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'Very Fast RAM'
1997\12\11@194746 by Sean Breheny

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I am looking for very fast ram/fifo buffer ICs for a possible digital
oscilloscope project. Mainly, I am wondering what the fastest commonly
available RAM or FIFO is, and how fast it is. By doing a search on the web,
the fastest I could find was about 3.3 or 4 ns access time. This would
probably work, but I am just wondering what exists. Does anyone know what
DSOs do to capture data at >2GS/s ?? I thought possibly they might parallel
several 5ns FSRAMs and let one recover while writing to the next, etc.
There still is a limit to how much one could do this, thought, because the
data has to be placed at the pins of the ram for a certain amt. of time.

Thanks,

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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1997\12\11@213647 by TONY NIXON 54964

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As far as I know, these fast CRO's, (if you can call that anymore),
sample a repetitive wave form over and over and build up a picture of
it by shifting the trigger point slightly on each scan. So in actual fact they
are
not sampling the wave form at GHz rates, maybe down to KHz rates.

I haven't kept up with the latest gear though, so I may be out of
date here.


Tony.


Just when I thought I knew it all,
I learned that I didn't.

1997\12\11@214307 by Sean Breheny

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At 01:00 PM 12/12/97 +1000, you wrote:
>As far as I know, these fast CRO's, (if you can call that anymore),
>sample a repetitive wave form over and over and build up a picture of
>it by shifting the trigger point slightly on each scan. So in actual fact
they
{Quote hidden}

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.

Sean


+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
.....shb7KILLspamspam@spam@cornell.edu
Phone(USA): (607) 253-0315

1997\12\11@221120 by Tom Handley

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  Sean, I've had a DSO project on the `back burner' for a long time.
I recently had a requirement to analyze IR remote signals so I'm going
to finish the project. Coincidentally, the recent issue of Circuit
Cellar Ink (#89 Dec 97) has two logic analyzer projects. One is a
ram-based 50Mhz design and the other is a FIFO-based 20Mhz design.
Both connect to a bidirectional PC parallel port. My design will be
PIC-based and connect to the serial port.

  - Tom

At 07:47 PM 12/11/97 -0500, you wrote:
{Quote hidden}

1997\12\11@222115 by Scott Newell

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>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.


Good luck,
newell

1997\12\11@223810 by Sean Breheny

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At 09:16 PM 12/11/97 -0600, you wrote:
{Quote hidden}

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 |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
shb7spamKILLspamcornell.edu
Phone(USA): (607) 253-0315

1997\12\11@234250 by Andrew Mayo

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If you read 'Analogue design, art science and personalities' (I think
this is the name) you'll see quite a bit of material on how TEK designed
high-speed vertical amplifiers.


{Quote hidden}

1997\12\11@234438 by Andrew Mayo

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yes, but couldn't you interleave A/Ds as well. i.e each A/D handles the
nth sample?. In front of the A/Ds we have a bunch of high-speed sample
and hold circuits, and we route the input signal to the appropriate A/D
in a 'round-robin' fashion.

Our friends at Los Alamos would have needed this sort of thing - as
someone at TEK said 'our customers needed scopes which would capture
events with a repetition rate of one every month, or so'.

{Quote hidden}

1997\12\11@235042 by Sean Breheny

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At 05:29 PM 12/12/97 +1300, you wrote:
>If you read 'Analogue design, art science and personalities' (I think
>this is the name) you'll see quite a bit of material on how TEK designed
>high-speed vertical amplifiers.
>
>

Thanks for the tip. I am still wondering, though, what in general do people
think of my idea? What am I forgetting? Any suggestions? Is it worth it?

Thanks,

Sean


{Quote hidden}

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
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Phone(USA): (607) 253-0315

1997\12\11@235051 by tjaart

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Scott Newell wrote:
{Quote hidden}

A nice trick if you want to fill up memory fast, is to use a PR counter
on the address line instead of binary counter.

--
Friendly Regards

Tjaart van der Walt
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1997\12\12@002811 by Michael S. Hagberg

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i've had this idea in the past.

if you have 8 banks of 80ns ram and rotated the active bank then writing
every 10ns could be done. this would give the required 80ns to setup the
memory for the next write. yes it does require alot of hardware, but it's
alot easier to buy a bunch of 80ns memory than it is to buy one 10ns memory.

just a thought. (if your totally confused, tell me and i will better
explain)

michael

1997\12\12@003729 by Ross McKenzie

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face
At 05:42 PM 12/12/97 +1300, you wrote:

>Our friends at Los Alamos would have needed this sort of thing - as
>someone at TEK said 'our customers needed scopes which would capture
>events with a repetition rate of one every month, or so'.

Do you always have to bring sex into the discussion? <g>

Regards,

Ross McKenzie
Melbourne Australia

1997\12\12@093609 by Keith Howell

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face
Tom Handley wrote:
> I've had a DSO project on the `back burner' for a long time.
> I recently had a requirement to analyze IR remote signals so I'm going to
finish the project.

IR carriers are only around 33-38kHz. IrDA is only 115 kBaud - 4 MBaud.
If you have some kind of ADC driven by your printer port, you could still have a
very useful gadget with a HUGE
memory. Which you can program in C instead of PIC assembly.

> Both connect to a bidirectional PC parallel port.
> My design will be PIC-based and connect to the serial port.

I'd stick with parallel if I were you. You will appreciate it when you want to
upload data into your PC.
And make sure you design for a commonly available cable such as the Laplink
transfer cable.
This is only 4 data bits + 1 or 2 handshake bits in either direction.
It suits all printer ports, whereas an 8-bit interface might exclude itself from
simpler PCs.
Either that or try using an LPT<->I2C interface.
This won't be slowed by baudrates, won't need a MAX232 chip, or the hassle of
baud rate generation.
The PIC UARTs don't look very accurate at high speeds, while I2C doesn't need
precise clocks.

I hear the very fastest scopes use a bit of co-ax cable as a delay line, and
digitise signals from points along
it. but that's stretching a budget.

1997\12\12@111127 by Bob Shaver

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part 0 1687 bytes
----------
From:   Sean Breheny
Sent:   Thursday, December 11, 1997 10:36 PM
To:     EraseMEPICLISTspamMITVMA.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 |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
RemoveMEshb7EraseMEspamEraseMEcornell.edu
Phone(USA): (607) 253-0315

1997\12\12@120140 by Darryl Newberry

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>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

I think I saw some hi-res video silicon CCDs recently announced. Perhaps
these could be used?

-dn

1997\12\13@103859 by Tom Handley

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  Keith, thanks for the info. I've done some simple logic analyzers and
A/Ds connected to the parallel port with rates around 500Ksps. I've been
wanting to do a more versatile, stand-alone, unit for a long time. I wanted
RS232 (or 485) so that I can get some distance between the DSO and PC. There
are a lot of ways to connect the two and I have'nt decided yet. I have some
IrDA eval boards and one possibility is IrDA with fiber or differential
drivers instead of IR. Note, this is an in-house project for my own use.
I was also looking at flash A/D's and using my Tek 465B as a front-end.

  - Tom

At 02:13 PM 12/12/97 +0000, you wrote:
>Tom Handley wrote:
>> I've had a DSO project on the `back burner' for a long time.
>> I recently had a requirement to analyze IR remote signals so I'm going to
> finish the project.
>
>IR carriers are only around 33-38kHz. IrDA is only 115 kBaud - 4 MBaud.
>If you have some kind of ADC driven by your printer port, you could still
have a
{Quote hidden}

from
{Quote hidden}

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