Forums

Sampler diodes with more barrier height?

Started by Joerg February 15, 2014
On 2/16/2014 10:03 AM, Maynard A. Philbrook Jr. wrote:
> In article <ldqf8e$f89$1@dont-email.me>, tmoranwms@charter.net says... >> >> "Robert Baer" <robertbaer@localnet.com> wrote in message >> news:DnVLu.407695$PK.221670@fx23.iad... >>> 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. >> >> A mere 1N4148 for instance is already in the same ballpark as others >> (i.e., 1-2pF CJO, or something like that), while being a massively better >> conductor (Vf ~ 0.7V up to, what, a few 100mA, as compared to that BAR18 I >> mentioned which goes resistive in the 10s of mA). But do PN junction >> diodes exist with short enough forward and reverse recovery times to >> actually handle a 200ps pulse? Could one made of GaAs or whatever be >> better? ...Could a very small LED be suitable? >> >> Tim > > Too bad the terahertz detector methods couldn't be utilized downward > abit. There seems to be more available with detecting that spectrum. > > Devices like the Golay cell, bolometer, pyroelectric effect, the photo > rectification (acoustic which I don't think would work any how).. > > As for pulsing lasers onto a semi for a detector, I would think the > semi would need to be of a large area for the Ghz, if it would work at > all? > > Jamie > > > etc.. >
Some of them can. My antenna-coupled MIM junctions worked from DC to at least 200 THz (1.5 microns in the NIR). They weren't what you'd call low-leakage devices, though. (I may get the chance to do some more this year.) Golay cells are very slow thermomechanical devices. Bolometers are limited to the thermal time constant of the device, and it's very very hard to get that as low as 1 ns. (I have some data at that speed, using a 20-ps laser.) Electro-optic and plasma-optic sampling has been done for at least 30 years. The very low phase noise of a good modelocked laser is a huge help, but it's pretty badly constrained by the need for the signal to be locked to the laser rep rate, rather than the other way round. I suppose if you had a high enough sample rate, you could do what John's LeCroy does--sample like crazy and do the clock recovery in software afterwards. 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
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. -- 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. >
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. -- Regards, Joerg http://www.analogconsultants.com/
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.
> Sampling pulses are usually created with SRDs and clipping lines. The really > fast samplers use NLTL shock lines. >
I don't have to go quite that fast, 100psec would be sufficient.
> SRD steps are usually pretty-fast steps that are made faster and cleaner with > steering diodes. See Agoston's patents. People worked very hard to build > samplers and step generators that had clean picosecond response. > > 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. >
We will have to SW-correlate a lot to clean this up. Mainly because the trailing edge of my pulse will look ugly as heck. Because I need it to also contain some lower frequency energy, down to 100MHz or so. -- Regards, Joerg http://www.analogconsultants.com/
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 :-) -- Regards, Joerg http://www.analogconsultants.com/
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 -- 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
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
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. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
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? 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