Scope trace fainter at faster Time/Div?

[LanceM]

An older 20 MHz dual channel Leader analog. Testing secondary ignition
pattern on car, using inductive clamp for trigger (#1 spark plug wire),
and capacitive p/u on inverted Chan 2 (on HT coil wire to distributor)

Is it normal for the trace signal to progressively & substantially dim
at faster Time/Div settings? Occurs on both Normal and Auto trigger. I
don't think adjusting the trigger level helped, but will retry that.

Thank you,
Lance

 

[Jim Yanik]

An older 20 MHz dual channel Leader analog. Testing secondary ignition
pattern on car, using inductive clamp for trigger (#1 spark plug wire),
and capacitive p/u on inverted Chan 2 (on HT coil wire to distributor)

Is it normal for the trace signal to progressively & substantially dim
at faster Time/Div settings? Occurs on both Normal and Auto trigger. I
don't think adjusting the trigger level helped, but will retry that.

Thank you,
Lance

I would think it is normal.The phosphor is energized less by a faster
sweep(lower duty cycle),thus less light emitted. Note that at very slow
sweeps,the brightness goes UP,if there is no dimming circuitry to protect
the phosphor.Some scopes had burned phosphor(lines) due to slow sweep
speeds at too high an intensity.Newer scopes have beam-regulating circuitry
to keep trace intensity fairly constant over a wide range of sweep speeds.

 

[[email protected]]

I would think it is normal.The phosphor is energized less by a faster
sweep(lower duty cycle),thus less light emitted. Note that at very slow
sweeps,the brightness goes UP,if there is no dimming circuitry to protect
the phosphor.Some scopes had burned phosphor(lines) due to slow sweep
speeds at too high an intensity.Newer scopes have beam-regulating circuitry
to keep trace intensity fairly constant over a wide range of sweep speeds.

Jim's right - the electron beam doesn't stay on the phosphor long enough
to excite it to high brightness. That's the advantage of a digital 'scope.

You might try a light hood - make one out of cardboard with a relief for
your nose and duct tape it to the 'scope.

Don

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