[time-nuts] OT - Portable Digital 'scope

[David]

Switched gain stages mean the bandwidth and transient response before
the ADC is going to change with different sensitivities unless both
are significantly limited which apparently is the case.

How do modern DSOs handle that?  I guess it would explain why I have
been told their front end calibration is so arduous.  If you lose the
calibration constants somehow, you might as well throw the
oscilloscope away.  Do any support user recalibration?

I agree that the floating input is a problem.  Maybe they got lucky
with that specific operational amplifier.

On Thu, 23 Feb 2012 13:25:51 -0800 (PST), Robert LaJeunesse
<rlajeunesse at sbcglobal.net> wrote:

>FWIW Rigol pushes their 40MHz Analog Devices part to 100 MHz without any problem 
>(seen in eevblog teardown). Yes it's sort of cheating, but if the part works 
>fine because all of the suppliers parts now yeild that fast due to an improved 
>process well, it saves a few dollars / quid / drachma...
>
>And the 40MSPS is over full temp range, likely this is not a problem for the DSO 
>203 which has NO temp rating.
>
>Yes the "72MHz analog" channel rating makes no sense for something sampling at 
>72MSPS, Nyquist says you get at most 36 MHz bandwith. I saw no anti-aliasing 
>filter (well, C9 or C11 and C73 do some roll off) so who knows what the screen 
>will actually show. Might make a decent 10MHz scope, at which point the use of 
>FET solid-state relays doesn't concern me so much.
>
>I was surprised that they use trimmers on the input to match the channels, 
>that's a nice touch but does add some loading. The U1B op-amp is disconnected 
>for higher input voltage ranges so it doesn't overload and distort the signal, 
>which is processed by the U1A amp for those ranges. Perhaps not good that the 
>U1B input is left effectively floating.
>
>________________________________
>From: Attila Kinali <attila at kinali.ch>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
>Sent: Thu, February 23, 2012 3:49:49 PM
>Subject: Re: [time-nuts] OT - Portable Digital 'scope
>
>On Thu, 23 Feb 2012 20:01:18 -0000
>"Rob Kimberley" <robkimberley at btinternet.com> wrote:
>
>> I'm looking at Item: 300658066641 on EBay, and wanted to know if anyone in
>> the group had any experience of this product. I know this is way off topic,
>> but as a group it's nice to know what's out there and possibly useful in our
>> mutual hobby.
>
>It depends what you want to use it for. If you just want to have
>something protable that can show you roughly what's going on,
>then this might be a good thing. For anything else, especially measurements,
>i wouldnt trust it further than i can throw it.
>
>It's actually quite nice that they put the schematics online too,
>so their claims can be verified.
>
>First that jumps out is that they are using a AD9288-40 as ADC.
>Note the -40 there? It means it's an 40Msps ADC. Ie the maximum
>usable BW you can have is 20MHz (actually a bit lower). The analog
>circuitry doesn't seem too bright either, but i guess you should be
>able to get the 20MHz. I havent checked the exact circuitry so
>i cannot say whether the input circuit does filter at 20MHz. But
>as they claim to have 72MHz analog bandwidth, i would be very carefull
>about aliasing problems
>
>The input circuitry isn't very impressive either and has an undefined
>input capacitance (there is a trimmer there) somwhere between 5pF and 30pF
>(plus stray capacitances). I find it a it strange, that they disconnect one
>input of the frontend opamp... it might anything from going into saturation
>to start bouncing around... 
>The analog switches are rather on the cheap side, nothing you'd expect
>in an DSO, but well.. for that price :-)
>
>Also notice that the connectors used look like SMB or MCX antenna
>connectors. Ie those would not be able to withstand many mating/break
>cycles.
>
>Overall, i'd say a good portable gauge, but not much more.
>
>For a more indepth analysis i'd have to take the schematics apart
>into a readable format... 
>
>            Attila Kinali