[time-nuts] 1 PPS 50-ohm driver

[Bill Byrom]

For most oscilloscopes with bandwidth no greater than around 1 GHz, the
response is Gaussian with a BT (bandwidth * risetime) product of about
0.35. So a 1 GHz oscilloscope has a risetime of about 350 ps. The 10%-
90% risetime of a series connected system is given by the square root of
the sum of the squares of the individual risetimes.
 
So if you had a 2 ns risetime pulse measured by a 350 ps (1 GHz BW)
scope, the expected displayed risetime would be:
sqrt( 4 + 0.1225) =  2.03 ns. So the scope bandwidth is unimportant in
this case.
 
More important is the oscilloscope probe (if used) and/or coaxial cable,
launch into the cable,  and connectors. The slow rise after the initial
edge in your scope image might be due to skin effect losses in the PC
board, coax cable, and/or scope probe (or errors in an uncalibrated
scope). But the rising edge looks great to me.
 
The problem with fast edges and fast login on the receiving end is that
reflections due to impedance mismatch cause great difficulties. For
example, if the load was a 50 ohm resistor in parallel with 20 pF to
ground, a negative-going reflection would travel back to the source. If
the round-trip propagation delay is 2 ns or less, the reflection will
end up causing a reduced risetime or ringing at the source. Your setup
has a good source match which should keep re-reflection to a minimum, so
there shouldn't be any significant re-reflection problems.
--
Bill Byrom N5BB
(Tektronix Application Engineer)
 
 
 
On Sun, Apr 17, 2016, at 09:11 PM, Stewart Cobb wrote:
> Sorry, newer scopes annotate a little better. Settings were 0.5
> V/div and
> 2
> ns/div. Trigger level is 1.4 V. Scope is HP 54720D with 54712A plugin
> (built in 50-ohm termination). The installed bandwidth of the
> plugin is
> somewhere between 1.0 and 1.5 GHz. I should have mentioned all that in
> the
> previous post.
>
> Cheers!
> --Stu
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