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Sampler diodes with more barrier height?

Started by Joerg February 15, 2014
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.

-- 
Regards, Joerg

http://www.analogconsultants.com/
I was browsing schottkies the other day and came across BAR18, highish 
voltage (70V), CJO max 2pF, stupidly high Vf (2V at only 40mA!).  How much 
leakage and CJO do you require?

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"Joerg" <invalid@invalid.invalid> wrote in message 
news:bma605Fp3t5U1@mid.individual.net...
> 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. > > -- > Regards, Joerg > > http://www.analogconsultants.com/
Tim Williams wrote:
> I was browsing schottkies the other day and came across BAR18, highish > voltage (70V), CJO max 2pF, stupidly high Vf (2V at only 40mA!). How much > leakage and CJO do you require? >
That's too big a diode, I need <0.3pF. Leakage should not be more than a few (very few) hundred nA but that should ideally hold to 80C or so. When the unit sits in the glistening sun and bakes. -- Regards, Joerg http://www.analogconsultants.com/
On 15/02/2014 22:24, Joerg wrote:
> Tim Williams wrote: >> I was browsing schottkies the other day and came across BAR18, highish >> voltage (70V), CJO max 2pF, stupidly high Vf (2V at only 40mA!). How much >> leakage and CJO do you require? >> > > That's too big a diode, I need <0.3pF. Leakage should not be more than a > few (very few) hundred nA but that should ideally hold to 80C or so. > When the unit sits in the glistening sun and bakes. >
BZX84C75 ? Cheers -- Syd
On 15/02/14 23:55, Syd Rumpo wrote:
> On 15/02/2014 22:24, Joerg wrote: >> Tim Williams wrote: >>> I was browsing schottkies the other day and came across BAR18, highish >>> voltage (70V), CJO max 2pF, stupidly high Vf (2V at only 40mA!). How >>> much >>> leakage and CJO do you require? >>> >> >> That's too big a diode, I need <0.3pF. Leakage should not be more than a >> few (very few) hundred nA but that should ideally hold to 80C or so. >> When the unit sits in the glistening sun and bakes. >> > BZX84C75 ? > > Cheers
Joke? Cheers, Jeroen Belleman
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. There are higher-barrier diodes in the Skyworks family. They have a great sample kit. What sort of risetime did you get? Are you doing an open-loop sampler? What the sampling pulse generator like? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Am 15.02.2014 22:54, schrieb Joerg:

> 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.
Sth. with a larger band gap like GaAs? Abuse a MGF1302 gate that you may have in your junk box? regards, Gerhard
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". -- Regards, Joerg http://www.analogconsultants.com/
Gerhard Hoffmann wrote:
> Am 15.02.2014 22:54, schrieb Joerg: > >> 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. > > Sth. with a larger band gap like GaAs? ...
I'll call Skyworks and see what else they can offer. Maybe something that isn't so mainstream at distributros but longterm available.
> ... Abuse a MGF1302 gate that you may have in your junk box? >
It needs to be something that's not unobtanium :-) Like the mountain bike frame I was eyeing and now it's out of stock. Hurumph! -- Regards, Joerg http://www.analogconsultants.com/
Am 16.02.2014 02:40, schrieb Joerg:
> Gerhard Hoffmann wrote: >> Am 15.02.2014 22:54, schrieb Joerg:
> I'll call Skyworks and see what else they can offer. Maybe something > that isn't so mainstream at distributros but longterm available. > > >> ... Abuse a MGF1302 gate that you may have in your junk box? >> > > It needs to be something that's not unobtanium :-)
MGF1302 & friends have been workhorses for ham EME for decades... Someone still makes GaAsFETs, no???? :-) Gerhard