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FM Modulator Design - RF Cafe Forums
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Najafi
Post subject: FM Modulator Design Posted: Sun Oct 01, 2006
3:52 pm
Lieutenant
Joined: Sun Oct 01, 2006 2:41 pm
Posts: 2
Hi all,
I am new here, I am trying to design an FM modulator
at 12MHz that could gives Deviation of +/-4KHz and distortion not more
that 2%. I tried direct modulation by using Varactor in parallel or
series Crystal. but I did not get more that 1 KHz. I tried 4046 with
Divider N =1 but did not succeeded. Any one have good idea might help.
Also it is very strange that most of RF perctical books avoid even discussing
this topic. can any one give me a good mathmatical design procedure.
Also I noticed some circuits in which used a parallel LC resonent
in parallel to crystal. I tried that too but it was not stabe, I do
not know why?.
Thank you
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nubbage
Post subject: FM Modulator DesignPosted: Tue Oct 03, 2006 6:12 am
General
Joined: Fri Feb 17, 2006 12:07 pm
Posts:
218
Location: London UK
The problem arises because a crystal
has a very high Q, and the capacitance in the equivalent circuit cannot
be easily modified by a varactor without seriously degrading the frequency
stability. Typically only a few hundred Hz deviation can be obtained.
There are some VXCO circuits that have overcome this problem, although
some trade-off occurs with average crystal stability. Do a search under
VXCO and you should find some circuits. You could also try the web site
"allaboutcircuits.com"
Another approach that has been very successful
is to use ceramic resonators at 12MHz in place of a conventional quartz
crystal. Most of the useful and detailed circuits for this requirement
can be found in Amateur Radio publications such as QST, Rad Com(UK)
and Dubus (Germany)
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Najafi
Post subject:
Re: FM Modulator DesignPosted: Sat Oct 14, 2006 4:05 am
Lieutenant
Joined: Sun Oct 01, 2006 2:41 pm
Posts: 2
[quote="nubbage"]......place
of a conventional quartz crystal. Most of the useful and detailed circuits
for this requirement can be found in Amateur Radio publications such
as QST, Rad Com(UK) and Dubus (Germany)[/quote]
any specific reference
please
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fred47
Post subject: VCXOPosted: Thu
Oct 19, 2006 4:00 pm
General
Joined: Wed Feb 22,
2006 3:51 pm
Posts: 104
Hi!
One readily-available reference
is the ARRL Handbook (almost any recent edition).
Most designers
don't try to modulate a crystal oscillator's frequency directly - even
if you can get enough deviation, there will be a (rather low) upper
limit to the modulating (baseband) frequency that you can use.
So what to do? The classic method is to use a varactor-tuned LC
oscillator in a phase-locked loop, with the VCO output divided down
to a relatively low frequency for comparison to a (stable, crystal-derived)
local reference.
Good Luck!
--Fred
Posted 11/12/2012
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