[time-nuts] Keysight N5511A - phase noise measurements down to theoretical-177 dBm/Hz

[Magnus Danielson]

Hi,

There is actually more approaches. Me and NIST showed you can setup a
cross-correlating interferometer, it actually works, but is so far hard
to maintain properties, but the principle works so we presented and
published it.

Another approach suggested by Enrico is to actually compensate with the
expected noise-level of the power-divider resistor, and you do not need
to match that extremely well to cover most of the noise.

Yet another being used when you do not need to go all the way down is to
have a common resistor and then use active buffers for power-splitting.
It's not perfect and there will be a common mode noise that remains, but
it will be robust to cancellation due to power-splitter noise of
anti-correlation, so for many purposes this is good enough.

Thus, there is three different methods to work around it and one
pragmatic to dodge it. So you can avoid cryogenic methods, but none of
these methods is perfect, we have not solved it completely.

Cheers,
Magnus

On 2019-08-21 11:16, Bruce Griffiths wrote:
> That should have been:
>
> Its only necessary (as NIST have shown) to cool the splitters to reduce the correlated or anti-correlated thermal noise between splitter outputs. Everything else can run at ambient temperature. 
>
> Bruce
>
>> On 21 August 2019 at 21:13 Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>>
>>
>> Its only necessary (as NIST have shown) to cool the splitters to reduce the correlated or anti-correlated noise between the outputs. Everything else can run at ambient temperature. 
>>
>> Bruce
>>> On 21 August 2019 at 18:49 ed breya <eb at telight.com> wrote:
>>>
>>>
>>> That's quite an impressive system. I guess it's a few generations beyond 
>>> my 11729C.
>>>
>>> One way to get overall performance to the limits of room temperature kT 
>>> noise level, is to lower the T where you work. I wouldn't be surprised 
>>> if some parts are TE-cooled, easily affordable in a big budget system. 
>>> My first thought was maybe a bunch of stuff in a cryogenic system, but 
>>> it looks like most pieces are modules in a rack mainframe, and not in a 
>>> special environment. But, within the modules, I could picture some 
>>> degree (PTP) of TE-cooling being included, giving some margin on the 
>>> capabilities.
>>>
>>> Ed
>>>
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