[SI-LIST] Re: How much scope memory needed?
- From: "Jon Keeble" <jkeeble@xxxxxxxxxxxx>
- To: <olaney@xxxxxxxx>
- Date: Sun, 15 Jul 2007 09:47:23 +1000
A good KPI for 'scopes is WPS (waveforms per second).
I kept my old analog scope for many years because it was better than many
digital scopes, even digital phosphor scopes (DPOs), at seeing what I DON'T
expect (which is what I really want to see) (especially with the lights off).
It can be quite difficult to get a value for WPS at any given displayed
horizontal time scale.
Obviously, in a stationary waveform, an anomoly that occurs once in 50,000
scans will on average be captured twice a second at 100,000 WPS.
BYW, Tektronix invented the term DPO, but don't seem to protected it in any way
because now everybody uses it (Agilent, LeCroy et al).
A large memory depth plays a role in extending the sampling rate at slower
'scan' rates, and so providing a larger ratio between the slowest and fastest
event viewable.
Regards
Jon Keeble.
----- Original Message -----
From: olaney@xxxxxxxx
To: joel@xxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Sent: Saturday, July 14, 2007 12:14 PM
Subject: [SI-LIST] Re: How much scope memory needed?
The amount of memory required for a good on-screen eye diagram is
actually rather modest -- a few hundred kilobytes at most. All that
other memory is so that you can capture lots and lots of waveform, or
equivalently, capture a long history for statistical accuracy. If the
sampling rate is sufficiently high, it also allows you to zoom in some
reasonable amount on what you have captured. But, capturing a few bit
periods worth of data to measure risetime, bit period, eye width, etc. is
not a memory intensive task. Unless you have a need, I'd get a megabyte
or two with the understanding that you can always upgrade later (make
sure you get a straight answer on this.) This is a case where 99 times
what you need for the average task gets used less than 1% of the time,
and is usually overkill at that. But hey, memory is (supposed to be)
cheap.
Orin Laney
On Fri, 13 Jul 2007 16:24:52 -0700 "Joel Brown" <joel@xxxxxxxxxx> writes:
> We are looking into purchasing a high speed scope to do signal
> integrity
> measurements (eye diagrams) on SATA and PCI-Express signals.
> At 40 GS/s how much waveform memory would be needed to get a good
> eye
> diagram and make other useful measurements?
>
> Options from various manufacturers range from 1M to 200M.
>
>
>
> Thanks - Joel
>
>
>
>
>
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