[time-nuts] Thunderbolt mounting

[Said Jackson]

Not small when you are trying to achieve a -160dBc noise floor. Then even a small fan can show up as a massive -120dbc or higher spur easily and ruin your day due to the microphonics of the crystal. Some time ago there was a discussion here about how entering a room measurably changed an ocxo frequency due to the steps causing vibration..

So we stabilize the temp but ruin our phase noise. Doesn't make sense to me.

We deal with big fans all the time, viz. Turboprop engines running at up to 2000rpm. Generates nasty massive spurs below 100hz.

Putting an ocxo in front of a fan is one of the worst things one can do, even if it is hiding in an enclosure such as a thunderbolt. It probably raises your ADEV very easily measurable at the fan turn on/off frequency as well.

Btw, the math of what -160dBc means in terms of power below carrier is very impressive. getting close to the thermal noise of a 50 ohm resistor.

Bye,
Said

Sent from my iPad

On Jul 12, 2012, at 21:38, Chris Albertson <albertson.chris at gmail.com> wrote:

> On Thu, Jul 12, 2012 at 6:56 PM, Said Jackson <saidjack at aol.com> wrote:
> 
>> Sorry guys, but using a fan on a thunderbolt is nuts. Not time nuts.
>> 
>> First you get ADEV humps as discussed here. Then the fan vibrations will
>> show up as massive spurs in the phase noise plots. Then the fan's
>> commutator emi will go everywhere. Then the fan may modulate the supply
>> voltage going into the thunderbolt if the same supply is used -not good.
>> 
>> 
> Have you actually tried using a fan and measured EMI and the supply voltage
> modulation.  I'm curious how much the fan pulled down the power supply.
> 
> Those things could happen but at what level?  Is the effect even
> measurable?   I think it might be like standing of a single sheet of paper
> so you are taller and can see farther.  Yes in theory it works but the
> effect is small.
> 
> Chris Albertson
> Redondo Beach, California
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