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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Fast sampler
Date: Wed, 05 Mar 2025 19:01:41 -0800
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On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2025-03-05 20:07, Joerg wrote:
>> On 3/5/25 5:00 PM, Phil Hobbs wrote:
>>> On 2025-03-05 19:15, john larkin wrote:
>>>> On Wed, 5 Mar 2025 18:20:47 -0500, Phil Hobbs
>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>
>>>>> Hi, All,
>>>>>
>>>>> Late last year we did a fast sampler/TDR with nice clean 60 ps edges.
>>>>>
>>>>> We're gearing up to actually sell them, so I did a short technical
>>>>> writeup on the design, which may be of interest.
>>>>>
>>>>> <https://electrooptical.net/News/a-high-performance-time-domain-reflectometer> 
>>>>>
>>>
>>>>
>>>> Neat. No step-recovery diodes.
>>>
>>> Well, 40 years does get you something sometimes. ;)
>>>
>> 
>> And those cheap yet blazingly fast RF transistors, thanks to cell phones 
>> and all. They make nice pulsers. But they are like the princess on the 
>> pea, very low Vce and if you go a smidgen above ... poof.
>> 
>> [...]
>> 
>
>They're not that bad, really--their betas are so high that BV_CEO is 
>lowish, but BV_CBO is 12 volts or more.  Their saturation behavior is 
>still pretty BJTish, though. ;)
>
>Cheers
>
>Phil Hobbs

I toyed with the idea of using a PHEMT as a series-switch fast
sample-and-hold.

Hey, here's another goofy idea: 

We used to make fast linear ramps, driving a comparator against a DAC,
as a programmable delay. But we got smarter and just used an RC
charging thing, and mucked the DAC codes with a polynomial to get our
delay.

But what if the comparator sees a fast RC on one input and a slow RC
on the other? The exponential curves cancel, and you get a nice slow
linear sampling timebase. If you don't quibble too much.