[time-nuts] State of the art of crystal oscillator measurements

Bob Camp kb8tq at n1k.org
Thu Aug 11 17:50:05 EDT 2016


The gotcha is that you don’t *have* to have a 50 ohm system. An output stage with
a narrow band tuned tank is one example of a very “not 50 ohms” system. 

There is also a whole debate around the “is 50 ohm source into 50 ohm load really 25 ohms”. 
That will give you a 3 db delta to bet beers about. The 3 db split of thermal noise between AM
noise and PM noise is fairly well accepted, but it still can be challenged. 

So is the floor -174 - 3 = -177 or is it something else …. 

If we have a -195.4 dbc/ Hz oscillator that puts out < 18 dbm in a 50 ohm system it’s not -177 or
it’s not a 50 ohm system. 


> On Aug 11, 2016, at 4:47 PM, John Miles <john at miles.io> wrote:
> Or rather -(177+DUT output power in dBm).  The minus sign makes the difference between the thermal floor and a nuclear war!
> -- john, KE5FX
> Miles Design LLC
>> Remember that L(f) is expressed in dBc/Hz, not dBm/Hz.  If it were dBm/Hz,
>> then kT would be the limit.  But in dBc/Hz terms, the limit is 177 + the DUT's
>> output power in dBm.
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