I'm not going to spend 13 minutes off my remaining life to
see exactly what was done in that video, but there is a trap
in this area.
If the signal isn't the right amplitude to fit on the dotted line
markers, the temptation is to change the vertical gain using
the VAR gain control. Unfortunately in many scopes that will
also change the scope's bandwidth and hence risetime.
(Essentially a BJT's Ft is a function of Ic).
The best way is either to change the input signal's amplitude
(but watch out for similar effects) or to use an external RF
50ohm attenuator.
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On 22/05/18 01:44, Siggi wrote:
Hey Lol,
Your nice old 485s come equipped with graticule markers specifically for
rise time measurement. See this video <> for
how to use them to get repeatable and comparable measurements between
scopes and signal sources.
Siggi
On Mon, May 21, 2018 at 19:46 lop pol via Groups.Io <the_infinite_penguin=
[email protected]> wrote:
On Mon, May 21, 2018 at 03:57 pm, Raymond Domp Frank wrote:
Hi lop pol,
Thanks for doing the tests. I don't understand the outcome, however.
Just looking at your images:
In the TD (tunneldiode pulser) image, assuming a TD rt of 100ps (<=125ps
specified), I see a 600ps rise time (rt), indicating about 590 MHz bw of
the
485 against a spec. of 350 MHz. Even for that 'scope, it seems very high
bw to
me. 30% faster than spec'ed would be "normal", but 70%? Even with a
theoretical rt of 0ps for the TD, the 485's bw would be about 580 MHz,
about
65% faster than specified for this model! It *could* be caused by a slow
1
ns/div. time base speed.
A second strange thing is the suggested rt of the PG506 (pg). I see a rt
of
1.6 ns. Assuming the specified rt for the 485 @ <= 1 ns (350 MHz), the
pg's rt
would be 1.24 ns, against a spec of < 1 ns. Not so good but possible,
since
the later (towards the top) part of the edge shows a speed reduction
(shoulder
rounding) that is not typical for this instrument. Also, the amplitude
on the
screen is a bit higher than used for the 485's specification, which could
result in it performing a bit slower. Also, using the pg's variable
attenuator
would reduce its rt. BTW, the "slower shoulder" could be a side effect of
using the attenuator pot to reduce the output signal!
So, two things I don't quite understand:
- Exceptionally fast (high bw) 485
- Exceptionally slow PG506.
Unless I'm mistaken in my observations, I don't see how to reconcile
them.
Raymond
I have added another 485 for our assessment.
067-0681-01 /g/TekScopes/photo/50828/2?p=Name,,,20,1,0,0
and pg506 fast rise
/g/TekScopes/photo/50828/3?p=Name,,,20,1,0,0
