At 09:16 PM 12/11/97 -0600, you wrote:
>>I was just looking at a few oscilloscope data sheets and, you are right,
>>many of these scopes sample at repetitive rates to achieve their peak
>>performance. However, several can do flat-out 1GS/s rates, the HP Infinium,
>>is one example.
>
>Yep, some can, some can't.  Another technique used in some of the older
>digital scopes was to clock the analog data into a fast ccd, and then clock
>it out to an adc at a much slower rate.  Philips and Tek both used this
>method, but they also had the ability to custom manufacture the required
>devices.
>
>Tek also made some digitizers using a video-camera like device that sweeped
>an electron beam across a 512 * 512 pixel diode target.  The charge on each
>'pixel' of the target could then be read out after the trace and either
>displayed (analog) or digitized and stored.  This topped out at something
>like the equivalent of 500 GSamples/second!
>
>As far as I know, all the newer scopes have gone to using fast flash ADCs.
>
>Seems to me the problem for the individual builder isn't finding fast ram
>(after all, you could interleave 16 sets of common 15 ns cache sram), but
>the front end and adc circuits.  Even a 300-400 MHz analog bandwidth is
>going to require some pretty 'tweaky' amps and attenuators, and lots o'
>gain-bandwidth product in the active devices.  Very fast flash converters
>aren't likely to be inexpensive, and their high (and variable) input
>capacitance isn't trivial to drive.
>

Well, my idea is somewhere in the neighborhood of 100MHz analog BW. I
realize that I don't have that much design experience to get good results
up to several 100s of MHz., but I don't see why 100MHz would be too hard.
There are lots of video amps and 100MHz op amps out there which are not too
expensive and (so it would seem to me) have BW around 100MHz. I have found
30MS/s ADCs for $10. I hope, and I don't see why it would be too hard, to
buffer the input of this ADC with a 100MHz op amp,  a (roughly) purely
resistive attenuator (made from a rotary switch and several resistors),
send the output of the ADC to some SRAM, clock the ADC with a PLL with
programmable dividing radio (to get adjustable sample rate with very
accurate time base), read the output either to a PC or to a microcontroller
(PIC :-) ) at a slower rate to feed to an LCD. I could use the multiple
sampling technique to digitize at more than 30MS/s as well as go flat out
at 30MS/s for up to 15MHz non-periodic stuff. I realize that this would not
be as good as a $1000 Fluke scopemeter or similar device, but for my
budget, I think it would make an interesting project and be able to
visualize waveforms for most hobby/student applications. Am I wrong/wasting
my money?


Thanks for the help,

Sean
+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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