Re: 12 and 16-bit oscilloscopes

John Devereux <jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

> "Joel Kolstad" <JKolstad71HatesSpam@xxxxxxxxx> writes:

>> "John Devereux" <jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote in message
>> news:87d56hkrd9.fsf@xxxxxxxxxxxxxxxxxxxxxxxx

>>> I would love to make a "scope" based on something like the
>>> AD7660. It's a 24 bit 2.5MHz sigma delta ADC PC does post
>>> processing for "triggering", averaging, spectrum analysis

>> Probably a handy option, but having a PC sift through 2.5MSps
>> just looking for an edge to trigger on seems like a bit of a
>> waste of the CPU. With some programmable logic (CPLD/FPGA) in the
>> same box as the ADC, you could perform triggering there and then
>> be able to use much lower-end PCs.

> Probably - I have not really looked at the numbers too seriously.

> But desktop CPUs are *very* fast, and edge detection is pretty
> easy. It would already be doing signal averaging and maybe digital
> filtering (especially on the slower "timebases"). So simple level
> detection would not be much extra work.

> Of course we could do all this in a bigger FPGA, but then where do
> you stop.

I regularly take one million or more samples with the Binary Sampler
and search the waveforms for different characteristics. Even with
smoothing, the response from the keyboard on a 466MHz cpu is almost
instantaneous. You should have little trouble finding a desired
starting point to display a waveform.

The software is Borland Pascal using inline assembly for the
critical sections. I run DOS in Flat Real Mode (FRM) and store the
data in Extended Memory. FRM gives instant access to memory above 1
Meg without having to go through Himem, which is painfully slow.

Doing the edge detect in software eliminates the problems with
hardware triggering and the extra jitter it adds. I plan to use this
technique for an inexpensive 5GHz Binary Sampling scope running on a
pc. I was going to use the pc sound card to transfer the waveform,
but it needs to have a response to DC. So a simple 16-bit ADC is
probably the way to go.

The AD7660 looks quite interesting. The 120dB SNR up to 78KHz is
impressive. It might be a good compliment to the inherent
noise-rejection capability of the Binary Sampler. This might produce
an inexpensive scope with unique capability and wide bandwidth.

Regards,

Mike Monett

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