Re: The reason for Tek delay lines


Tom Lee
 

Early scopes had to use lumped delay lines because the speed of light (even diminished by the dielectric) is much too fast for a practical length of coax to give you sufficient delay.

Coax — if defined loosely enough to include not-really-coaxial — was used in most scopes after the lumped delay line era, once scopes got fast enough that large delays were unnecessary. To keep volume small, the coax was thin and thus lossy. The need for equalization to undo the frequency-dependent loss and dispersion made for some fun. The special helical lines used in, e.g., the 485 allowed for much shorter cables, at the expense of requiring more heroic equalization. Rather than traveling along the helix, very high frequency signals would surf from turn to turn, causing the step response to wiggle before the main edge makes it through. Compensating for this “pre-shoot” requires more L’s and C’s.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 2, 2021, at 3:33 PM, John Williams <books4you4@mail.com> wrote:

As far as I know coax was never used in any delay lines inside Tektronix scopes. When the technology developed specialized delay cable was constructed as solid state drivers with higher impedance were developed. The 519 delay line may have been made from existing coax, but take a look at the size. This is my understanding anyway. The technology was developing by Tektronix engineers as they went, and Howard had to agree with everything. What the hell, it certainly worked.




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