Path: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Fast sampler
Date: Fri, 7 Mar 2025 03:29:49 +1100
Organization: A noiseless patient Spider
Lines: 138
Message-ID: <vqcii6$31tkb$2@dont-email.me>
References: <4eaced48-83bc-4f69-5bdf-a26650bd583a@electrooptical.net>
 <ulphsj9i8gnii3im5u1jduoq22vbgklmqp@4ax.com>
 <e058d585-07e2-2560-7041-7a8ca8b2f4d8@electrooptical.net>
 <m2sapqF25baU2@mid.individual.net>
 <6aac874a-8283-026b-1af5-33295c858e67@electrooptical.net>
 <mi3isjt25p7nu6hij8fucuf4j1e110bhnc@4ax.com> <vqb4f8$2n0i3$1@dont-email.me>
 <vqcctn$3134o$1@dont-email.me> <1pejsj1m286nn5ggclpfi3drkn2j5mnf28@4ax.com>
 <aa25d93f-ed88-62e7-9b08-2e438bc28302@electrooptical.net>
 <4nhjsj5ufkdfqo8jb0tsgqjd301jounknh@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Thu, 06 Mar 2025 17:29:59 +0100 (CET)
Injection-Info: dont-email.me; posting-host="1625736ff246add36f256a47399a0949";
	logging-data="3208843"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1+tJlexj0p8affYGOQqEw6hh4LfXolSzQ4="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:Ocw+DUQtRvjQcDxodm64tXFzhwA=
In-Reply-To: <4nhjsj5ufkdfqo8jb0tsgqjd301jounknh@4ax.com>
Content-Language: en-US
X-Antivirus-Status: Clean
X-Antivirus: Norton (VPS 250306-6, 7/3/2025), Outbound message
Bytes: 7047

On 7/03/2025 3:06 am, john larkin wrote:
> On Thu, 6 Mar 2025 10:54:20 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 2025-03-06 10:24, john larkin wrote:
>>> 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.
>>>
>>
>> The line receivers don't seem to have any significant amount of kickout,
>> either--we can sweep the Rx pulse across the Tx pulse with no apparent
>> funnies due to interaction.  Have you folks seen any kickout issues?
>>
>> Of course the kickout might be delayed, I suppose.
>>
>> Cheers
>>
>> Phil Hobbs
> 
> We didn't test for that. When a comparator fires, all sorts of stuff
> happens downstream, that could jostle adjacent channels.
> 
> Our comparators typically drive a 1 ns Tiny Logic flipflop as the next
> step in the signal chain.
> 
> Standard ECL wasn't as fast as 15 cent Tiny parts are now.

But it's current steering logic and the supply rails stay a lot cleaner 
than you see with CMOS switches.

And what's Standard ECL now? It was Motorola 10k back when I was young, 
and Motorola/Philips/Fairchild 100k a few years later. Motorola ECLinPs 
took over a about when I stopped using it, about when I started posting 
here, some twenty years ago.

-- 
Bill Sloman, Sydney