[time-nuts] New topics (was Re: He is aTime-Nut Troublemaker....)

Magnus Danielson magnus at rubidium.dyndns.org
Wed Dec 24 17:48:20 UTC 2008


Didier skrev:
> Chuck,
> 
> I am quite familiar with how to calculate a voltage or power ratio in dB,
> but refering to the first issue, when you combine two oscillators, does the
> noise improve by 3dB?

If you look back at my previous post, when combining the two outputs, 
the noise raises by 3 dB, but the signal strength raises by 6 dB, giving 
a net effect of lowering the noise by 3 dB relative the signal strength.

You will find this described in many places for transistors. It occurs 
for instance in the MAT-0x series of datasheets among several other places.

Cheers,
Magnus

> Didier
> 
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com 
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of Chuck Harris
>> Sent: Wednesday, December 24, 2008 10:02 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] New topics (was Re: He is aTime-Nut 
>> Troublemaker....)
>>
>> Didier wrote:
>>
>>>> Square root of 2 is about 1,414 or about 3,01 dB.
>>> I am always confused when considering noise, is it 10*log(p1/p0) or 
>>> 20*log(p1/p0)?
>> A moment's reflection on why the 10 log  vs. 20 log, might help.
>>
>> The conversion from a power ratio to dB is:
>>
>> dB = 10 log (P1/P2)
>>
>> Remember that Power = VxV/R, so:
>>
>> P1/P2 = (V1xV1)/(V2xV2), the R's cancelling.
>>
>> So,
>>
>> dB = 10 log [(V1^2)/V2^2)] or, 10 log[(V1/V2)^2]
>>
>> If we want to express this as a ratio of voltages, rather 
>> than a ratio of powers (there's a pun in there somewhere ;-), 
>> we need to take the square root of (V1/V2)^2 outside of the log.
>>
>> To do this, we need to remember that log[X^2] = 2 log X, so:
>>
>> dB = 10 log[(V1/V2)^2] = 20 log[V1/V2]
>>
>> A couple of things to note:
>>
>> 1) dB's are dB's.  3dB represents the doubling of a power ratio,
>>     6dB represents the doubling of a voltage ratio.
>> 2) Convention says that if -dB's are loss, and +dB's are 
>> gain, but that
>>     is just convention.
>>
>> -Chuck Harris
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, 
>> go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
> 
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



More information about the time-nuts mailing list