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Failed to connect to MySQL: (1203) User howardkn already has more than 'max_user_connections' active connectionsPath: ...!local-1.nntp.ord.giganews.com!Xl.tags.giganews.com!local-4.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Thu, 06 Mar 2025 15:24:21 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: Fast sampler Date: Thu, 06 Mar 2025 07:24:21 -0800 Message-ID: <1pejsj1m286nn5ggclpfi3drkn2j5mnf28@4ax.com> References: <4eaced48-83bc-4f69-5bdf-a26650bd583a@electrooptical.net> <6aac874a-8283-026b-1af5-33295c858e67@electrooptical.net> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Lines: 102 X-Trace: sv3-gzt6ESBeRwbHvznX2wCaUhMazb/7lB3tjbMgUNXYTQ7JxWeMvQN2alUrXkFRc+T/fQdZHsPsGup6+F9!RUA6K83WmBhRsK5KuobFVAJ8cSZRUHUP1qlvdu7UL3SnSqVB4IkBjuOSndiXC/dL0V7/9MbXLlG1!Szo66g== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 5210 On Thu, 6 Mar 2025 14:53:43 -0000 (UTC), Phil Hobbs wrote: >Phil Hobbs wrote: >> john larkin wrote: >>> On Wed, 5 Mar 2025 20:10:08 -0500, Phil Hobbs >>> 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 >>>>>>> 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. >>>>>>>> >>>>>>>> >>>>>>>> >>>>>> >>>>>>> >>>>>>> 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.