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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: Fast sampler
Date: Thu, 06 Mar 2025 07:24:21 -0800
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On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
>> john larkin <jl@650pot.com> wrote:
>>> 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.
>> 
>> They work well for that. A couple of years back, we did a POC for the Navy
>> that used several SAV551pluses—100 ps is doable. The main problem is that
>> their voltage gain is lowish, so you don’t get as much speedup as with a
>> BJT.   
>> 
>> And of course they’re 10x the price. 
>> 
>>> 
>>> 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.
>> 
>> Not sure about that.  For the proto, I used a ramp from an arb to make the
>> threshold—the sampling loop converged at each point, so I wound up with a
>> 10**7:1 zoom—10 us per picosecond. 
>> 
>> The fast bit was all over before the slow bit moved perceptibly. 
>> 
>
>The other issue is that the prop delay depends on the overdrive. Since
>we’re comparing a ramp to a fixed threshold, which that basically means how
>far the ramp rises during the time required for the positive feedback to
>get going. 
>
>So we still need an online calibration. Fortunately that isn’t hard—an
>open-circuited bit of coax is enough. It doesn’t have to be done often. 
>
>Cheers 
>
>Phil Hobbs 

I did caution about quibbling too much. One issue is that the LVDS
line receivers have a bunch of offset as the common-mode voltage
approaches the positive supply rail. And of course the esd diodes are
nonlinear capacitors. And things always ring a little. Geez, nobody's
perfect.

One of my guys did a bunch experiments using an LVDS receiver as the
comparator in a picosecond-resolution delay circuit. We use a 16-bit
DAC and a 4th order polynomial and calibrate the polynomial for every
channel. Our P500 has, I recall, nine of those.

https://highlandtechnology.com/Product/P500

When I was young and foolish, I used to do time delays with linear
ramps and ECL comparators.