[time-nuts] Opamp Phase noise: OPA653 additional measurements

Bob Camp lists at rtty.us
Tue Apr 17 12:06:23 EDT 2012


Hi

Tossing a beach towel over it usually does a pretty good job out to 10
seconds.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, April 17, 2012 8:46 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Opamp Phase noise: OPA653 additional measurements

I'll try shielding from air currents (usually effective to a little 
below 1Hz).
The Opamp is thermally connected (metal tab under package is soldered 
to) to one of the power planes so the thermal time constant is a little 
longer than it would otherwise be.

Bruce


Bob Camp wrote:
> Hi
>
> My *guess* is that your 1Hz data may be as much from ambient changes as
from the op amps themselves. No data to back that up on these specific
parts, just a guess.
>
> Bob
>
> On Apr 17, 2012, at 7:28 AM, Bruce Griffiths wrote:
>
>    
>> Not yet, I just used the pair of TI evaluation boards which I obtained
for another project.
>>
>> I intend to mount the various components (splitters, 180 degree hybrids
etc) on a thick aluminium plate to improve the mechanical and thermal
stability of the system.
>>
>> Bruce
>>
>> Bob Camp wrote:
>>      
>>> Hi
>>>
>>> Did you do any temperature stabilization on the op amps?
>>>
>>> Bob
>>>
>>> On Apr 17, 2012, at 5:25 AM, Bruce Griffiths wrote:
>>>
>>>
>>>        
>>>> Subsequent measurements using an improved technique has allowed the
true flicker phase noise characteristics of  a pair of OPA653 opamps to be
determined.
>>>>
>>>> Opamp
>>>>     Gain 1x (6db folled by 6 db attenuator consisting of a 50 ohm
resistor in series with the output of the opamp and the 50 ohm load
resistor)
>>>>     Signal input level: + 10dBm
>>>>
>>>> OFFSET    Opamp PN (dBc/Hz)    Measurement system PN (dBc/Hz)
>>>>
>>>> 1                -148                                -161
>>>>
>>>> 10              -161                                -172
>>>>
>>>> 100            -164                                -177
>>>>
>>>> 1000          -164                                -179
>>>>
>>>> Source: 10MHz low phase noise OCXO removed from a Thunderbolt amplified
by a QBH9816 amplifier.
>>>>
>>>> Conclusion: FET opamp PN in the flicker region can be quite low despite
its somewhat higher baseband flicker noise than that of some bipolar input
opamps.
>>>>
>>>> Bruce
>>>>
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