John Larkin 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. > > Yikes, looks like the Zetex varicaps are gone. >Yup :-(> Skyworks has a varicap that swings about 80 pF, maybe 50 usable with > your swing. Maybe combine that with a few switched fixed caps? >I thought about massively paralling but then the low end gets tough, and it starts to consume real estate. Got it done with a mux now.> Maybe use some big grungy power diode that wasn't born to be a > varicap? >Probably, as long as it's not a big fat one. But the issue is that I'd need to be able to simulate because this one has to wrap up soon.> There must be a way to do your function without needing a variable > capacitor. What switcher chip are you using? >LTC3721-1.> 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. >Also, one has one watch EMI with such a long string. Anything that is forward conducting will pick up stuff, the nastier ones are usually GSM cell phones. My wife just cooked homemade Sauerbraten, with dumplings and all the trimmings. Yummmmm! -- 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 10/22/2012 8:55 PM, Joerg wrote:> legg wrote: >> 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? >> > > It's going to present a capacitance to it which can be varied via a > higher voltage. Works nicely, I've got it running on LTSPice with an old > Siemens varicap model. But ... only Avnet Europe and Wuhan have stock left. >BB201s are 95 pF at 1 volt, and come in duals, so you can get 200 pF-ish out of one SOT23. You have to watch out for varactor specs--some headline their maximum capacitance, others their minimum. 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 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●October 22, 20122012-10-22
On 10/22/2012 3:29 PM, Joerg wrote:> miso wrote: >> On 10/22/2012 2:22 PM, 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. >>> >> >> There are plenty of hoarders that have those caps. I picked them up >> years ago when I figured they would go extinct. They same goes for 10 >> turn pots of the panel variety with the dials on them. (Good old Mike >> Quinn in San Leadro by the Oakland airport would save panels with those >> pots and junk the rest of the product.] Most radios are synthesized >> these days, so there is no market for those caps. >> > > Yeah, I know but I was hoping there'd be at least one other market. > Seems like there ain't. > > >> 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. >Is this pin where you see the sawtooth? Seems to me with a high impedance current source, you can steal the current. Now a triangle waveform is a different case. So you have tried a high impedance current source?
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 18:02:03 -0700, Joerg <invalid@invalid.invalid> wrote:>krw@att.bizzzzzzzzzzzz wrote: >> 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. >> > >What's nuts about this?Single source because you want a crappy behavioral model to make you feel good. There *are* other vendors out there, with perfectly good products.>>>>> 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? >> > >There will be a loop going. That needs a tad more resources than just a >timer.<shrug>> >>> 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. ;-) >> > >Yeah, I usually did get my MIPS so far. But every time I step into a uC >guys cubicle they ask "So you want to eat up the last timer now?" or >something.<shrug> Handle it.>>> 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. > > >Well, I can't spend time bench-testing a new switcher chip that doesn't >have a decent (meaning behavioral) SPICE model. My stuff usually goes >CAD to production these days, with no lab bench time before the >prototypes are done. On this one I only had to bench test the load and >almost got sick when I looked at the results.You think you're alone? Get the manufacturer to guarantee the design.
Reply by ●October 23, 20122012-10-23
Have you looked at Big Fat Schottkies? The 30V 40A types are in the 10nF at zero bias, and obviously will handle the puny 50mA reset pulse. :) Can't say I ever got great results using schottky capacitance, I think their Q is crap or something (which is something to think about when everyone touts "zero reverse recovery losses"!). It also varies rather quickly with voltage, maybe too rapidly for the chip. Well, you can bracket the C range with series/parallel caps anyway, and maybe a higher voltage schottky, or a beefy junction diode, will have a more gentle curve. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "Joerg" <invalid@invalid.invalid> wrote in message news:aelrpnF24rtU1@mid.individual.net...> 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/
Reply by ●October 23, 20122012-10-23
On a sunny day (Mon, 22 Oct 2012 14:22:49 -0700) it happened Joerg <invalid@invalid.invalid> wrote in <aelrpnF24rtU1@mid.individual.net>:>Folks, > >Looked at Digikey and some others. Where are those huge capacitance >varicaps? The ones with several hundred pF of range for AM radios.They are upstairs in the attic here.
Reply by ●October 23, 20122012-10-23
Joerg a �crit :> 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. >Medium size Mosfets? -- Thanks, Fred.
Reply by ●October 23, 20122012-10-23
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.Reading between the lines on the LTC3721-1 datasheet, it seems that the CT pin generates the upward part of the timing ramp with a (roughly) 150uA current source. The downward part of the ramp is from a much stronger current sink. In an earlier post, Miso suggested sinking some current out of the CT pin to reduce the frequency, to which you replied "... the chip immediately gets sea-sick when you do anything DC to that pin." Had you tried *sourcing* a small (a few uA) current into CT to increase the frequency? Regards, Allan
Reply by ●October 23, 20122012-10-23
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.http://www.weisd.com/test/GenericParts_WEISD_view.php?editid1=BB112 Regards, Boris Mohar Got Knock? - see: Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca void _-void-_ in the obvious place
Reply by ●October 23, 20122012-10-23
On Mon, 22 Oct 2012 17:55:55 -0700, Joerg <invalid@invalid.invalid> wrote:>legg wrote: >> 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? >> > >It's going to present a capacitance to it which can be varied via a >higher voltage. Works nicely, I've got it running on LTSPice with an old >Siemens varicap model. But ... only Avnet Europe and Wuhan have stock left.The whole tuning range of the hyperabrupt parts was achieved by a 6-8V span of bias. If the ramp amplitude is a signifigant percentage of this, you risk rectification and other effects. RL
