Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
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My Hobby Website:
FM Modulator Design - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
platforms dominating public commenting venues. RF Cafe Forums began sometime around
August of 2003 and was quite well-attended for many years. By 2010, Facebook and
Twitter were overwhelmingly dominating online personal interaction, and RF Cafe
Forums activity dropped off precipitously. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: FM Modulator Design Posted: Sun Oct 01, 2006
Joined: Sun Oct 01, 2006 2:41 pm
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?.
Post subject: FM Modulator DesignPosted: Tue Oct 03, 2006 6:12 am
Joined: Fri Feb 17, 2006 12:07 pm
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)
Re: FM Modulator DesignPosted: Sat Oct 14, 2006 4:05 am
Joined: Sun Oct 01, 2006 2:41 pm
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
Post subject: VCXOPosted: Thu
Oct 19, 2006 4:00 pm
Joined: Wed Feb 22,
2006 3:51 pm
One readily-available reference
is the ARRL Handbook (almost any recent edition).
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)