[SI-LIST] Re: HW Debugging
- From: olaney@xxxxxxxx
- To: jsantosfernandez@xxxxxxxx
- Date: Fri, 2 Nov 2007 11:01:04 -0700
A 1 GHz o'scope would be fine if you were dealing with 600 MHz sine
waves, but for digital signals you need enough harmonics to adequately
represent the rise and fall times, overshoot, ringing, etc. The maximum
pattern rate will be 300 MHz (10101010...), and the 1 GHz scope will give
you the third harmonic, which is at least enough to read the bit pattern,
but a 2 GHz scope will get you (barely) to the 7th harmonic, at which
point you can start to believe that what you see onscreen represents
what's happening at the probe tip. Speaking of probes, do not make the
mistake of matching scope and probe bandwidths -- a 1 GHz probe on a 1
GHz scope yields a cascaded bandwidth around 707 MHz. A 1 GHz scope with
a 2 GHz probe gives 894 MHz in cascade, etc. My personal choice is an HP
54006A probe that offers a 6 GHz bandwidth. (986 MHz on a 1 GHz scope,
1.9 GHz on a 2 GHz scope). The bugaboo of high speed probes is getting
an adequate ground. That makes you a fan of topside ground planes or
strategically located bypass caps. If you have the luxury of building in
coaxial test points that can directly accept 50 ohm cable, life can be
good - just use a cable instead of a probe. The tiny (and cheap) SMT
connectors designed for cell phones, for instance, can be sprinkled
around for debug and production test if you are foresighted enough, and
pay for themselves in ease of test. The trick is to place a resistor in
series so that the presence of the probe does not affect normal
operation, lest you wind up with a circuit that works perfectly only
while test equipment is attached. The 54006A probe uses a 450 ohm
resistor for 10:1 voltage division into a 50 ohm scope input. The total
of 500 ohms at the tip is high enough for probing typical 50 ohm signal
traces with minimal disturbance. Of course, the resistor has to be
properly implemented to get good fidelity.
That's all about using a scope, which is often a primitive tool. Others
on this list can chime in with more advice. I see a few other answers
have already flowed in.
Orin
On Fri, 2 Nov 2007 13:04:54 +0100 "Santos Fernandez, Jesus"
<jsantosfernandez@xxxxxxxx> writes:
> Hello all,
>
>
> We are designing a PCB with fastest transmission clock speed of
> 600MHz.
>
> We have 1GHz oscilloscopes (and below)
>
>
>
> Do you think we will need additional test equipment for any required
> design testing?
>
> If you think so, will you, please, suggest which minimum equipment
> will
> be necessary?
>
>
>
> Thank you in advance,
>
> J. Santos
>
>
>
>
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