From: Jeff Liebermann
Subject: Re: AN: New Logic Analyzer, 500MHz, $166
Date: Sun, 17 Nov 2002 12:18:08 -0800
Organization: Committee to Maintain and Independent Xenix
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On Sun, 17 Nov 2002 19:00:50 -0000, "John Jardine"
>Offhand, as there is stray capacitance between the connecting wires, I would
>have thought it better if the wires are looked at as a distributed
>transmission line. In which case at 500Meg the wires may have a simple
>'resistive' impedance of a few hundred ohms. But of course ... the
>100kohms//10p meter input impedance is in no way going to terminate that
>line, which still makes the makers claim of a 500Mbit/S measurement,
Ummmm.... Using a transmission line to reduce losses and prevent
reflections largely relies on the assumption that the load is
terminated with some semblence of a known impedance. Otherwise, you
get reflections and waveform distortion issues. Whether its with a
simulated ladder line or coax makes little difference. What you're
suggesting is the equivalent re-inventing the oscilloscope probe for
this logic analyzer. It's a nice idea, works well with 10x
attentuation up to about 200MHz, but will fail the size, cost,
useability, and performance test. [Insert nightmare image of 8ea
scope probes hung on a USB dongle].
Please note that 1X (no attenuator inside) scope probes are limited to
about 30MHz at cable lengths of about 1 meter. To get a -3dB
bandwidth of 500MHz, my guess(tm) is that the scope probe would need
to be very very very short. Of course, the alternative is an active
probe, which is what should have been implimented in the first place.
Anyway, my points are that:
1. 30 cm test leads are rediculously long.
2. Performance at 500MHz is not possible with 30 cm test leads.
3. Performance bandwidth and loading reactances should be specified
with some sensible test leads included.
Ever see the price of a 500MHz scope probe? Shudder...
Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
(831)421-6491 pgr (831)336-2558 home