[time-nuts] 74AC gates phase noise

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Feb 25 23:04:22 UTC 2010


John Miles wrote:
> -178 is about the same broadband floor that you see from the higher-end
> Wenzel ULN parts.  These can be custom-ordered in the $2000-$2500 range in
> single quantities.  Pascall's product line seems relatively similar.
> Obviously the carrier needs to be over +4 dBm to get to -178 dBc/Hz at 25C,
> but it can certainly be done. :)
>    
Only if the noise figure of the following amplifier is 4dB or so.
With no extra amplification is used one only needs a signal level of 
+1dBm to achieve a phase noise floor of -178dBc/Hz if the output is 
extracted through the crystal in such a way that the thermal noise of 
the load dominates.
Half of the -174dBm/Hz (at 300K) thermal noise contributes to the  phase 
noise the other half contributes to amplitude noise.
Thus the contribution of the load thermal noise to the phase noise floor 
( dBc/Hz) is -177 + Pout (dBM).
> What you get for $10,000 and up are the BVAs and similar ultrastable OCXOs.
> These have unremarkable phase noise but ungodly stability, around 10x better
> than the best 10811s and E1938As.
>
> The most economical way to get into the -170 dBc/Hz range with commercial
> oscillators seems to be the Valpey-Fisher parts.  These are in the $100
> neighborhood.
>
> -- john, KE5FX
>
>    
Bruce
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
>> Behalf Of Rick Karlquist
>> Sent: Thursday, February 25, 2010 1:34 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] 74AC gates phase noise
>>
>>
>> Martyn Smith wrote:
>>      
>>> Garry,
>>>
>>> Are you also saying your 100 MHz OXCO is making -178 dBc/Hz?
>>>
>>> Unless your 100 MHz OXCO is some special $10k type, there's no way your
>>> OXCO
>>> has a -178 dBc/Hz floor noise?
>>>
>>> Regards
>>>
>>> Martyn
>>>        
>> Can you explain this further?
>>
>> It isn't clear to me that there is any fundamental reason
>> why this noise floor is not possible, especially if other
>> specs (like aging) can be relaxed.  Also, the noise floor
>> tends to be limited by issues having to do with physics,
>> not economics.  How would having $10,000 to spend get
>> around the physics issues?
>>
>> I think I could approach this noise floor by taking a 10811
>> and bringing the signal directly out of the grounded base
>> buffer, and then readjusting the ALC to double or triple
>> the crystal current.  The biggest unknown is at what point
>> the crystal sustains damage.  The 10811 is definitely not
>> optimized for phase noise floor.  The designers were well
>> aware of this.  But phase noise floor isn't everything.
>>
>> Rick Karlquist N6RK
>>
>>
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>>      
>
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