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@JohnU's answer to What are different ways to 'elongate' an electrical pulse? begins:

For a crazy idea on catching super-fast pulses look up the (very old) Tektronix 545 analogue sampling scope that fires a pulse "backwards" up a delay line against the input signal coming the other way, it's mad genius. It's a 30MHz scope that can display GHz signals.

and end with:

Edit: Can't find detail on the 545 but here's a link to Jim Williams explaining why "old [Tek] scopes are better" and a few specs: Reading Jim Williams - 3.9GHz bandwidth & 10uV per division sounds pretty groovy to me.

Can we help @JohnU out here?

Question: How did a delay line enable a 30 MHz "(very old) Tektronix 545 analogue sampling scope" to display GHz signals?

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Because the delay line allowed the trigger circuit to trigger, and then the incoming waveform could be displayed.

Basically, the trigger circuit has some inhrent delays. By passing the input signal through a delay line, it gives time for the trigger to activate and therefore show analog(ue) signals 'before' the trigger. The delay line alone didn't increase the BW. The highest BW scopes of the time had no input amplifiers, and the input signal directly drove the deflection plates of the CRT. They were not high impedance inputs.

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  • \$\begingroup\$ Thanks! I'm just curious; roughly when was "of the time"? 1950's? 1980's? \$\endgroup\$
    – uhoh
    Oct 24, 2022 at 23:18
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    \$\begingroup\$ Some history on the 545 can be found at this link. \$\endgroup\$
    – qrk
    Oct 24, 2022 at 23:27
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Old Tektronix scopes with plug-in capability (such as the 545) were able to display GHz signals by the use of sampling plug-ins such as the 1S2. All of the high frequency circuits (such as the high speed samplers) were in the plug-in. A high bandwidth scope was not required.

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