[time-nuts] OT - Portable Digital 'scope

[Azelio Boriani]

Actually, undersampling does use the alias effect to bring down the RF
carrier. That is, the direct sampling radio concept cannot avoid the
aliasing: it is exploited to avoid, for example, to sample a 2GHz carrier
modulated with a 20MHz signal with a 4Gsample/second ADC (by the way, does
it exist?). A simple 20Msample/second ADC would be enough. Yes, to analyze
an analog signal in real time I doubt you can use this method, if the
signal is periodic maybe... you can advance the sampling trigger a bit,
cycle by cycle, and reconstruct the whole signal after this little amount
has covered a full cycle.

On Thu, Feb 23, 2012 at 10:56 PM, Jim Lux <jimlux at earthlink.net> wrote:

> On 2/23/12 1:25 PM, Robert LaJeunesse 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.
>>
>
> That doesn't mean you couldn't use a sampler running at, say, 50 MSPS to
> look at a 110 MHz signal (something we actually do in a radio).
>
> There are lots of ADCs out there that have RF bandwidths of much more than
> the sample rate, intended for use in direct sampling receivers. What
> performance really depends on is how good the sample/hold or track/hold is
> and what the sample jitter is.  (and of course, whether there's a stage in
> front to keep unexpected signals from aliasing in)
>
> There are several ADCs out there that have GHz bandwidths and max sample
> rates in the 100MSPS range.
>
>
>
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