Beware that analog design at 100Mhz is much more difficult than at 20 or even 50 MHz.  Maintaining signal quality through the input buffers, circuit board traces, sample-and-hold circuit and into a fast A/D requires careful design and much attention to PCB layout details.  Cross-talk, clock bleed-through and general noise are big problems.  It can be done (<g>) but it's not something you can throw together and expect to work.

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From:   Sean Breheny
Sent:   Thursday, December 11, 1997 10:36 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        Re: Very Fast RAM

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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