[time-nuts] OT - Portable Digital 'scope
hmurray at megapathdsl.net
Thu Feb 23 23:42:27 UTC 2012
> 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...
The trick is that Nyquist still holds. You can capture a 1 GHz signal with a
40 MHz ADC as long as the signal bandwidth is only 20 MHz.
It may get tricky to build an anti-aliasing filter with those parameters.
As Jim said, the analog front end and sample-hold needs the raw bandwidth and
the clock driving the sample-hold needs good accuracy.
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