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 <[email protected]> wrote:
Hi
> On Mar 25, 2016, at 1:55 PM, jimlux <[email protected]> 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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