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FM Modulator Design - RF Cafe Forums

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Post subject: FM Modulator Design Posted: Sun Oct 01, 2006 3:52 pm


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


Post subject: FM Modulator DesignPosted: Tue Oct 03, 2006 6:12 am


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 ""

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)


Post subject: Re: FM Modulator DesignPosted: Sat Oct 14, 2006 4:05 am


Joined: Sun Oct 01, 2006 2:41 pm
Posts: 2
[quote="nubbage"] 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


Post subject: VCXOPosted: Thu Oct 19, 2006 4:00 pm


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
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!

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