[time-nuts] OT - Portable Digital 'scope

Jim Lux jimlux at earthlink.net
Thu Feb 23 23:16:27 UTC 2012


On 2/23/12 2:22 PM, Azelio Boriani wrote:
> 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.

Well.. a 40 MSPS ADC for a 20 MHz wide signal, but yes.

There *are* multi Gsample/second ADCs out there.  ADC12D1800, 12 bit, 
3.6 GSPS.
ADS5400 12 bit 1GSPS
etc



  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.
>
That's the way the "sampling head" used for microwaves works (back in 
the day, when tunnel diodes were a new and exotic thing, for instance). 
Still used to today on scopes where they call it things like "equivalent 
time sampling"...

basically a real fast sample and hold, and a not so fast ADC.


There's also clever schemes with fast S/H and multiple interleaved 
ADCs.. but accounting for the tracking errors between ADCs is always a 
chore.

And, various "sub-band-coding" approaches where you subdivide the 
frequency into sub-bands, digitize them in parallel and do fancy math to 
recombine it into a single stream of samples.  If the channelization and 
ADC clocks are cleverly chosen and derived from the same reference, you 
can do quite well (& compensate for differences among ADCs)


The high performance radar processing world is full of this kind of 
thing (look up STAP: space/time adaptive processing).




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