varicaps?

I wonder if it's practical to use a MOSFET's Coss as a varicap. I haven't seen it done.

-----------------------

The on-line catalogs are full of them.

The world of RF lives on them.

..... Phil

-----------------------

Yes

-- Kevin Aylward

Standard technique in ASIC design

-- Kevin Aylward

I can find Skyworks singles in the nasty little SC79 package, or even smaller. Not many duals. The MMBVs are mostly obsolete.

Cool. I should have googled it first, there's a few examples out there.

Sure is. But you only get a few volts range before it turns on, and it's controlled by Vds, so turn-on is obviously a bit of a problem. Still, that leaves you will the full negative-gate bias range. And it's very stable with respect to Vgs, AFAIK, good news for linearity.

Tim

There are pots of low-pF ones on Digikey. The 1000-pFish ones (MVAM108 etc.) are long gone, and the tens-to-hundreds range is declining. My fave MV209 is a distant memory, but BB201s are still in stock.

Cheers

Phil Hobbs

I need a small range, maybe a change of 0.2 pF over 5 volts bias swing. The c-b junction of some small transistor might work. I think varicaps have special doping profiles to tune the c-v curve, but I'll be working at fairly high voltages, 10V maybe, and that looks to be out of the hyperabrupt zone. I'd like a constant voltage-frequency curve, but that ain't going to happen.

I'm tuning a 150 MHz LC oscillator a few thousand PPM, part of a bizarre digital PLL.

Looks like I can get some low-capacitance Skyworks singles in SC79.

Thank you. You are always so friendly and helpful.

onsdag den 1. april 2020 kl. 23.01.15 UTC+2 skrev John Larkin:

According to my Boonton, a BFT25 c-b junction only goes from about 0.7 pF to around 0.65 from 0 to 6 volts. There's probably a bit of fixture capacitance too. Not a very good varicap.

I want a delta-c around 0.15 pF roughly, maybe 0.2. My dac+opamp could go to 20 volts.

A power Zener will have larger area and a good range of reverse-bias with low leakage. Realistically, though, probably ANY diode other than a RF type will have significant capacitance and likely can do this.

Because Rbb can be high in a 'small transistor', I'd try a cheap zener for tuning a high-Q oscillator.

Up to about 30pf they'll likely remain. Skyworks and such. Those with large capacitances lost their market.

You won't get much capacitance range out of those. For really large capacitances you could use Z5U ceramic caps. They lose a lot of capacitance when approaching their max DC voltage so can act as a poor man's varicap.

-------------

News to me.

... Phil

My car doesn't even receive AM, only FM, Sirius XM and Slacker Internet radio. Oh, and bluetooth from your phone. But no AM radio. I think they could do that 100% in software if they wanted to. It's only 1600 kHz max frequency.

--

Can only think in 1s and 0s.

..... Phil

This guy is constantly screaming at others, I can only imagine how much he screams at himself. I feel sorry for you. You have my sympathy and condolences.

Am 02.04.20 um 02:41 schrieb Phil Allison:

Here in Europe it has disappeared without a trace.

The masses moved to FM 40 years ago and now to a digital mode which is harder to sell to the people.

I think I have posted here already the pointer to the clip where they blew up the towers of our local 1.6 MW monster on 1422 KHz. Europawelle Saar, it used to be an easy victim for my first detector receiver when I was a kid.

Gerhard