Yes, which is why I specifically redefined coax to include non-coax. :) And your supposition is absolutely correct — the helical lines misbehave so badly that they’re only practical below a few hundred MHz. At some point the equalization circuitry becomes so complex that regular old cable starts to look attractive, despite the length/weight.
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The variable part of the delay lines used in the 7A19 and 7A29 with that option are a very nice mechanical design. I like the “trombone slider” arrangement used to vary the path length.
Sent from my iThing, so please forgive typos and brevity.
On Feb 2, 2021, at 4:49 PM, Ed Breya via groups.io <firstname.lastname@example.org> wrote:
It may be good to be a little more specific in order to avoid confusion of terminology between "delay line" and coax and the differential straight or helical delay lines often seen in analog scopes. As I recall, one 500 series scope I had (maybe 585?) had a differential delay line that looked "old-school" in style (braided wire outer shield, I don't know if helical) compared to say the 7000 series ones (flat twin-lead, with gapped copper foil shield). As I recall, the 7K (except 7104 which I don't know) all have these straight twin lines, and are thus quite long for the delay, compared to helical types, which I don't think are capable of good enough fidelity much above the hundred MHz class. The 2200 series 100 MHz scopes managed to go helical, and have very compact lines - a few turns wrapped around the back end of the CRT. Regular coax is used in the 7S14, for instance, since its sequential sampling system needs a delay line (random sampling and DSOs don't). There's a big roll of bare (presumably to save volume) coax squeezed in there for each channel. The 7A19 and probably 7A29 use some semi-rigid coax, to provide for the variable (on one channel) delay option. High grade single coax is still the best delay line for electrical signals, but of course can get very large for long delays.