[time-nuts] Reverse isolation

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 9 22:08:20 EDT 2009


Didier

A 10-1000mW of single mode ECDL helps.
But then you need a modulator.
You would also need to avoid frying any photomixer at the other end.

Bruce

Didier wrote:
> But not necessarily lowest noise
>
> Didier 
>
>   
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com 
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of Don Latham
>> Sent: Sunday, March 08, 2009 8:48 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Reverse isolation
>>
>> Best isolation is via a piece of fiber optics.
>> Don
>>
>> Bruce Griffiths
>>     
>>> Lux, James P wrote:
>>>       
>>>>> Actually, it depends on the distance from the hot carrier, the 
>>>>> amplitude of the hot carrier and the particular VNA in 
>>>>>           
>> question. My 
>>     
>>>>> VNA allows me to change filters as well as integration time.
>>>>>
>>>>>           
>>>> And especially, where is the selectivity in the VNA's 
>>>>         
>> receiver?  You 
>>     
>>>> might have a 10 Hz filter, but if the receiver's front end is wide 
>>>> open, then the dynamic range requirement might be a challenge to 
>>>> meet.
>>>>
>>>> Consider a 30 dB gain amplifier claiming 120dB reverse isolation..
>>>>
>>>> You put in -30dBm and get 0dBm out in the forward direction.  
>>>> Meanwhile, you inject 0dBm into the output of the 
>>>>         
>> amplifier (from the 
>>     
>>>> VNA) and you expect to see -120dBm appearing at the input. 
>>>>         
>>  Depending 
>>     
>>>> on how good the directional coupler/bridge is that feed the port 1 
>>>> reflected power receiver, you might be facing measuring a -120dBm 
>>>> signal next to a -30dBm signal, which is challenging...
>>>>
>>>> Assuming the DUT is linear (hah!) one could just apply load with 
>>>> different reflection coefficients to it when measuring S11.  This 
>>>> might even be harder, though... With -120dBm reverse isolation, 
>>>> you're basically looking at seeing the effect of adding a coherent 
>>>> signal 90 dB down to the signal you're already measuring for S11. 
>>>> That's not going to result in a huge change in amplitude or phase 
>>>> (e.g. About 1 part in 1E4 or 1E5)..
>>>>
>>>> Maybe for this list, though.. Measuring phase shifts of 
>>>>         
>> 1E-5 radian..
>>     
>>>> Walk
>>>> in the park, as it were.
>>>>
>>>> Jim
>>>>
>>>>
>>>> _______________________________________________
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>>>>
>>>>
>>>>         
>>> Jim
>>>
>>> Its even more challenging when the isolation amplifier has 
>>>       
>> a nominal 
>>     
>>> gain of 0dB coupled with a reverse isolation of 120dB or more.
>>> In this case one would be looking at measuring phase shifts on the 
>>> order of 1urad or less.
>>>
>>> In this case one could try applying say a +10dBm signal to 
>>>       
>> the input, 
>>     
>>> modulate the load reflection coefficient and look for a modulation 
>>> frequency related sideband at the input port.
>>>
>>> Bruce
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to 
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>>>
>>>       
>> -- 
>> Dr. Don Latham AJ7LL
>> Six Mile Systems LLP
>> 17850 Six Mile Road
>> POB 134
>> Huson, MT, 59846
>> VOX 406-626-4304
>> www.lightningforensics.com
>> www.sixmilesystems.com
>>
>>
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>>     
>
>
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