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Trimmer / Variable capacitors

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hermannhelmholz

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Greatings,

I need a variable capacitor / trimmer. I tried many different types and they where all of low quality. Do you guys know of a company that produce trimmer for around 20MHz approx. that has a high Q / low losses? I need one from 10 to 100pF and one from 50pf to 200pF that have linear adjustement. Had anyone ever try the trimmers of Sprague-Goodman :

http://www.spraguegoodman.com/pdfs/500.pdf

http://spraguegoodman.com/pdfs/402.pdf

thanks.
 

The mica type (your first listed) will be the highest Q types you can get. The only other quality type are "Johansen", these use air dielectric, so won't be available in your capacities?
Frank
 
Those parts bring back old memories of using them in the mid 70'~80's so I looked them up.
Johanson, same range 1~30pF air tuner gold plated.
Sprague Goodman make them if you can afford them... $5~$25 ea.
If you need $1 solution, you need to redesign your requirements. Is this for a XTAL filter, VCXO or what?
 

Is is for some sort of oscillator. What do you think a can expect for a Q-factor at 20 MHz for the mica and for the air cap?

thanks!
 

More important what Q do you need for this oscillator?
What price?
What pulling range?
What temp stability?
What initial error at room temp?

I estimate design and Q with cost as follows; at 10MHz

Xtal 10K to 1million ( $0.25 ~$100 )
Ceramic 1K ($0.25)
LC 100
 

Thanks for your reply,

I need:

Q > 400
price < 20$
pulling range : one from 10 to 100pF and one from 50pf to 200pF
temp stability : I will use it at room temperatur
What initial error at room temp : resonable

regards

- - - Updated - - -

There are Crystal Oscillators (XTAL) that are variable?
 

Yes of course all oscillators are variable with voltage controlled capacitor diode and other methods.
I was reporting Q of the Crystal not the tuning cap. and error in ppm of each part. used in an osc .. read and answer again pls

You certainly do not need high Q caps to tune the motional capacitance of a XTAL oscillator... Can you post schematic or partial?
 

Concepts for making your own tuning capacitor, in case the commercial ones are too pricey:



**broken link removed**

http://www.crystalradio.net/beginners3/cap.shtml

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

'Tuning capacitor C1 is a sandwich composed of three metal disks separated by two CD-ROMS.'



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

http://motivationtolearn.org/wordpress/?p=1130#more-1130

Photograph of someone's homemade tuning capacitor:

90_1342837308.jpg
 

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