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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Fast sampler
Date: Thu, 6 Mar 2025 14:53:43 -0000 (UTC)
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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 


-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics