On Mon, 22 Oct 2012 18:58:22 -0500, Tim Wescott wrote:> 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.Ooh Ooh! Not a Miller capacitance! You want an Anti-Miller capacitance, because then your amplifier output voltage range can be the same as the input voltage range! i_in = (Vin - Vo) * C * s Vo = K * Vin i_in = Vin * (1 - K) * C * s So, the effective capacitance at Vin is (1 - K) * C. Use a variable-gain amplifier, and don't blame me if you go insane before you get it to remain stable under all operating conditions. C || .-----||------. | || | | | | | Vin | |\ | | | \ | o-------o----|K >-----' Vo | / i_in --> |/ (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) -- 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
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 Oct 22, 8:06=A0pm, John Larkin <jlar...@highlandtechnology.com> wrote:> On Mon, 22 Oct 2012 14:22:49 -0700, 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. > > 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 =A0 =A0 =A0 =A0 Highland Technology, Inc > > jlarkin at highlandtechnology dot comhttp://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 =A0 acquisition and simulation- Hide quot=ed text -> > - Show quoted text -Hey, maybe some cheap photodiode could be used as a varicap? (I have no ideas as to the cheap part.) George H.
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 17:21:01 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Oct 22, 8:06�pm, John Larkin <jlar...@highlandtechnology.com> >wrote: >> On Mon, 22 Oct 2012 14:22:49 -0700, 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. >> >> 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 comhttp://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- Hide quoted text - >> >> - Show quoted text - > >Hey, maybe some cheap photodiode could be used as a varicap? >(I have no ideas as to the cheap part.) > >George H.Good idea. Phil might know of a large-area part. -- 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
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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20122012-10-22
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?>>>> 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.>> 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.>> 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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20122012-10-22
On 10/22/2012 3:31 PM, Joerg wrote:> Nico Coesel wrote: >> 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 >> >> How about using a bad quality ceramic capacitor? >> > > They don't make Z5U this small. Only for larger capacitors. >And the problem with using an alternator diode instead is...? -- Many thanks, Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss: http://www.tinaja.com/whtnu.xml email: don@tinaja.com Please visit my GURU's LAIR web site at http://www.tinaja.com
Reply by ●October 22, 20122012-10-22
Tim Wescott wrote:> On Mon, 22 Oct 2012 18:58:22 -0500, Tim Wescott wrote: > >> 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).Lasse had suggested something similar but I am dealing with a sawtooth and a brutal reset pulse.>> 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. >>About 25V is the highest I can provide, else it'll be more complicated than my mux is now.>> I get my name on the patent if it works. > > Ooh Ooh! Not a Miller capacitance! You want an Anti-Miller capacitance, > because then your amplifier output voltage range can be the same as the > input voltage range! > > i_in = (Vin - Vo) * C * s > > Vo = K * Vin > > i_in = Vin * (1 - K) * C * s > > So, the effective capacitance at Vin is (1 - K) * C. Use a variable-gain > amplifier, and don't blame me if you go insane before you get it to > remain stable under all operating conditions. > > C > > || > .-----||------. > | || | > | | > | | > Vin | |\ | > | | \ | > o-------o----|K >-----' Vo > | / > i_in --> |/ > (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) >Sounds scary :-) Plus it would need to stomach a 50mA reset pulse and if the behavior ain't 100% capacitive the switcher chip gets upset. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20122012-10-22
On Mon, 22 Oct 2012 16:38:28 -0700, Joerg <invalid@invalid.invalid> wrote:>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".I receive all ON product notices (spec, process, material, source, discontinuance, resurection etc etc etc) changes, and there's nothing on SVC383. Did you ask the ON recommended vendor - Rochester Electronics? They also have no stock, but they shouldn't give you a phoney reason for this. Digikey still lists their supplier for SVC389 as Sanyo Semiconductor (USA), so their info for both parts may be dated. Did you actually ask Digikey? Looks like the usual minimum purchase of a reel, for items not recently stocked. The issue is more likely a Sanyo-based supply chain one - strictly sales related. I'd sniff around previously reliable Sanyo distribution outlets (not excluding Digikey). SVC389 was also an RS Component line item. RL
Reply by ●October 22, 20122012-10-22
Joerg <invalid@invalid.invalid> wrote:> >>> 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.Sounds like all the fun went away from your job :-) I re-instated my etching setup for testing chips. OTOH fast turnaround services aren't that expensive. In most cases some undocumented features get revealed. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
Reply by ●October 22, 20122012-10-22
Joerg wrote:> Les Cargill wrote: >> 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. >>> >> >> >> Could something like this be a start? >> >> http://www.timeelectronics.com/products/category/decade-boxes/1071-capacitance-box >> >> >> Gets pretty close - 10 pF resolution if I read that right. >> > > Then we'd have to ship a bearded professor in SMT size with every board, > for the self-alignment routine :-) >Ach! <rends garment>. -- Les Cargill
