jack said:
Hi,
I have a 60 Mhz tech digital oscope and was trying to find out what
is the shortest pulse that would be faithfully reproduced on this
scope. Would a pulse with a rise time of 1 ns and decay of 8 to 10 ns
be faithfully reproduced?. As a second question,if my scope can't do
No.
this does anyone know of a circuit that could scale every thing up so
I could view a pulse like this on this scope and (by knowing the scale
factor of the circuit) be able to deduce the real timing information
of the pulse? Thanks for any help. jack
Risetime and pulse width are two different things, but for sure if your scope is
too slow for the rise time, you'll never see a narrow pulse.
It depends on how your scope rolls off after 60MHz but in general, the rule
relating frequency and rise time is:
F ~= 0.35 * (1/Tr).
Tr is the rise time of the scope, F is the bandwidth. For example, a 1GHz scope
has a rise time of 350ps. In general, to get accurate measurements, you want
your instrument to be 10 times faster than the signal you are viewing. So the
1GHz scope can directly accurately view 3.5ns rise times.
The fun starts when trying to define rise time. The standard is 10-90% of the
amplitude, but sometimes you'll see it defined as 0-50%. Stick to 10-90%.
A 60MHz scope will have a rise time of around 0.35/F ~= 6ns. This is a bit too
slow to glean any useful info if you know your rise time is in the 1ns range.
You can still make width measurements if the pulse is a lot wider than 6ns
though. But the constant error that the scope introduces will increase in
proportion as the pulse narrows.
If you had a 1GHz scope, you'd be closer to the required bandwidth (1GHz = 350ps
rise time)
There is an equation relating the rise time of the scope to the rise time of the
signal:
Real rise = sqr( (measured**2) - (equipment**2) )
In other words, the real rise time is the square root of the differences of the
squares of your equipment and the measured time.
*But*, in order for this to work, you have to be able to measure something, so
you need your scope to be about a third of the rise time of your expected rise
time, to get an error of about 10% with that formula.
So, you need a minimum of 1GHz bandpass to measure 1ns rise times.
Your 60MHz bandwidth scope's rise time is 18 times too slow.
Sorry...