[time-nuts] Spectrum Analyzer Specifications --> phase noise test set

[Bob Camp]

Hi

The sensitivity of BNC connectors goes up quite a bit as they wear out. Both sides of 
the connection are subject to wear. Replacing both sides is often the only solution when
they get noisy. That said, “screw down” connectors are a better way to go. 

The “capacitive loading” termination of the mixer is something that a number of us 
tried to reproduce when the paper came out. Even after fairly extensive conversations 
with the authors, the effect seems be quite difficult to reproduce. It certainly is not a 
“general solution” to the problem. 

Bob


> On Mar 25, 2016, at 5:55 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> As long as BNC connectors are avoided as their phase shift isnt that stable.. A small mechanical disturbance will change it significantly. Actually low noise PN measurement systems can be very sensitive to cable movement. Bolting modules to a metal baseplate helps a lot as does using intermodule connections comprising SMA(m)-SMA(m) barrels rather than cables.
> There's also the question of mixer port termination.The nist paper by Walls and Stein indicates that capacitive termination of the IF port may be effective in reducing noise whilst maintaining a flat response fro dc to around 50KHz.Small value resistors in series with the RF and LO ports are then useful in reducing the VSWR.
> Bruce
> 
> 
>    On Saturday, 26 March 2016 10:10 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
> 
> 
> Hi
> 
>> On Mar 25, 2016, at 1:55 PM, jimlux <jimlux at earthlink.net> wrote:
>> 
>> On 3/25/16 5:07 AM, Bob Camp wrote:
>>> Hi
>>> 
>>> The reverse isolation issue is indeed one of the weaknesses of this setup. For testing
>>> OCXO’s isolation is not a big deal. A normal OCOX has very good output buffering
>>> to give it the stability you are after. If you are running (maybe) a VCO with no buffering, that
>>> assumption falls apart. The VCO will  / can injection lock through the mixer. In that
>>> case you *do* need an amp to provide enough isolation to prevent the injection lock.
>>> 
>> 
>> 
>> But if someone were building a little module for a cheap and cheerful noise analyzer, then the buffer amp would be a separate module.
> 
> That’s how I have always done it in the past. The need for the bufferer is rare enough that including it in the 
> basic analyzer module is not cost effective. The HP 3048 has the same basic issue (isolation) and they made
> the same decision there. 
> 
> Bob
> 
> 
>> 
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