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    Trimmer / Variable capacitors

    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.

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    Re: Trimmer / Variable capacitors

    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


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    Re: Trimmer / Variable capacitors

    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?
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



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    Re: Trimmer / Variable capacitors

    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!



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    Re: Trimmer / Variable capacitors

    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
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



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    Re: Trimmer / Variable capacitors

    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?



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    Re: Trimmer / Variable capacitors

    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?
    A good design question lists your overall requirements™ The best question deserves a better answer. ™
    ... Tony Stewart EE since 1975



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    Re: Trimmer / Variable capacitors

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

    http://antiqueradios.com/albums/temp...itorscheme.jpg

    http://braincambre500.freeservers.co...0capacitor.htm

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

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

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

    http://www.hpfriedrichs.com/rr-cdrom.htm

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

    http://motivationtolearn.org/wordpre...1130#more-1130

    Photograph of someone's homemade tuning capacitor:




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    Re: Trimmer / Variable capacitors

    The Air trimmers are my choice. I will try one of thoses : http://www.surplussales.com/Variable...VarTrimm5.html and maybe try to build the second example given by BradtheRad ( http://braincambre500.freeservers.co...0capacitor.htm ). What do you mean, the Q Factor of this home-made capacitor can be?

    thanks a lot for your help.



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