legg wrote:> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg <invalid@invalid.invalid> > wrote: > >> Folks, >> >> Looked at Digikey and some others. Where are those huge capacitance >> varicaps? The ones with several hundred pF of range for AM radios. All >> gone lalaland by now? >> >> Even the ones I found in the 100pF range are either obsolete or not >> recommended for new designs. >> >> What I am trying to do: I need to control a switcher chip in frequency >> because I've got a very resonant load to deal with. Unfortunately it >> sets the frequency with a timing cap. I'll have to somehow vary that >> between 750pF and 2000pF. Can also be digital but then with a >> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >> >> Doing it with caps and a mux chip or two has its own challenges. The >> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >> high. Talking about the a rock and a hard spot here. >> >> Oh, and cost is not very important. If a diode or two or three are >> needed that cost $5 a pop that's ok. > > SVC383 ex-Sanyo, is still in the ON catalog, with no product notices > re last run or anything. Dual 500pf. > > http://www.onsemi.com/pub_link/Collateral/ENN6264-D.PDF >Thanks, but I've been through that as well. Here is the bad news: http://www.digikey.com/product-detail/en/SVC383T-TL-E/869-1202-2-ND/2165536 Quote "Obsolete item; call Digi-Key for more information". -- Regards, Joerg http://www.analogconsultants.com/
Large capacitance varicaps, where are thee?
Started by ●October 22, 2012
Reply by ●October 22, 20122012-10-22
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 16:30:01 -0700, Joerg <invalid@invalid.invalid> wrote:>krw@att.bizzzzzzzzzzzz wrote: >> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg <invalid@invalid.invalid> wrote: >> >>> Folks, >>> >>> Looked at Digikey and some others. Where are those huge capacitance >>> varicaps? The ones with several hundred pF of range for AM radios. All >>> gone lalaland by now? >> >> http://www.angelfire.com/electronic2/index1/VariableCap.html >> >> About half way down. >> >>> Even the ones I found in the 100pF range are either obsolete or not >>> recommended for new designs. >> >> No kiddin? ;-) >> >>> What I am trying to do: I need to control a switcher chip in frequency >>> because I've got a very resonant load to deal with. Unfortunately it >>> sets the frequency with a timing cap. I'll have to somehow vary that >>> between 750pF and 2000pF. Can also be digital but then with a >>> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >> >> Use a regulator with an external clock. That's what we do (all of our >> switchers are synchronized). >> > >Well, I need one from LTC because it must be simulated. The load is >really ugly yet must be well regulated with some unorthodox loop >elements in there. Linear only has the 3721 and 3723 for push-pull. Some >older ones as well but they have the same engine type in them.Nuts.>>> Doing it with caps and a mux chip or two has its own challenges. The >>> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >>> high. Talking about the a rock and a hard spot here. >>> >>> Oh, and cost is not very important. If a diode or two or three are >>> needed that cost $5 a pop that's ok. >> >> A regulator with an external clock shouldn't be an issue, then. > > >If LTC had a suitable one. We have a processor so sync'ing is no big >deal except that I usually get the looks if I request processor MIPS.Setting a timer is too many MIPS?>Analog dudes aren't really entitled to those.Analog dudes certainly are everywhere I've worked. If they want resources, they get them. In fact, they get almost all the MIPS. ;-)>Anyhow, it's not a show stopper, I can use a small varicap plus mux. The >mux is like an umpteen-speed gear shifter, not very pretty. If big >varicaps were still available it would become very easy. But I guess >even if I found one the risk is too great that it's going to be >obsoleted in a few years.That's too ugly to contemplate.
Reply by ●October 22, 20122012-10-22
langwadt@fonz.dk wrote:> On 23 Okt., 01:16, Joerg <inva...@invalid.invalid> wrote: >> langw...@fonz.dk wrote: >>> On 23 Okt., 01:08, Joerg <inva...@invalid.invalid> wrote: >>>> langw...@fonz.dk wrote: >>>>> On 23 Okt., 00:30, Joerg <inva...@invalid.invalid> wrote: >>>>>> langw...@fonz.dk wrote: >>>>>>> On 22 Okt., 23:23, Joerg <inva...@invalid.invalid> wrote: >>>>>>>> Folks, >>>>>>>> Looked at Digikey and some others. Where are those huge capacitance >>>>>>>> varicaps? The ones with several hundred pF of range for AM radios. All >>>>>>>> gone lalaland by now? >>>>>>>> Even the ones I found in the 100pF range are either obsolete or not >>>>>>>> recommended for new designs. >>>>>>>> What I am trying to do: I need to control a switcher chip in frequency >>>>>>>> because I've got a very resonant load to deal with. Unfortunately it >>>>>>>> sets the frequency with a timing cap. I'll have to somehow vary that >>>>>>>> between 750pF and 2000pF. Can also be digital but then with a >>>>>>>> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >>>>>>>> Doing it with caps and a mux chip or two has its own challenges. The >>>>>>>> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >>>>>>>> high. Talking about the a rock and a hard spot here. >>>>>>>> Oh, and cost is not very important. If a diode or two or three are >>>>>>>> needed that cost $5 a pop that's ok. >>>>>>>> -- >>>>>>>> Regards, Joerg >>>>>>>> http://www.analogconsultants.com/ >>>>>>> possible to force you own triangle in? >>>>>> Not on this one. It issues a major reset pulse of almost 50mA and if >>>>>> that sees any obstructions it all goes bonkers. It already does when you >>>>>> tough it with a 10k resistor, like the princess on the pea. >>>>>> -- >>>>>> Regards, Joerg >>>>>> http://www.analogconsultants.com/ >>>>> this too crazy?http://www.falstad.com/circuit/e-capmult.html >>>> Not crazy at all but there can't be an R3 in my case. Falls apart when >>>> it gets into the tens of ohms. It's sawtooth, not triangular. >>>> -- >>>> Regards, Joerg >>>> http://www.analogconsultants.com/ >>> I'm just guessing, diode across R3 ? >> No, there is more or less a wire. >> > > I meant add diode so discharge is fast >I have tried some tricks like that before and the LTC3721 really does not like anything on its timing terminal other than a clean capacitance. Well, I'll just multiplex it. The good old sledgehammer method. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20122012-10-22
George Herold wrote:> On Oct 22, 6:27 pm, Joerg <inva...@invalid.invalid> wrote: >> George Herold wrote: >>> On Oct 22, 5:23 pm, Joerg <inva...@invalid.invalid> wrote: >>>> Folks, >>>> Looked at Digikey and some others. Where are those huge capacitance >>>> varicaps? The ones with several hundred pF of range for AM radios. All >>>> gone lalaland by now? >>>> Even the ones I found in the 100pF range are either obsolete or not >>>> recommended for new designs. >>>> What I am trying to do: I need to control a switcher chip in frequency >>>> because I've got a very resonant load to deal with. Unfortunately it >>>> sets the frequency with a timing cap. I'll have to somehow vary that >>>> between 750pF and 2000pF. Can also be digital but then with a >>>> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >>>> Doing it with caps and a mux chip or two has its own challenges. The >>>> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >>>> high. Talking about the a rock and a hard spot here. >>>> Oh, and cost is not very important. If a diode or two or three are >>>> needed that cost $5 a pop that's ok. >>>> -- >>>> Regards, Joerg >>>> http://www.analogconsultants.com/ >>> Hmm.. I have no idea Joerg. >>> So silly ideas, can you parallel a bunch of small ones? >>> What's the frequency? Could you use a cruddie Y5/Z5 ceramic? >>> (You've most likely already been down both these roads.) >> Yup :-( >> >> They don't make Z5U this small. Paralleling gets to be tough for space >> reasons because I'd need a boatload. > > OK so how 'bout a few Z5U in series? >That would make a whole big Christmas ornament :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 14:22:49 -0700, Joerg wrote:> Folks, > > Looked at Digikey and some others. Where are those huge capacitance > varicaps? The ones with several hundred pF of range for AM radios. All > gone lalaland by now? > > Even the ones I found in the 100pF range are either obsolete or not > recommended for new designs. > > What I am trying to do: I need to control a switcher chip in frequency > because I've got a very resonant load to deal with. Unfortunately it > sets the frequency with a timing cap. I'll have to somehow vary that > between 750pF and 2000pF. Can also be digital but then with a > granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. > > Doing it with caps and a mux chip or two has its own challenges. The > ADG-series from AD is around 11pF per pin, otherwise their Rdson is too > high. Talking about the a rock and a hard spot here. > > Oh, and cost is not very important. If a diode or two or three are > needed that cost $5 a pop that's ok.In the off chance that you haven't thought of it yet: 1N4001 diodes. xxx3055 or other fairly low-voltage "moose" power BJT -- use the B-C junction. One of the "used to be common, now it's boutique" obsolete semiconductor houses. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 14:22:49 -0700, Joerg wrote:> Folks, > > Looked at Digikey and some others. Where are those huge capacitance > varicaps? The ones with several hundred pF of range for AM radios. All > gone lalaland by now? > > Even the ones I found in the 100pF range are either obsolete or not > recommended for new designs. > > What I am trying to do: I need to control a switcher chip in frequency > because I've got a very resonant load to deal with. Unfortunately it > sets the frequency with a timing cap. I'll have to somehow vary that > between 750pF and 2000pF. Can also be digital but then with a > granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. > > Doing it with caps and a mux chip or two has its own challenges. The > ADG-series from AD is around 11pF per pin, otherwise their Rdson is too > high. Talking about the a rock and a hard spot here. > > Oh, and cost is not very important. If a diode or two or three are > needed that cost $5 a pop that's ok.OK. This is obscenely bizarre, and any design and implementation details are left to your poor tortured brain: You can make a gyrator to turn a capacitance into an inductance via the good graces of an op-amp. The effective inductance depends on the capacitance and the gain. So -- consider yourself challenged to make a similar gizmo that either takes a capacitance and multiplies it by a variable amount, based on a gain that you can adjust, or that takes an inductance in a "gyrator" circuit that presents a capacitive load (and make it stable, heh heh heh). -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 15:29:00 -0700, Joerg <invalid@invalid.invalid> wrote:>miso wrote: >> On 10/22/2012 2:22 PM, Joerg wrote:<snip>>> If you are not conflating sawtooth with triangle wave, then maybe you >> could shunt some current from the osc pin, which would slow down the >> frequency. Pick your external cap for the highest frequency needed, then >> bleed with some DAC based circuit or even a resistive trimpot. The >> 10-turn PCB mounted resistors still exist. >> >> The difficulty will be in the compliance of the current source used for >> bleeding. > > >That won't be a problem but the chip immediately gets sea-sick when you >do anything DC to that pin.What do you think a varactor will do, with a sawtooth bias? RL
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 18:53:38 -0500, Tim Wescott wrote:> On Mon, 22 Oct 2012 14:22:49 -0700, Joerg wrote: > >> Folks, >> >> Looked at Digikey and some others. Where are those huge capacitance >> varicaps? The ones with several hundred pF of range for AM radios. All >> gone lalaland by now? >> >> Even the ones I found in the 100pF range are either obsolete or not >> recommended for new designs. >> >> What I am trying to do: I need to control a switcher chip in frequency >> because I've got a very resonant load to deal with. Unfortunately it >> sets the frequency with a timing cap. I'll have to somehow vary that >> between 750pF and 2000pF. Can also be digital but then with a >> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >> >> Doing it with caps and a mux chip or two has its own challenges. The >> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >> high. Talking about the a rock and a hard spot here. >> >> Oh, and cost is not very important. If a diode or two or three are >> needed that cost $5 a pop that's ok. > > OK. This is obscenely bizarre, and any design and implementation > details are left to your poor tortured brain: > > You can make a gyrator to turn a capacitance into an inductance via the > good graces of an op-amp. The effective inductance depends on the > capacitance and the gain. > > So -- consider yourself challenged to make a similar gizmo that either > takes a capacitance and multiplies it by a variable amount, based on a > gain that you can adjust, or that takes an inductance in a "gyrator" > circuit that presents a capacitive load (and make it stable, heh heh > heh).D'oh. Use the Miller capacitance of a gain-modulated stage. A series cap, a transistor with a controlled bias voltage, a B-C cap (to be "Millered"), and your analog circuits brilliance to actually make it work. Have fun getting it through a circuit design review. And finding a voltage supply big enough for the amp. I get my name on the patent if it works. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 14:22:49 -0700, Joerg <invalid@invalid.invalid> wrote:>Folks, > >Looked at Digikey and some others. Where are those huge capacitance >varicaps? The ones with several hundred pF of range for AM radios. All >gone lalaland by now? > >Even the ones I found in the 100pF range are either obsolete or not >recommended for new designs. > >What I am trying to do: I need to control a switcher chip in frequency >because I've got a very resonant load to deal with. Unfortunately it >sets the frequency with a timing cap. I'll have to somehow vary that >between 750pF and 2000pF. Can also be digital but then with a >granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. > >Doing it with caps and a mux chip or two has its own challenges. The >ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >high. Talking about the a rock and a hard spot here. > >Oh, and cost is not very important. If a diode or two or three are >needed that cost $5 a pop that's ok.Yikes, looks like the Zetex varicaps are gone. Skyworks has a varicap that swings about 80 pF, maybe 50 usable with your swing. Maybe combine that with a few switched fixed caps? Maybe use some big grungy power diode that wasn't born to be a varicap? There must be a way to do your function without needing a variable capacitor. What switcher chip are you using? Now you've got me thinking about varicaps in series with fixed caps, with the diodes sometimes working as varicaps and sometimes forward conducting like PIN diodes. Could get messy to program. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 18:53:38 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Mon, 22 Oct 2012 14:22:49 -0700, Joerg wrote: > >> Folks, >> >> Looked at Digikey and some others. Where are those huge capacitance >> varicaps? The ones with several hundred pF of range for AM radios. All >> gone lalaland by now? >> >> Even the ones I found in the 100pF range are either obsolete or not >> recommended for new designs. >> >> What I am trying to do: I need to control a switcher chip in frequency >> because I've got a very resonant load to deal with. Unfortunately it >> sets the frequency with a timing cap. I'll have to somehow vary that >> between 750pF and 2000pF. Can also be digital but then with a >> granularity of 5pF. The sawtooth voltage across it is 2.5Vpp. >> >> Doing it with caps and a mux chip or two has its own challenges. The >> ADG-series from AD is around 11pF per pin, otherwise their Rdson is too >> high. Talking about the a rock and a hard spot here. >> >> Oh, and cost is not very important. If a diode or two or three are >> needed that cost $5 a pop that's ok. > >OK. This is obscenely bizarre, and any design and implementation details >are left to your poor tortured brain: > >You can make a gyrator to turn a capacitance into an inductance via the >good graces of an op-amp. The effective inductance depends on the >capacitance and the gain. > >So -- consider yourself challenged to make a similar gizmo that either >takes a capacitance and multiplies it by a variable amount, based on a >gain that you can adjust, or that takes an inductance in a "gyrator" >circuit that presents a capacitive load (and make it stable, heh heh heh).Start with the biggest cap value needed, 2000 pF. Connect one end of that to chip terminal A, the one that expects a cap to ground. Now pick off the voltage from A, run through an MDAC, buffer that, and drive the low end of the cap. With mdac gain = 0, A sees the full 2000 pF. With MDAC gain = 0.999, it sees 2 pF. Gain 0.5 nets 1000 pF. Like that. But the buffer does have to handle all the AC current in the cap. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
