NTC capacitors

Not many people still make these.

Everything else in the world seems to have a positive TC: pc boards, varicaps, inductors.

I want a 2 pF N3300, and probably won't find one.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin
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A quick search found these guys.

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Not clear what the ranges are.

George H.

Reply to
George Herold

I emailed them earlier this morning. No answer yet.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

Oh lordy. Varicap and a control circuit, if you're desperate.

I wonder if someone will pick up the ball.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

Varicaps have big positive TCs that vary with the voltage-capacitance, which I guess some sort of polynomial could mostly fix. A nice little NTC cap would be nice.

There are lots of N3300 caps around, but they tend to be huge multi-kilivolt things for some reason.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

AFAIK only some larger ones are still available non-custom in N3300. Even Vishay Cera-Mite only goes down to 33pF.

Is there no other way to do the circuit?

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Regards, Joerg 

http://www.analogconsultants.com/
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Joerg

I always check DigiKey when someone complains about stuff not being available. I found a bunch of 40kV doorknob caps, but figured that advising you of this fact wouldn't be funny enough.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

It's a 600 MHz oscillator that I want to phase-lock with a very small varicap. The frequency tempco is around +50 PPM/K, mostly from FR4 capacitance, which has a horrible PTC.

If the varicap has enough pull range to do the phase lock over the uncompensated temp range, I have to have a bigger varicap, and the varicap itself has a big positive TC that varies with voltage. What a mess.

An NTC cap would be great, but it probably doesn't exist.

We use some 18 pF N750 caps in another place, and we have to special order them by the reel.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

How about using a bigger one in series instead of shunt?

Cheers

Phil Hobbs

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Phil Hobbs

2pF is the domain of roll your own, eg an offcut of wire. What insulator expands with temp?

NT

Reply to
tabbypurr

** LOL - my thoughts exactly.

Used to be called a "gimmick" capacitor.

.... Phil

Reply to
Phil Allison

but still maintains constant epsilon-r with temperature.

Reply to
Ralph Barone

FR4 is indeed horrible in TC and I assume other PCB materials would be too costly here.

I never tried that but you might want to look into using MOSFETs as variable capacitors (the gate to channel capacitance):

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Regards, Joerg 

http://www.analogconsultants.com/
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Joerg

If you're phase locking to an external reference then so what if your circuit has a tempco- all you should be concerned about is that it stays within hold in.

Reply to
bloggs.fredbloggs.fred

That's the issue, staying in lock. To keep the phase noise down, I want a very high-Q resonator and a narrow DAC+varicap pull range. If the varicap covers both the optimum pull range and another few thousand PPM for temperature changes, it's a bigger varicap and makes more noise and contributes its own big, variable positive TC to the existing positive TCs.

I can't explain the details in public, partly because it's proprietary and partly because it's too complex. I was hoping that some RF dudes might know more about NTC caps.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

I wrote a little Windows program to explore padding NTC caps, like Tim's thing but a different cap arrangement.

Tim's:

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Mine:

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I want minimal added capacitance and I need about -2.5 fF/K to compensate my FR4 capacitance. Given that, the programs are about the same... no additive padding is desired.

My stocked 18pF N750 does that, but, padded, adds 7.5 pF to my resonator, which pulls it down over 100 MHz, which isn't good. That's the problen with working at 600 MHz; everything gets so danged sensitive.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

What's the oscillator configuration? If it's a Colpitts, you can use an overtone selection network (series LC) as one of the divider capacitors, and use the inductor's positive tempco to give you an effectively negative capacitor tempco.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
Principal Consultant 
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Phil Hobbs

FR4 expands with temp, which suggests a negative capacitance TC, but a real PC board has a positive TC around 1000 PPM/K.

I guess an FR4 cap has a thickness that expands, reducing C, but an area that expands faster, increasing C. But I think the material Er dominates.

Hey, on a multilayer board I could include a test capacitance made out of FR4. That could be the sensor for compensating the parasitic FR4 capacitance, or the sensor for ovenizing that section of the board.

Some plastic caps seem to have positive TC, some negative. Teflon undergoes a phase change right at room temp, so its TC is insane.

Water has a serious NTC. I need a water capacitor!

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

It is a grounded-base Colpitts. I'll look into your idea. Thanks.

I looked into putting an inductor directly across the resonator, which raises the frequency and cancels the gross FR4 capacitance, but that adds yet another negative frequency TC.

This is an insanely complex detail of an insanely complex instrument. I'll show it to you some day.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Did you consider "tampering" with that mechanism by slotting the FR4 cap? A brute force method would be to pre-heat that area and have a uC or whatever hold it at that elevated temperature. Just the slotted cap area. This should result in a tempco near zero.

Servo is another method but makes the already complex board even more complex. I'd just regulate temperature because that only requires some dissipative element, an NTC and a couple of vacant port pins on the uC. Plus maybe a transistor but possibly the 10-20mA a uC can sink might be enough. McGyver would probably PWM a FET with the Rdson in the right range so it acts as the controlling and dissipative element at the same time.

Problem is, most of us RF dudes know and have built with controlled-tempco caps but this was when we still had all our hair and all our teeth. Meantime the market for this stuff has shriveled so much that you can't easily buy small caps like that anymore. Unless you are willing to pay a small ceramics place for a custom run.

Look at the bright side. At least you have a genuine water elevator.

Things could be worse. I recently started brewing beer again and realized (what I should have known all along) that yeast triggers a hefty positive tempco process. Starts fermenting -> raises wort temperature -> fermentation gets more wild -> wort temperature rises even more -> fermentation goes berserk ... had to rig a fridge to keep it at 68F.

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Regards, Joerg 

http://www.analogconsultants.com/
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Joerg

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