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Load Capacitance

Hello Everyone,

I have a couple of surplus crystals, 10.7 MHz. Is there a way to find out the Load Capacitance for them ? The people I bought them from don't know.

What I am doing is a "Can I make this work" project. I have a stable audio oscillator, a working varactor and an iffy collpits, crystal, RF oscillator, 10.7 MHz. I am trying to come up with a working Frequency Modulated Oscillator.

I am trying to get the RF Osc. more stable. The project doesn't want to work yet, getting the RF Osc. working reliably seemed like a good place to start.

You seem to be a group of helpful people. Would you, please, try to help me out ?

You help will be Gratefully Appreciated

Thank You,

--

Wayne
Reply to
Wayne
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"Wayne"

** The specified load C is only relevant to getting the specified frequency accuracy - along with series or parallel resonance.
** I assume you intended to "pull" the crystal with the varactor diode.

Not very much frequency change is possible that way - a rule of thumb is expect 20ppm per pF.

Plenty of info on this topic on the web.

.... Phil

Reply to
Phil Allison

Thanks for the reply Phil,

That has been my intention. Like I say, all I intend is make it work. I will do More research, web wise,and through the books I do have.

Again, Thanks,

--

Wayne
Reply to
Wayne

How much frequency modulated?

If you want something you can hear on a broadcast FM receiver -- you can't get there from here, at least not with a crystal oscillator. If you want to generate narrow-band FM, like for amateur radio or commercial comms, then that's generally done with an oscillator feeding a reactance modulator, with the correct preemphasis on the audio so that it comes out FM on the other side.

I believe that at one time broadcast-quality FM was generated with an LC oscillator, phase locked to a crystal source with a low-bandwidth loop to keep the carrier frequency correct, and modulated with the audio. Nowadays it's just as likely to be all done with direct digital synthesis, but it's not something I've kept up with.

--
www.wescottdesign.com
Reply to
Tim Wescott

Thanks Tim,

I am not that experienced, less so with RF. I am trying to take some building blocks, put them together and see if they work. I am not trying to transmit symphony quality sound across the country. :-) I am trying to see my freq. meter, or scope deviate like I think they should. Like Phil says, I shouldn't expect much..

Thank you for some more insight,

Wayne

Reply to
Wayne

"Tim Wescott"

** Nonsense.

A suitable varactor diode circuit will pull a 10.7 MHz ( fundamental) crystal by about 5 kHz.

Sees pages 3 to 5 of this link for typical pulling curves:

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Then one multiplies the frequency up to the FM broadcast band using two stages of tripling or 9 times.

THEN the frequency shift is also 9 times greater.

Be VERY audible on any FM receiver.

.... Phil

Reply to
Phil Allison

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