[time-nuts] time-nuts] OP-Amps for 10MHz distribution...?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Mar 2 06:19:30 UTC 2012


John Miles wrote:
> Apologies if this is a dupe -- I'm not sure it ever made it to the list:
>
>    
>> The attached plot indicates the phase noise performance obtainable with
>> a wideband FET (OPA653) input opamp.
>> With a 10MHz +9dBm input, the phase noise floor is around -163dBc/Hz at
>> 1kHz offset and around -154dBc/Hz at 10Hz offset.
>> A quieter test source would be useful particularly for offsets below 10Hz.
>>      
> What was the test setup?
I used an undisciplined Thunderbolt (powered by an open frame linear 
supply) together with the splitter it came with.
The amplifier was connected to one splitter output and the amplifier 
output drove the input channel whilst the other splitter output drove 
the reference channel.

> Ideally the 10 MHz test source won't influence the
> measurement if you use a splitter to drive both the input and reference
> jacks, with the DUT in the input arm.   (I'm assuming that's what you're
> doing, or you wouldn't have been able to see 1 Hz PN down to -140 dBc/Hz.)
>
> In reality a noisy test source can still degrade a 2-port measurement, but
> this will usually be seen at wideband offsets where the broadband noise
> decorrelates due to differences in the input and reference paths other than
> those caused by the DUT's additive noise.  I usually use a 10 MHz LPF for
> 2-port measurements even when the test source is a good crystal oscillator,
> just to be on the safe side.
>    
Now that I have suitable filters, I'll try using a filter between the 
Tbolt and the splitter.

> If you let it run overnight, does the close-in measurement floor drop any
> further?   It looks like there's still a lot of instrument influence below
> 100 Hz.  Higher signal levels will help quite a bit, as well.
>
>    
I'll try a longer run using a higher level input (need to unearth the 
2N5943 transformer feedback prototype amplifier) to the splitter late 
next week

> -- john
>
>
>
>    
I'll also try a setup using a quadrature hybrid and a minicircuits phase 
detector to measure phase noise.
The recent NIST paper comparing the performance of several mixers 
indicates that the close in phase noise of this setup should be somewhat 
lower.

Bruce



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