On 2/17/2014 3:34 PM, Joerg wrote:> Phil Hobbs wrote: >> On 02/17/2014 12:39 PM, Joerg wrote: >>> Phil Hobbs wrote: >>>> On 02/17/2014 11:25 AM, Phil Hobbs wrote: >>>>> On 02/17/2014 11:05 AM, Joerg wrote: >>>>>> Phil Hobbs wrote: >>>>>>> On 02/17/2014 10:20 AM, Joerg wrote: >>>>>>>> Phil Hobbs wrote: >>>>>>>>> On 02/17/2014 01:55 AM, miso wrote: >>>>>>>>>> Phil Hobbs wrote: >>>>>>>>>> >>>>>>>>>>> On 2/15/2014 8:01 PM, Joerg wrote: >>>>>>>>>>>> John Larkin wrote: >>>>>>>>>>>>> On Sat, 15 Feb 2014 13:54:05 -0800, Joerg >>>>>>>>>>>>> <invalid@invalid.invalid> >>>>>>>>>>>>> wrote: >>>>>>>>>>>>> >>>>>>>>>>>>>> Folks, >>>>>>>>>>>>>> >>>>>>>>>>>>>> After John Larkin sent me SMS7621 Schottky RF diodes I played >>>>>>>>>>>>>> around >>>>>>>>>>>>>> with them in the lab. While they perform nicely at room >>>>>>>>>>>>>> temperature >>>>>>>>>>>>>> everything goes to pots at a slightly elevated temperature. At >>>>>>>>>>>>>> ballpark >>>>>>>>>>>>>> 50C the leakage current is already painfully high for use as a >>>>>>>>>>>>>> sampler >>>>>>>>>>>>>> diode. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Question: Is there a fast RF diode or sampling diode in a >>>>>>>>>>>>>> reasonable >>>>>>>>>>>>>> cost range (low single-digit Dollars) with more barrier >>>>>>>>>>>>>> height? In >>>>>>>>>>>>>> samplers one doesn't care so much about Vf but unfortunately >>>>>>>>>>>>>> many >>>>>>>>>>>>>> such >>>>>>>>>>>>>> diodes are marketed as RF detectors where Vf does matter. I >>>>>>>>>>>>>> guess >>>>>>>>>>>>>> that's why they leak so much when the temps go up. >>>>>>>>>>>>> >>>>>>>>>>>>> The classic 2-diode feedback sampler doesn't much care about >>>>>>>>>>>>> leakage. >>>>>>>>>>>>> Within about a microsecond after the sampling pulse, the >>>>>>>>>>>>> delta-v >>>>>>>>>>>>> glitch >>>>>>>>>>>>> is amplified in a charge amp, usually back up to 100% sampling >>>>>>>>>>>>> efficiency, and dumped into a slow s/h. That voltage, with the >>>>>>>>>>>>> diode >>>>>>>>>>>>> back-biases, is applied back onto the diodes for the next shot. >>>>>>>>>>>>> That >>>>>>>>>>>>> makes the overall sampling process ultra-linear and allows >>>>>>>>>>>>> it to >>>>>>>>>>>>> work >>>>>>>>>>>>> down to very low trigger rates. >>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> If push comes to shove I'll have to use a 2-diode sampler and >>>>>>>>>>>> figure >>>>>>>>>>>> out >>>>>>>>>>>> a balun for driving. I could also use a follow-up S&H on my >>>>>>>>>>>> single-diode >>>>>>>>>>>> sampler but it's a real estate and cost issue. Essentially my >>>>>>>>>>>> range >>>>>>>>>>>> gate >>>>>>>>>>>> will be around 300psec and the sampling cannot happen more than >>>>>>>>>>>> about >>>>>>>>>>>> once per usec. So around 3000:1. Delta-V is a few tens to >>>>>>>>>>>> hundreds >>>>>>>>>>>> of uV. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> There are higher-barrier diodes in the Skyworks family. They >>>>>>>>>>>>> have a >>>>>>>>>>>>> great sample kit. >>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> I think I'll have to talk to the guys there. Things like >>>>>>>>>>>> Schottky >>>>>>>>>>>> barrier height aren't in the datasheets but their FAEs should >>>>>>>>>>>> know. >>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>>> What sort of risetime did you get? Are you doing an open-loop >>>>>>>>>>>>> sampler? >>>>>>>>>>>>> What the sampling pulse generator like? >>>>>>>>>>>>> >>>>>>>>>>>> >>>>>>>>>>>> I haven't tried the diodes in a sampler yet because the sampler >>>>>>>>>>>> only >>>>>>>>>>>> exists on paper so far. >>>>>>>>>>>> >>>>>>>>>>>> The sim shows about 120psec rise time. Open loop sampler. >>>>>>>>>>>> Essentially an >>>>>>>>>>>> RF BJT driven hard by a fast logic chip and then the collector >>>>>>>>>>>> signal is >>>>>>>>>>>> capacitively coupled into the sampler (fractions of a pF). >>>>>>>>>>>> >>>>>>>>>>>> One of the challenges with a post-S&H is that there seem to be >>>>>>>>>>>> no ICs >>>>>>>>>>>> for that. The SMP04 still needs 4usec, too long, and it costs an >>>>>>>>>>>> arm >>>>>>>>>>>> and >>>>>>>>>>>> a leg. It's a pity they don't sell the fast samplers from ADCs >>>>>>>>>>>> sans the >>>>>>>>>>>> ADC. "I'd like to have burger and fries but without the burger". >>>>>>>>>>>> >>>>>>>>>>> There are lots of nice analogue muxes with very low charge >>>>>>>>>>> injection, >>>>>>>>>>> and the dual-gate MOSFET trick works even better. >>>>>>>>>>> >>>>>>>>>>> Cheers >>>>>>>>>>> >>>>>>>>>>> Phil Hobbs >>>>>>>>>>> >>>>>>>>>> >>>>>>>>>> I'm all ears for the dual gate MOSFET trick. Are they using one of >>>>>>>>>> the >>>>>>>>>> gates >>>>>>>>>> to shield the charge injection of the other gate? >>>>>>>>>> >>>>>>>>>> Back in the day, much effort went into complimentary switches in >>>>>>>>>> charge >>>>>>>>>> transfer circuits. All sorts of games regarding clock rates >>>>>>>>>> (not too >>>>>>>>>> fast, >>>>>>>>>> not to slow), etc. >>>>>>>>>> >>>>>>>>>> >>>>>>>>> It's pretty simple--you connect the input to the source and G2, >>>>>>>>> storage >>>>>>>>> cap to D, sample pulse to G1. (It works best with parts that bias >>>>>>>>> properly with V_G2S = 0.) Works great with 3N201s. ;) >>>>>>>>> >>>>>>>>> You usually wouldn't do it that way because of the resistance >>>>>>>>> nonlinearity, but you don't care much about that in a sampler, >>>>>>>>> assuming >>>>>>>>> the sample gate is several time constants wide. >>>>>>>>> >>>>>>>> >>>>>>>> Unfortunately it hardly ever is. Mostly the time constant is 10x to >>>>>>>> 100x >>>>>>>> the sample gate width. So one either has to live just with the small >>>>>>>> delta-V as a sampled signal for processing or let this run over a >>>>>>>> hundred cycles or so and accumulate in a post sampler. Or use the >>>>>>>> feedback trick from the old HP days that John described to make the >>>>>>>> sampler cap ratchet up. >>>>>>>> >>>>>>>> The ratcheting should be easier with the dual-gate circuit though >>>>>>>> because there won't be any leakage. Might not even need the >>>>>>>> feedback or >>>>>>>> post-sampling at all. But capacitances are probably quite high, >>>>>>>> or at >>>>>>>> least higher than 0.25pF. >>>>>>>> >>>>>>>> Possibly the nonlinearityy could be servoed out by running a 2nd >>>>>>>> dual-gate in a slower fashion. That's how I sometimes did >>>>>>>> controllable >>>>>>>> delay circuits. It would be nice to have a dual-dual-gate FET for >>>>>>>> that >>>>>>>> but having them close to each other with some serious copper on the >>>>>>>> inner layers could work. >>>>>>>> >>>>>>> >>>>>>> Right, it's the second sampler I'm talking about. Normally getting a >>>>>>> 100-ns TC and a 1-us window isn't so hard. >>>>>>> >>>>>> >>>>>> Looks like I may have to live with single-diode and then there's >>>>>> only a >>>>>> few tens of nsec available. It might have to be discrete again since >>>>>> even what they call fast (ADG820 and similar) isn't that great in >>>>>> speed. >>>>>> At least in the reasonable price range. >>>>>> >>>>>> >>>>>>> In a fast sampler running inside a sampling loop (where the previous >>>>>>> sample is added to the bias voltages), the diode conditions are kept >>>>>>> pretty nearly constant from sample to sample. >>>>>>> >>>>>> >>>>>> Or one just lives with the delta-V without a loop. Could be averaged >>>>>> locally or digitally later since AD converters have become so cheap. >>>>>> >>>>> >>>>> Assuming that one has a sufficiently accurate model of diode behaviour >>>>> under dynamic conditions. Feedback is nice that way. >>>>> >>>>> A BF908 has a reverse transfer capacitance (G1-D) of typically 0.03 pF. >>>>> The G2-D capacitance would be more of a worry, but one more cascode >>>>> stage should fix that. >>>> >>>> Or run G2 from the buffer output, which would also fix it. Have to try >>>> that. >>>> >>> >>> True, but if you need a buffer or a charge amp with a beefy output you >>> might as well use a Schottky diode there, too :-) >>> >> >> It only has to drive a 3-pF capacitance, and it's 3 pF to the input and >> not to the sample pulse. Plus when it's off, it's really off. But that >> part will run out of steam above a few hundred megahertz. >> >> My original suggestion was a cascode made out of two pHEMTs, e.g. >> SKY65050s. It would have much the same virtues at much lower >> capacitance, and has gain up to 12 GHz or so. The Skyworks ones are >> much nicer than the Avago ones for this sort of job, because their >> output conductance is a lot lower, i.e. they have a really high Early >> voltage. >> > > They are nice but most likely they'd have to be adjusted in production > for the upper gate bias (where the to-be-sampled signals would go in). > The datasheet mentions about 30% total tolrance in the pinchoff. I guess > I could automate that. The other thing is whether it'll be stable with > one riding on the drain of the other. With GHz devices that can be like > trying to balance a garden hose on the tip of a finger.I've used them as the bottom of a cascode with a BFP640 SiGe:C on top. The BFP640 is a 40-GHz device, but works fine with a 5-ohm bead in its base lead. I haven't tried a pair of SKY65050s. One especially nice thing about them is that there's a particular voltage (within operating bias) where the gate current goes to zero. I don't know how consistent that is from device to device, but in the devices I have, the leakage definitely changes sign somewhere in the normal bias region. There are lots of things you can do with that.> > Wish they had a SPICE model for the SKY65050, there's only ADS and > S-Parameters on their site. > > http://www.skyworksinc.com/Product.aspx?ProductID=348I wound up using an ATF38143 for that particular project, because its 1/f corner was at 10 MHz rather than the SKY65050's 50 MHz. Its drain impedance is only a few hundred ohms, though, which makes it a miserable device for anything fancy (other than a cascode). The pHEMT/SiGe cascode is a magical device--300 pV noise, many GHz BW, effectively infinite Early voltage. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Sampler diodes with more barrier height?
Started by ●February 15, 2014
Reply by ●February 17, 20142014-02-17
Reply by ●February 17, 20142014-02-17
Phil Hobbs wrote:> On 2/17/2014 3:34 PM, Joerg wrote: >> Phil Hobbs wrote: >>> On 02/17/2014 12:39 PM, Joerg wrote: >>>> Phil Hobbs wrote: >>>>> On 02/17/2014 11:25 AM, Phil Hobbs wrote:[...]>>>>>> >>>>>> Assuming that one has a sufficiently accurate model of diode >>>>>> behaviour >>>>>> under dynamic conditions. Feedback is nice that way. >>>>>> >>>>>> A BF908 has a reverse transfer capacitance (G1-D) of typically >>>>>> 0.03 pF. >>>>>> The G2-D capacitance would be more of a worry, but one more >>>>>> cascode >>>>>> stage should fix that. >>>>> >>>>> Or run G2 from the buffer output, which would also fix it. Have to >>>>> try >>>>> that. >>>>> >>>> >>>> True, but if you need a buffer or a charge amp with a beefy output you >>>> might as well use a Schottky diode there, too :-) >>>> >>> >>> It only has to drive a 3-pF capacitance, and it's 3 pF to the input and >>> not to the sample pulse. Plus when it's off, it's really off. But that >>> part will run out of steam above a few hundred megahertz. >>> >>> My original suggestion was a cascode made out of two pHEMTs, e.g. >>> SKY65050s. It would have much the same virtues at much lower >>> capacitance, and has gain up to 12 GHz or so. The Skyworks ones are >>> much nicer than the Avago ones for this sort of job, because their >>> output conductance is a lot lower, i.e. they have a really high Early >>> voltage. >>> >> >> They are nice but most likely they'd have to be adjusted in production >> for the upper gate bias (where the to-be-sampled signals would go in). >> The datasheet mentions about 30% total tolrance in the pinchoff. I guess >> I could automate that. The other thing is whether it'll be stable with >> one riding on the drain of the other. With GHz devices that can be like >> trying to balance a garden hose on the tip of a finger. > > I've used them as the bottom of a cascode with a BFP640 SiGe:C on top. > The BFP640 is a 40-GHz device, but works fine with a 5-ohm bead in its > base lead. I haven't tried a pair of SKY65050s. > > One especially nice thing about them is that there's a particular > voltage (within operating bias) where the gate current goes to zero. I > don't know how consistent that is from device to device, but in the > devices I have, the leakage definitely changes sign somewhere in the > normal bias region. There are lots of things you can do with that. >Probably the leakage is very low to begin with. Unlike the Schottky diodes with low barriers. John: Just to to Skyworks and the are sending me samplers of their DMJ2824. Around 0.1pF, amazing. Seems those are the only ones with high Schottky barrier and thus their Vf is more like a silicon diode. Big downside is that they cost a ton. I've also asked Avagotech but there you have to fill out a form so that takes time. Their HSMS286x series is low in Vf so I assume their temp drift in leakage current is pretty bad.>> >> Wish they had a SPICE model for the SKY65050, there's only ADS and >> S-Parameters on their site. >> >> http://www.skyworksinc.com/Product.aspx?ProductID=348 >Skyworks confirmed this afternoon that their is indeed no SPICE model :-(> I wound up using an ATF38143 for that particular project, because its > 1/f corner was at 10 MHz rather than the SKY65050's 50 MHz. Its drain > impedance is only a few hundred ohms, though, which makes it a miserable > device for anything fancy (other than a cascode). >In this case I could live with that.> The pHEMT/SiGe cascode is a magical device--300 pV noise, many GHz BW, > effectively infinite Early voltage. >Maybe on the next project. I need to press on with this one and get that part finalized at least in SPICE. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●February 17, 20142014-02-17
On Mon, 17 Feb 2014 12:22:20 -0800, Joerg <invalid@invalid.invalid> wrote:>John Larkin wrote: >> On Mon, 17 Feb 2014 10:49:14 -0800, Joerg <invalid@invalid.invalid> wrote: >> >>> John Larkin wrote: > >[...] > >>> >>>> It would be interesting to see what could be done with MiniCircuits parts >>>> (baluns and mixers) to make an open-loop bridge sampler. >>>> >>> I mostly came away disappointed when canvassing the commercial >>> offerings. That was the same during my masters project in the mid-80's. >>> I was designing a CCD camera that would beat the daylights out of the >>> camera the CCD manufacturer was offering (and it did). After a >>> frustrating search I just wound my own bridge sampler transformers. >>> Which I should have done in the first place, would have saved me time. >>> For the diodes I used matched quads, I believe from HP. Four individual >>> diodes per pouch. >> >> Skyworks has a medium-barrier bridge quad, the SMS3930-021, and a high-barrier >> version, SMS3931. Both 0.3 to 0.5 pF per diode. Might work as an open-loop >> bridge sampler. >> > >That may be better than the SMS7621 which also comes in a bridge quad. >I'll call Skyworks this afternoon to see whether their higher barrier >types are better in leakage current at higher temps. The board has to >perform at least to 60C and I want to keep it simple, ideally without >much fancy post-sampling and loop stuff.Really, the EL/EP52 flipflop 1-bit sampler looks like a good bet here. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●February 17, 20142014-02-17
Lasse Langwadt Christensen wrote:> Den tirsdag den 18. februar 2014 00.44.55 UTC+1 skrev John Larkin:[...]>> >> If you look at Agoston's Tek samplers, it's mind-boggling to think >> >> about how many hours they had to have spent to get it as clean as it >> >> is. There are strange wirebonds all over the place, notched >> >> microstrips, bizarre shapes, scary stuff. It must have been a >> >> nightmare in production and test, too. >> >> >> >> https://dl.dropboxusercontent.com/u/53724080/SD-24/SD-24.zip >> > > I visited Rohde & Schwarz in Munich many years ago and saw the place > where they made stuff like that > > the guy there had a German saying something like: > the higher the frequency the longer the face >Or like an Italian sports car where the boss in the shop said: "Oh, only Giuseppe can do those". -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●February 17, 20142014-02-17
John Larkin wrote:> On Mon, 17 Feb 2014 12:22:20 -0800, Joerg <invalid@invalid.invalid> > wrote: > >> John Larkin wrote: >>> On Mon, 17 Feb 2014 10:49:14 -0800, Joerg <invalid@invalid.invalid> wrote: >>> >>>> John Larkin wrote: >> [...] >> >>>>> It would be interesting to see what could be done with MiniCircuits parts >>>>> (baluns and mixers) to make an open-loop bridge sampler. >>>>> >>>> I mostly came away disappointed when canvassing the commercial >>>> offerings. That was the same during my masters project in the mid-80's. >>>> I was designing a CCD camera that would beat the daylights out of the >>>> camera the CCD manufacturer was offering (and it did). After a >>>> frustrating search I just wound my own bridge sampler transformers. >>>> Which I should have done in the first place, would have saved me time. >>>> For the diodes I used matched quads, I believe from HP. Four individual >>>> diodes per pouch. >>> Skyworks has a medium-barrier bridge quad, the SMS3930-021, and a high-barrier >>> version, SMS3931. Both 0.3 to 0.5 pF per diode. Might work as an open-loop >>> bridge sampler. >>> >> That may be better than the SMS7621 which also comes in a bridge quad. >> I'll call Skyworks this afternoon to see whether their higher barrier >> types are better in leakage current at higher temps. The board has to >> perform at least to 60C and I want to keep it simple, ideally without >> much fancy post-sampling and loop stuff. > > Really, the EL/EP52 flipflop 1-bit sampler looks like a good bet here. >Yes, it sure is tempting. But I have to wrap up soon and differential stuff with both inputs close together can cause nasty oscillation. Then I'd be in hot doodoo because we essentially have to commit straight from SPICE to board. In some of the fast comparators that's almost guaranteed. It's like standing on that huge tower at an olympic pool. I did finally jump but there was a trainer down there who could fish me out if I screwed up. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●February 18, 20142014-02-18
On Sunday, February 16, 2014 7:07:03 PM UTC-5, John Larkin wrote:> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> wrote: ><snip>> > This is the classic 2-diode feedback sampler: > > https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPGThanks! I didn't really understand what you guys were talking about. So the diodes are kept "almost on" by the feedback? Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps? Hmm well not a voltage. A current proportional to the input voltage? "Scratch scratch" I've printed a copy, I'll stare at it over lunch. (I guess I should think about currents. (?)) George H.> > which is pretty much what's in the HP185 scope from 1962. The analog switch, > > just before the integrator, can be any decent CMOS part; it will be on for a > > couple of us after each sample. The opamps can be any cheap jfet dual. There are > > no expensive parts here... the SRD might cost 75 cents. > > > > The signal at A is a small bump that's the difference - a few per cent - between > > the input and the output at the sampling instant. That gets amplified and gated > > into the integrator as the baseline for the next shot. Loop gain can be 1, or > > less than 1 for "smoothing". Any drift is in the integrator, and schottky diode > > leakage hardly matters. > > > > B1 and B2 represent the voltages that back-bias the sampling diodes. In the Tek > > full-bridge 7S14, they were literally 1.3 volt mercury batteries... which died > > in a few years and were hell to replace. Bandgaps would work fine. > > > > > > > > -- > > > > John Larkin Highland Technology Inc > > www.highlandtechnology.com jlarkin at highlandtechnology dot com > > > > Precision electronic instrumentation
Reply by ●February 18, 20142014-02-18
On Tue, 18 Feb 2014 06:41:56 -0800 (PST), George Herold <gherold@teachspin.com> wrote:>On Sunday, February 16, 2014 7:07:03 PM UTC-5, John Larkin wrote: >> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> wrote: >> ><snip> >> >> This is the classic 2-diode feedback sampler: >> >> https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG > >Thanks! I didn't really understand what you guys were talking about. > >So the diodes are kept "almost on" by the feedback?No, they are back-biased by a volt or so. The bias tracks the last-sampled value.>Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps?The signal level info is actually stored in C1 and C2. If the signal (at the sampling instant) is equal to the feedback voltage, C1 and C2 receive equal amounts of charge and there is no net signal into the amps. If not, one of the caps charges more than the other, and you get a signed glitch to work with. The voltage glitch at A may be just a few per cent of the difference between the signal level and the output... low "sampling efficiency." The feedback loop boosts the signal and makes everything linear. Joerg wants to use a full-bridge sampler with no feedback. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Reply by ●February 18, 20142014-02-18
John Larkin wrote:> On Tue, 18 Feb 2014 06:41:56 -0800 (PST), George Herold <gherold@teachspin.com> > wrote: > >> On Sunday, February 16, 2014 7:07:03 PM UTC-5, John Larkin wrote: >>> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> wrote: >>> >> <snip> >>> This is the classic 2-diode feedback sampler: >>> >>> https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG >> Thanks! I didn't really understand what you guys were talking about. >> >> So the diodes are kept "almost on" by the feedback? > > No, they are back-biased by a volt or so. The bias tracks the last-sampled > value. > >> Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps? > > The signal level info is actually stored in C1 and C2. If the signal (at the > sampling instant) is equal to the feedback voltage, C1 and C2 receive equal > amounts of charge and there is no net signal into the amps. If not, one of the > caps charges more than the other, and you get a signed glitch to work with. The > voltage glitch at A may be just a few per cent of the difference between the > signal level and the output... low "sampling efficiency." The feedback loop > boosts the signal and makes everything linear. > > Joerg wants to use a full-bridge sampler with no feedback. >Probably just single-diode, the full bridge needs too much "stuff" for this design and there is no negative supply. Linearity isn't a concern and, provided each delta-V step is at least several millivolts, there isn't much to be gained in SNR by a charge accumulation method versus just averaging or lowpass-filtering these delta-Vs into an RC or something. Of course, then the sampling pulse must come from a very clean source. Now I just need a high Schottky barrier (or at least middle barrier) diode that does not leak so much and won't cost an arm and a leg. The latter seems to be a problem. Worst case I'll have to live with a solution where nearly all the charge leaks out during each cycle and I average the resulting "sawteeth". Lowers the SNR but not by much. I wonder how they do that on the cheap cable testers. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●February 18, 20142014-02-18
On 2/18/2014 9:18 AM, John Larkin wrote:> On Tue, 18 Feb 2014 06:41:56 -0800 (PST), George Herold <gherold@teachspin.com> > wrote: > >> On Sunday, February 16, 2014 7:07:03 PM UTC-5, John Larkin wrote: >>> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> wrote: >>> >> <snip> >>> This is the classic 2-diode feedback sampler: >>> >>> https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG >> Thanks! I didn't really understand what you guys were talking about. >> >> So the diodes are kept "almost on" by the feedback? > No, they are back-biased by a volt or so. The bias tracks the last-sampled > value. > >> Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps? > The signal level info is actually stored in C1 and C2. If the signal (at the > sampling instant) is equal to the feedback voltage, C1 and C2 receive equal > amounts of charge and there is no net signal into the amps. If not, one of the > caps charges more than the other, and you get a signed glitch to work with. The > voltage glitch at A may be just a few per cent of the difference between the > signal level and the output... low "sampling efficiency." The feedback loop > boosts the signal and makes everything linear. > > Joerg wants to use a full-bridge sampler with no feedback. > >Hi, John - Are those transmission lines where it looks like the pulse generator output is shorted to ground? Thanks, John S
Reply by ●February 18, 20142014-02-18
On Tue, 18 Feb 2014 07:58:39 -0800, Joerg <invalid@invalid.invalid> wrote:>John Larkin wrote: >> On Tue, 18 Feb 2014 06:41:56 -0800 (PST), George Herold <gherold@teachspin.com> >> wrote: >> >>> On Sunday, February 16, 2014 7:07:03 PM UTC-5, John Larkin wrote: >>>> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> wrote: >>>> >>> <snip> >>>> This is the classic 2-diode feedback sampler: >>>> >>>> https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG >>> Thanks! I didn't really understand what you guys were talking about. >>> >>> So the diodes are kept "almost on" by the feedback? >> >> No, they are back-biased by a volt or so. The bias tracks the last-sampled >> value. >> >>> Then with the sampling pulse they (the diodes) are like switches and transmit the input voltage to the caps? >> >> The signal level info is actually stored in C1 and C2. If the signal (at the >> sampling instant) is equal to the feedback voltage, C1 and C2 receive equal >> amounts of charge and there is no net signal into the amps. If not, one of the >> caps charges more than the other, and you get a signed glitch to work with. The >> voltage glitch at A may be just a few per cent of the difference between the >> signal level and the output... low "sampling efficiency." The feedback loop >> boosts the signal and makes everything linear. >> >> Joerg wants to use a full-bridge sampler with no feedback. >> > >Probably just single-diode, the full bridge needs too much "stuff" for >this design and there is no negative supply. Linearity isn't a concern >and, provided each delta-V step is at least several millivolts, there >isn't much to be gained in SNR by a charge accumulation method versus >just averaging or lowpass-filtering these delta-Vs into an RC or >something. Of course, then the sampling pulse must come from a very >clean source. > >Now I just need a high Schottky barrier (or at least middle barrier) >diode that does not leak so much and won't cost an arm and a leg. The >latter seems to be a problem. Worst case I'll have to live with a >solution where nearly all the charge leaks out during each cycle and I >average the resulting "sawteeth". Lowers the SNR but not by much. > >I wonder how they do that on the cheap cable testers.Cable TDRs are slow, aren't they? The Tek field-level TDRs (1500 series) had full-bridge sampler front-ends, with risetimes in the hundreds of ps. http://www.ko4bb.com/Manuals/Tektronix/Tektronix_-_1503C_Metallic_Time_Domain_Reflectometer/TEK%201503C%20Service_EN.pdf Story: Tek once had a scope assembly plant in Guernsey, Channel Islands, for european tax reasons. (Channel Islands has a unique legal status and was occupied by the Nazis in WWII.) Later things changed, so they pulled out. But they let the islanders start up Polar Instruments http://www.polarinstruments.com/ Last time I checked, they were still using the 15xx-series TDR boards, which are actually slow for PCB work. Good book: https://en.wikipedia.org/wiki/The_Guernsey_Literary_and_Potato_Peel_Pie_Society -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
