Forums

Large capacitance varicaps, where are thee?

Started by Joerg October 22, 2012
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/
On Oct 22, 5:23=A0pm, 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.) George H.
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. 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.
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? -Lasse
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? -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... nico@nctdevpuntnl (punt=.) --------------------------------------------------------------
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. -- Regards, Joerg http://www.analogconsultants.com/
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. -- Regards, Joerg http://www.analogconsultants.com/
langwadt@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/
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. -- Regards, Joerg http://www.analogconsultants.com/
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. -- Les Cargill