Specifications

The rear panel has two BNC connectors:

• Y-Axis Output Jack - a buffered signal of one of the channels (channel 2 on the 2125A) is

available with an output impedance of 50 Ω. It can act as a preamplifier with the same

bandwidth of the scope (one use is to amplify a low-level signal for a frequency counter).

• Z-Axis Input Jack (also called "External Blanking Input") - a voltage can modulate the intensity

of the CRT electron beam. The signal levels are often TTL such that 0 volts turns the beam off

and 5 volts causes the maximum intensity. Older scientific instruments sometimes used XY

mode with the z-axis input to provide a raster-type display similar to how an analog television

behaves.

Theory of operation

Cathode ray tube (CRT)

The basis of the operation of an analog oscilloscope is the cathode ray tube (CRT), a special type of

vacuum tube. The following picture is a schematic representation of a CRT:

Figure 6

The glass envelope G is the vacuum container; inside the envelope, most of the air is removed to

eliminate electron collisions with air molecules. The filament A is heated by current flowing through it.

This heated filament causes electrons to be emitted. A positive potential is put on plate B with respect

to the filament

Since electrons have small mass, the electron beam is capable of being positioned rapidly over the

phosphor screen. When this is done rapidly enough, you see a continuous trace of light

which accelerates the electrons towards and through the hole in B, resulting in the

electron beam E. Voltages on deflecting plates C and D cause the electron beam E to impact at

different positions on the phosphor screen F because of electrostatic deflection (more sophisticated

designs may use both electrostatic and magnetic deflection). When electrons impact the phosphor's

atoms, the atoms change into an excited state. When the excited states decay back to the ground

state, the atoms emit photons of visible light. This light is what lets you see the trace on the

oscilloscope screen. The light emission from the phosphor happens over a period of time called the

persistence, which distinguishes phosphorescence from fluorescence, which happens immediately.

The filament is called the cathode because of its negative potential with respect to the accelerating plate B. The

electron beam can be seen in some tubes because of collisions with residual air molecules. In the late 1800's,

these beams were called "cathode rays" -- hence the name "cathode ray tube".

. The scope's

If you have an analog scope, you can measure the point at which your eyes and brain just start to see the trace

licker. People see flicker at frequencies from below 30 Hz to 60 Hz or so.

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