Craig Sawyers <c.sawyers@...>
I am not sure what floodFlood guns are inherent in the way an (analog) storage scope works.
The storage CRT has the normal phosphor, and behind that an electrically
insulating storage screen (usually a phosphor, but its role here is to
release electrons, not photons). In some CRT's this can be physically
attached to the phosphor, and in others slightly behind it. Whatever the
physical arrangement, the storage "mesh" has a negative voltage applied, and
the property that when a high electron energy writing beam electron hits it,
it releases more than one secondary electron. Further back in the CRT there
is a collector, which has a positive voltage, and soaks up the secondary
electrons. This means that the -ve voltage is reduced where the writing
beam has hit.
At the same time the flood guns at illuminating the entire surface of the
mesh (there are also a bunch of collimation electrodes on the CRT to make
sure that the electron illumination is uniform). Where the voltage on the
storage mesh is reduced (IOW where the trace has gone) the flood gun
electrons pass through, and strike the front phosphor, causing the light
that you see. Elsewhere, the storage mesh is sufficiently negative to repel
the flood gun electrons, meaning that none get to the front phosphor,
meaning that the screen is dark.
So what you see on a storage oscilloscope screen, when in storage mode, is
the flood gun electrons striking the phosphor - not the gun electrons!
This process is called bistable storage, because where a writing beam has
passed the flood electrons all pass through - so a spot on the front
phosphor is either bright or dark - with no adjustment possible to the
brightness of the stored trace. Also, it is a stable state - the energy
diagram for the electrons on the storage mesh has a minimum where there is a
written spot, and it stays there almost indefinitely (typically hours to
When a trace is erased, the voltages on the flood gun, collector and storage
mesh are changed so that the uniform -ve charge is restored to the storage
Variable persistence mode uses a different part of the electron energy
diagram, where the electron energy is not stable. That means that there is
a tendency for the entire screen to fade positive (ie to all go bright). To
prevent this, the screen is erased and rewritten many times a second.
Consequently, if the displayed waveform changes, the previous one fades out
over a period of seconds as the erase cycle fades it out. The "persistence"
control varies the erase frequency - with a high frequency, the trace erases
very quickly. With a low frequency, the trace fades slowly but at the
expense of slight fade positive in the background - so the entire screen
gets a glow.