Large capacitance varicaps, where are thee?

[Joerg]

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.

 

[George Herold]

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.

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.

 

[miso]

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.

 

[[email protected]]

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.

possible to force you own triangle in?

-Lasse

 

[Nico Coesel]

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?

 

[Joerg]

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.

 

[Joerg]

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.

 

[Joerg]

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.

 

[Joerg]

How about using a bad quality ceramic capacitor?

They don't make Z5U this small. Only for larger capacitors.

 

[Les Cargill]

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.

 

[[email protected]]

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.

this too crazy? http://www.falstad.com/circuit/e-capmult.html

-Lasse

 

[Joerg]

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.

 

[Joerg]

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

 

[[email protected]]

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).

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.

 

[[email protected]]

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.

I'm just guessing, diode across R3 ?

-Lasse

 

[Joerg]

I'm just guessing, diode across R3 ?

No, there is more or less a wire.

 

[George Herold]

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?

George H.

 

[[email protected]]

No, there is more or less a wire.

I meant add diode so discharge is fast


-Lasse

 

[Joerg]

http://www.angelfire.com/electronic2/index1/VariableCap.html

About half way down.


No kiddin? ;-)


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.

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.
Analog dudes aren't really entitled to those.

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.

 

[Joerg]

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
http://www.digikey.com/product-detail/en/SVC383T-TL-E/869-1202-2-ND/2165536

Quote "Obsolete item; call Digi-Key for more information".