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Sampler diodes with more barrier height?

Started by Joerg February 15, 2014
John Larkin wrote:
> On Sun, 16 Feb 2014 15:38:43 -0800, Joerg <invalid@invalid.invalid> 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. >>> >> I know, but I was hoping that I didn't have to piece it all together and >> someone would have made a nice IC for that. I can't be the only one. > > This is the classic 2-diode feedback sampler: > > https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG >
Thanks.
> 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. >
I'd have to run the post-sampler faster because my whole signal period is under 1/2usec.
> 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. >
True, with this scheme the transformer isn't so critical anymore either because it's output gets spiffed up by the SRD.
> 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. >
Yikes. I've seen that before, where they generated the bias for a tube with a battery. A red one with a black cat on there. I guess those must have been cheaper than resistors half a century ago. Needless to say, it leaked all over the place. -- Regards, Joerg http://www.analogconsultants.com/
Phil Hobbs wrote:
> On 2/16/2014 6:45 PM, Joerg wrote: >> John Larkin wrote: >>> On Sat, 15 Feb 2014 17:01:36 -0800, Joerg <invalid@invalid.invalid> >>> 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). >>> >>> Be wary of the BJT response. I've pulsed all sorts of BJTs, including >>> some of >>> the 45 GHz SiGe parts, and have never seen especially fast output >>> edges. Phemts >>> are different; they can switch fast. >>> >> >> I've done it with BFS17 a lot. They can really pulse. What one cannot >> rely on too well is the "let go" phase, of course. I need to bang a >> slope of around 100psec onto a load. That requires some fast BFP-series >> part. >> > > That's interesting. From a quick scan of the BFS17 datasheet, it > doesn't look as though it's anything special, at least not on the scale > of the BFR505 or BFG520. What do you like about it especially? >
Because they had manners and didn't oscillate so easily, like John said. But the main reason was that the price was right, around 3c. That's hard to beat. Haven't used on in a few years now but I still have lots of them. -- Regards, Joerg http://www.analogconsultants.com/
Am 17.02.2014 00:45, schrieb Joerg:
> John Larkin wrote: >> I've done a little playing with software deconvolution for TDR. The idea is to >> build a fast but ugly sampling or TDR system and clean up its step response with >> a software FIR filter, the problem being how to train the filter. >> >> https://dl.dropboxusercontent.com/u/53724080/Sampling/TDR_Decon_demo.jpg >> >> I have an algorithm that seems to work well, although I've never really >> understood it. It just seemed like a reasonable thing to do.
HP had the deconvolution software in their 54750A scopes & associated TDRs. There is a PSPL app note that compares industry standard samplers that talks about that. regards, Gerhard
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: > > > > >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. > > >> > > > > > >I know, but I was hoping that I didn't have to piece it all together and > > >someone would have made a nice IC for that. I can't be the only one. > > > > This is the classic 2-diode feedback sampler: > > > > https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG > > > > 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
What is that mess used for?
On Sun, 16 Feb 2014 18:18:22 -0800 (PST), bloggs.fredbloggs.fred@gmail.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: >> >> >> >> >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. >> >> >> >> >> > >> >> >I know, but I was hoping that I didn't have to piece it all together and >> >> >someone would have made a nice IC for that. I can't be the only one. >> >> >> >> This is the classic 2-diode feedback sampler: >> >> >> >> https://dl.dropboxusercontent.com/u/53724080/Sampling/2-diode-sampler.JPG >> >> >> >> 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 > >What is that mess used for?
Nothing that you'd ever be involved with. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
On Mon, 17 Feb 2014 02:50:42 +0100, Gerhard Hoffmann <dk4xp@arcor.de> wrote:

>Am 17.02.2014 00:45, schrieb Joerg: >> John Larkin wrote: >>> I've done a little playing with software deconvolution for TDR. The idea is to >>> build a fast but ugly sampling or TDR system and clean up its step response with >>> a software FIR filter, the problem being how to train the filter. >>> >>> https://dl.dropboxusercontent.com/u/53724080/Sampling/TDR_Decon_demo.jpg >>> >>> I have an algorithm that seems to work well, although I've never really >>> understood it. It just seemed like a reasonable thing to do. > >HP had the deconvolution software in their 54750A scopes & >associated TDRs. There is a PSPL app note that compares >industry standard samplers that talks about that. > >regards, Gerhard >
I think that one used the Bracewell Transform, sort of an FFT that is its own inverse. The classic deconvolution was an FFT-divide-reverseFFT, since convolution in the time domain is multiplication in the frequency domain. But the FFT thing has noise and divide-by-zero problems. Imagine a graphic equalized where you have to crank some of the gains to infinity, to make zero bins come out flat. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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.
Phil Hobbs wrote:
> On 2/16/2014 7:13 PM, John Larkin wrote: >> On Sun, 16 Feb 2014 18:57:20 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2/16/2014 6:45 PM, Joerg wrote: >>>> John Larkin wrote: >>>>> On Sat, 15 Feb 2014 17:01:36 -0800, Joerg <invalid@invalid.invalid> >>>>> 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). >>>>> >>>>> Be wary of the BJT response. I've pulsed all sorts of BJTs, >>>>> including some of >>>>> the 45 GHz SiGe parts, and have never seen especially fast output >>>>> edges. Phemts >>>>> are different; they can switch fast. >>>>> >>>> >>>> I've done it with BFS17 a lot. They can really pulse. What one cannot >>>> rely on too well is the "let go" phase, of course. I need to bang a >>>> slope of around 100psec onto a load. That requires some fast BFP-series >>>> part. >>>> >>> >>> That's interesting. From a quick scan of the BFS17 datasheet, it >>> doesn't look as though it's anything special, at least not on the scale >>> of the BFR505 or BFG520. What do you like about it especially? >>> >>> Cheers >>> >>> Phil Hobbs >> >> I like BFS17s because they are pretty fast but no too fast. They are >> less likely >> than really fast parts to oscillate in nanosecond current-steering >> sorts of >> apps. But Ft is only about 2 GHz. > > So sort of the big brother of the BC817? >
The "Sports Edition" of the BC847 :-) -- Regards, Joerg http://www.analogconsultants.com/
John Larkin wrote:
> On Sun, 16 Feb 2014 15:47:20 -0800, Joerg <invalid@invalid.invalid> wrote: > >> John Larkin wrote: >>> On Sun, 16 Feb 2014 12:56:11 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2/15/2014 9:28 PM, Robert Baer wrote: >>>>> Joerg 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. >>>>>> >>>>> Schottky, like Germanium diodes are inherently leaky (very similar >>>>> that way) because of the low Vf. >>>>> A standard plain silicon diode is far better and has a higher barrier >>>>> height; thousands to choose from, and a fair number of fast ones at >>>>> decent prices. >>>>> >>>> Not at picosecond speeds, there aren't. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> PN diodes neither turn on nor turn off in much less than a ns. >>> >> By that time my pulse is already east of Hangtown :-) > > I'm still disappointed by the pulse response of microwave NPN transistors, even > the SiGe stuff. I'm thinking that a PN junction works a lot faster in > small-signal mode (where it's already been on for a while) than going from off > to on in one whack. PIN diode concept, carriers or something. >
But then they could not claim many GHz of bandwidth in amplifier configurations. Usually that is used at serious output swings in cable distribution systems and the like. One guy got almost 200psec in transitions out of the old BFR92. I've never managed to quite get that from a BFS17. Normally you aren't supposed to saturate them but it's tough to walk the fine line between that and not enough amplitude. They can't do very sort pulses, just a fast pull-down, unfortunately. So the other side of the pulse doesn't look like much to write home about. -- Regards, Joerg http://www.analogconsultants.com/
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. 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