Load Capacitance

W

Wayne

Guest
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
 
"Wayne"
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.
Click to expand...
** The specified load C is only relevant to getting the specified frequency
accuracy - along with series or parallel resonance.


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.
Click to expand...

** 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
 
"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:7u3pf5Fg57U1@mid.individual.net...
"Wayne"

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.

** The specified load C is only relevant to getting the specified
frequency accuracy - along with series or parallel resonance.


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 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
Click to expand...

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
 
On Wed, 17 Feb 2010 20:04:16 -0500, Wayne wrote:

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,
Click to expand...
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
 
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:A-ednfKGK4sUK-HWnZ2dnUVZ_s1i4p2d@web-ster.com...
On Wed, 17 Feb 2010 20:04:16 -0500, Wayne wrote:

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,

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
Click to expand...


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
 
"Tim Wescott"

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.
Click to expand...
** 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:

http://www.raltron.com/cust/tools/appnotes/Crystal%20Mini%20Primer%20-%20Part%201.pdf

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
 
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