DC pulses on AC-coupled oscilloscope

[John Marsh]

Hello all,
I have an old, AC-coupled oscilloscope which has no
calibrated V/cm Y-axis. I want to use a simple 555 pulse generator,
whose peak pulsed DC output is 9V. With this signal into the
oscilloscope, what exactly am I seeing on the screen? @Scope is
Heathkit/Daystrom 10-12U.
Viva valves,
John Marsh.

 

[Bob Masta]

Hello all,
I have an old, AC-coupled oscilloscope which has no
calibrated V/cm Y-axis. I want to use a simple 555 pulse generator,
whose peak pulsed DC output is 9V. With this signal into the
oscilloscope, what exactly am I seeing on the screen? @Scope is
Heathkit/Daystrom 10-12U.
Viva valves,
John Marsh.

What you are seeing is an AC-coupled pulse, which means
that "it depends". Depends upon the time constant of the
input coupling and the pulse parameters. For example,
if you make your pulse into a 50% duty cycle square wave
and run the frequency up, you'll get a decent looking square
wave on the scope. As any portion (high or low) of the wave
gets too long for the time constant, you'll see the wave peak
and then decay away with a standard RC time constant decay
until the next transition. Note that this applies to the "off"
state as well as the "on" state of the pulse, so you see an
"up decay" after an off-transition. Once you get familiar
with what you are looking at, you can probably make pretty
decent measurements just by looking at the peaks.






Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

[John Marsh]

Dear Bob,
Thanks for your help. So:-I need to lower the frequency of
the pulses and check that there is no decrease in indicated amplitude.
If there is, I need a higher frequency for my work.

I think I learn more using cheap gear that needs a little effort.
£7.50 well spent!