Large capacitance varicaps, where are thee?

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.

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

The difficulty will be in the compliance of the current source used for bleeding.

possible to force you own triangle in?

-Lasse

How about using a bad quality ceramic capacitor?

Yup :-(

They don't make Z5U this small. Paralleling gets to be tough for space reasons because I'd need a boatload.

Yeah, I know but I was hoping there'd be at least one other market. Seems like there ain't.

That won't be a problem but the chip immediately gets sea-sick when you do anything DC to that pin.

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.

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

Could something like this be a start?

Gets pretty close - 10 pF resolution if I read that right.

this too crazy?

-Lasse

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.

Then we'd have to ship a bearded professor in SMT size with every board, for the self-alignment routine :-)

About half way down.

No kiddin? ;-)

Use a regulator with an external clock. That's what we do (all of our switchers are synchronized).

A regulator with an external clock shouldn't be an issue, then.

I'm just guessing, diode across R3 ?

-Lasse

No, there is more or less a wire.

OK so how 'bout a few Z5U in series?

George H.

I meant add diode so discharge is fast

-Lasse

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.

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.

Thanks, but I've been through that as well. Here is the bad news:

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