[time-nuts] OT - Portable Digital 'scope

David davidwhess at gmail.com
Thu Feb 23 23:39:52 UTC 2012

Sampling oscilloscopes and digital storage oscilloscopes that support
equivalent time sampling do this very thing.  My Tektronix 2230 with a
20 MS/sec flash converter has a bandwidth of 100 MHz and a 2 GS/sec
equivalent time sampling rate.  A 7854 with a 500 kS/sec sampler (at
10 or 11 bits) and is good to at least 400 MHz.  A 7T11 samples at 50
kS/sec but has a 14 GHz bandwidth with an S-4 sampling head.

All of the above examples rely on repetitive signals and use one or
another form of time to voltage conversion.  The 2230 directly
measures the time difference between the trigger and sample clock with
a time to voltage converter.  The 7854 simultaneously samples the
signal and the sweep.  The 7T11 sequentially or randomly triggers the
sampler at different sweep positions.

The DS203 is relying on digital triggering after the ADC so equivalent
time sampling is possible but subject to aliasing of the reconstructed
trigger waveform itself.  I presume each acquisition record is aligned
with the waveform record before being merged.  Some more recent high
performance DSOs work this way.

On Thu, 23 Feb 2012 23:22:01 +0100, Azelio Boriani
<azelio.boriani at screen.it> 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. 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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