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

Bob Camp kb8tq at n1k.org
Fri Sep 5 21:00:57 EDT 2014


Oddly enough (and yes it is odd) you can modulate an oscillator well outside the crystal’s bandwidth. The bigger issue is that the EFC does not pull the crystal very far on a normal OCXO. The FM modulation index drops to very small numbers pretty fast as you go up in modulation frequency. 

You typically only worry about modulation sidebands that are above the phase noise floor. Since phase modulation sidebands go down as 1/Fmod on an FM modulator (for small modulation index) they get pretty low pretty fast. 

If your OCXO has an EFC range of 0.1 ppm at 10 MHz, it will swing 1 Hz p-p (+/- 0.5 Hz) for the full EFC voltage. At 5 Hz, you have a modulation index of 0.1. Of course if you are multiplying to 10 GHz, the index could be quite large. This gets back to the “this all depends on what you are doing”. 

If your EFC is 5V, a reasonably quiet signal would have noise below 0.5 mV. That’s already 80 db down. A very quiet supply should be in the < 5 nV / sqrt(Hz) range.  That would put the noise down 180 db. 

It’s unlikely that your OCXO has a phase noise spec of -180 dbc / Hz at 10 Hz. We may already be done …

To bring all the numbers together:

At 1 Hz the modulation will do a sideband X db down at your desired frequency.

You will drop 20 db by the time you get to 10 Hz simply due to the 1/F FM->PM.

You are 80 or 180 db down (depending on your EFC) beyond that. 

So you are at X - 20 - 80  or 100 db below your 1Hz sideband. (noisy EFC)

Chances are (unless you are at microwaves), you are well below the phase noise floor already. 

You are X - 20 - 180 or 200 db below the 1 Hz sideband (quiet EFC). 

Even at microwaves, you are below the phase noise floor. Most likely you are below it by 80 db.

Bottom line - it’s not all that hard to get a quiet enough EFC voltage. 


On Sep 5, 2014, at 7:32 PM, Alex Pummer <alex at pcscons.com> wrote:

> 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
> 73
> Alex
> 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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