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

[Bruce Griffiths]

There are many ways to achieve a reasonably low phase noise floor when 
measuring the additive phase noise of amplifiers etc:

1) Use an interferometer to null the carrier and amplify the 
interferometer output being careful to ensure that the residual carrier 
at the output of the amplifier isnt sufficient to induce significant 
flicker phase noise. Enrico Rubiola has published many papers on this 
technique.
Since the test source phase noise may be much higher than that of the 
amplifier under test especially at low offsets, the carrier suppression 
has to be quite high to ensure that the residual oscillator phase noise 
at the doesnt exceed that added by the amplifier under test.

2) Use duplicate systems to amplify and measure the phase noise together 
with cross correlation.

3) Use a low noise mixer with the LO and RF inputs in phase quadrature.
     Low pass filter the IF output then amplify and digitise the 
baseband signal with a high resolution ADC

OR

4) Use a technique akin to that used in the TSC5120A (avoids the need 
for achieving accurate quadrature between the LO and RF inputs to a mixer).
For a brief overview read the TSC5120A manual and the various associated 
patents.

A combination of techniques is usually necessary to achieve ultra low 
instrument noise floors.

The measurement setup is somewhat adhoc and it should be feasible to 
reduce the instrument phase noise floor to around -200dBc/Hz or so with 
additional refinements.

Bruce

Azelio Boriani wrote:
> Very good. I'm fascinated by the figures... how can such phase noise levels
> be measured and, in general, being able to "sense" the most faint signal.
>
> On Tue, Apr 17, 2012 at 11:25 AM, Bruce Griffiths<
> bruce.griffiths at xtra.co.nz>  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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