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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Sep 6 12:52:35 EDT 2014

Hi Bob,

Agreed. I often find that modulations eats your margin out.

PWM is interesting in this regard. PWM has the property that the lowest 
frequency has the highest amplitude and the overtones then decay with 
1/f from that. For a given clock rate, as you add a bit of PWM 
precision, you half the PWM repetition rate and thus move the frequency 
down... where we are more sensitive to the modulation it causes, and the 
1/f slope of the oscillator does not help.

I designed a PWM-like signal that has reversed PWM spectrum so that the 
highest frequency has the strongest amplitude. The 1/f of the oscillator 
integration makes the modulation flat among the different bits and much 
easier to handle phase-noise wide.

Another approach is sigma-delta style modulation, which noises out the 
amplitude. Higher-degree sigma-delta needs to avoid idle-tones for 
optimum result.

Thus, paying attention to these details pays of with simplifying the 
effort to achieve good phase-noise properties.

There is more dangers that can occur in PWM-space, but this should be 
enough of a starting-point.


On 09/06/2014 01:39 PM, Bob Camp wrote:
> Hi
> Yes indeed, as you go below 1 Hz (or 1 radian/sec) all the things that “help” you roll off wise now hurt you. If you are worried about sidebands inside 1 Hz, you need to change a sign here and there. The only thing that saves you is that the noise floor is now coming up pretty fast.
> If you modulate a crystal oscillator, the loaded frequency of the crystal is changed to accomplish the modulation. When your FM swings 100 Hz high, your crystal is tuned 100 Hz high. When your modulation swings 100 Hz low, your crystal is tuned 100 Hz low. The Q has no impact in this case. No I did not believe it worked that way until I did it …. Since then I’ve built a *lot* of VCXO’s with modulation bandwidths >> than their crystal Q bandwidths. The biggest problem comes from crystal spurs rather than crystal Q.
> Bob
> On Sep 6, 2014, at 6:09 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>> Bob,
>> On 09/06/2014 03:00 AM, Bob Camp wrote:
>>> Hi
>>> 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.
>> Since the oscillator integrate frequency into phase, you have a 1/(2*pi*f) factor. The typical LaPlace model for an oscillator is Ko/s, where Ko is the input sensitivity of the oscillator.
>> A more complete model needs to include the Q of the crystal, naturally, unless you are "in-band" of that Q where it has less drastic properties.
>>> Bottom line - it’s not all that hard to get a quiet enough EFC voltage.
>> Agreed.
>> I've found that thinking about systematic noises of low frequency (i.e. comparator frequency and overtones) as well as loop dynamics is what one should think about. Lack of DAC resolution hurts.
>> Cheers,
>> Magnus
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