rickman <gnuarm@gmail.com> wrote:

>On 3/15/2013 6:06 AM, Nico Coesel wrote:
>> rickman<gnuarm@gmail.com>  wrote:
>>
>>> On 3/14/2013 12:03 PM, Nico Coesel wrote:
>>>>
>>>> Its a factor 16 attenuation you don't need to do in hardware. So if
>>>> the hardware attenuator does 1:1.5, 1:10 and 1:100  (which is doable
>>>> with 2 relays) you save quite some circuitry. 8 bits is probably more
>>>> then enough. It will be hard to get the response so flat that more
>>>> than 8 bits actually adds accuracy of the readout.
>>>
>>> I've worked with 8 bit scopes and the vertical clearly shows steps which
>>> I find interfere with making reasonable measurements.  That's why I said
>>
>> That has more to do with how the software shows the signal.
>> Interpolation can solve a lot.
>
>That may be.  The OP was talking about debugging the sinc reconstruction 
>and I've been thinking a little bit about just how useful that is.  Is 
>there a downside to sinc reconstruction, other than the work required? 

It is useable if you have at least 5 samples per period. So that is
0.2fs. The whole problem though is not the number of samples per
period. According to sampling theory the signal is there but it just
needs to be displayed properly so the operator can see a signal
instead of some 'random' dots. With the proper signal reconstruction
algorithm you can display signals up the the Nyquist limit (0.5fs).

>I was thinking an aliased signal might interfere with this, but now that 
>I give it some thought, I realize they are two separate issues.  If you 
>have an aliased tone, it will just be a tone in the display whether you 
>use sinc reconstruction or not.

In my design I used a fixed samplerate (250MHz) and a standard PC
memory module. 1GB already provides for more than 2 seconds of storage
for 2 channels. That solves the whole interference issue and it allows
to use a proper anti-aliasing filter. With polynomal approximation I
could reconstruct a signal even when its close to the Nyquist limit. I
tested it and I could get it to work for frequencies up to 0.45fs.

-- 
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------