[Radio Wrangler]

A scope isn't going to have any better bandwidth than the Y amplifier, but the CRT also has a high frequency roll-off.

The speed of the electrons passing a pair of deflection plates is high, but still finite. Each electron spends a little time under the influence of the Y plates. During this time, the Y signal changes. The amount of deflection achieved is the integral of the signal over the time the electron was between the plates.

If the Y signal frequency is high, some of the signal starts to cancel what its earlier self did. Eventually when the time in the deflectors equals a full cycle, we get complete cancellation and we see no signal on the screen. Go higher still and the signal grows again, but never as big as before. The frequency respponse is a (sin(x))/x set of bounces.

This is the same mechanism as sets the rolloff of a magnetic tape replay head due to the finite gap and the recorded wavelength.

If we fix the problem by speeding the electrons up with more accelerating voltage, or if we cut the length of the plates, thebandwidth improves but as the electrons are being deflected for less time and we get less deflection at all frequencies. The CRT deflection plates become less sensitive and then we need more voltage swing from the Y amplifier and it gets harder to reach the wanted bandwidth. So faster scopes run hotter.

You just can't win.

So you cheat. Divide long Y plates into several pairs of short 'platelets'. Connect each pair to subsequent tappings on a balanced pair of delay lines. Put a terminating resistor on the end of the line to prevent echoes. Your CRT now has the deflection sensitivity of the length of all the plates added together. It has the bandwidth of the length of one platelet. Neat?

You can do a similar trick by distributing several valves down transmission lines rather than having them in simple parallel. You can build a much faster Y amp this way and now you know why some Tek scopes had a hundred valves in them.

Scopes are fascinating, and the programs are better than on TV.

David