[time-nuts] OCXO Voltage Input? (Bob Camp)

Alex Pummer alex at pcscons.com
Fri Sep 5 19:32:23 EDT 2014

it is not so easy to FM modulate a crystal oscillator, since the crystal 
has a high Q therefore the modulation bandwidth of a crystal oscillator 
is very narrow example: Q = F/dF -> df = F/Q if F = 10MHz,  Q = 60,000 
dF = 166Hz
On 9/5/2014 1:10 PM, Hal Murray wrote:
> dan at irtelemetrics.com said:
>> If I had 10Mhz or some other high frequency on the EFC line, would a typical
>> OCXO respond to that?
> Some VCXOs actually specify their bandwidth.  High audio is sometimes useful.
>   I haven't seen anything beyond that, but I'm just listening to discussions
> like this one.  There could well be applications that use a higher frequency.
> One application is correcting for mechanical vibrations.  This is interesting
> in radar used on helicopters.  (They do Doppler filtering to remove clutter.
> The lower speed of objects that can get through the filter depends on the
> clock stability.)
> PCs often FM modulate their clocks.  It's a hack to get past the FCC EMI
> requirements.  It spreads a spike in the frequency domain into a blob with a
> lower peak.  I think 30 KHz is typical.  The PCI specs were tweaked to allow
> this so they probably say something about the legal frequency limit.
> PCs probably don't use expensive OCXOs, but that technology might get used in
> other applications.
> How do FM modulators work?

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