[SI-LIST] Re: SDA bandwidth

Salut Jean,
regarding the bandwidth, you are correct to consider the rise time rather 
than the bit period (although of course they are not completely unrelated, 
since the rise time must alway be significantly shorter than one bit 
period). Lacking additional information, assuming a rise time of 30 - 40% 
of the bit interval is often a good guess, in your case that gives 30 - 
40ps. The corresponding equivalent bandwidth is then approx. 8 - 11 GHz. 
Of course there is still considerable signal energy above that limit, so a 
better number is the knee frequency, 0.5/Tr, i.e. around 17 GHz for a 30ps 
rise time signal. And at the same time, you'd get 30% attenuation of that 
frequency if you chose your scope bandwidth equal to this frequency, which 
is quite substantial, so you may want to choose a somewhat higher 
bandwidth scope than that. Unfortunately even high-end real-time scopes 
top out around 20 GHz today, so you'll be running into technical 
limitation and may have to settle for less than ideal (ideal would be a 
bandwidth of 3 - 6 times your highest frequency of interest). You can get 
equivalent time sampling scopes (Agilent, LeCroy, and Tektronix all have 
such scopes & sampling plugins with analog bandwidth up to 50 ... 100 
GHz), but of course they can't do single-shot capture of a data stream. On 
the upside, there is virtually no limit to the timing resolution this type 
of scope can achieve (sub-ps).

As for timing resolution with a real-time scope, your assumption is far 
too pessimistic. True, a 60 GS/sec scope will only give you 6 sample 
points per 100ps interval, but then the scope does interpolation between 
the curves. The simplest type would be linear interpolation ("connect the 
dots with straight lines"), already that would give you much better 
resolution for your edge crossings than 100ps/6. Modern scopes all use 
better interpolation methods than that (sin(x)/x interpolation), which 
will give you quite accurate edge crossing information and allow jitter 
measurements down to single picoseconds and below. Another way to see it 
is that the scope doesn't just measure low and high (like a logic analyzer 
would), rather it gives you finely resolved voltage information at each 
sample instant, so you have more information in the voltage direction 
which then allows to determine the crossing point more precisely as well.

There are other differences between real-time scopes and equivalent time 
sampling scopes that will be important for you - single shot capture 
capability, influence of trigger jitter, ability to decompose timing 
jitter into its components etc. As a shameless plug, my book "Digital 
Timing Measurements"  (available from Amazon.com, ISBN-10: 0387314180, 
ISBN-13: 978-0387314181) talks at length about these considerations in a 
(I hope!) easy-to-understand manner. There are also a lot of apps notes 
out that discuss these topics.

Hope that helps

Wolfgang






gypse <gypse@xxxxxxxxx> 
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Subject
[SI-LIST] SDA bandwidth






Hi all,

Have you got an idea of the required features (bandwidth + sampling rate) 
for a
SDA to analyse 10Gbits/s serial links ?

Bandwidth:
Thumbs rules for the bandwidth would lead me to choose 5 times the 
fundamental
frequency to see the 5th harmonics ie 25 GHz. But high speed devices have 
a rise
time of only 40ps, so the estimated bandwidth of such signals would rather 
be
0.5/30ps = 12 Ghz ... So, what to choose ?

Sampling rate:
Am I wrong if I say that with a sampling rate of 60 Gsamples/s, I
will capture an eye with only 6 points and therefore I will be able to 
measure
jitter with an accuracy of only 16 %. Should I need a higher sampling rate 
?

Thanks.
Jean.

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