On 7/12/22 8:53 AM, Erik Kaashoek via time-nuts wrote:
I'm struggling with the noise floor.
First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with
shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with
1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz
was still -140dBc/Hz
The setup was simplified to this schematic:
http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
What's the noise contribution of the resistors? V = sqrt(4*k*T*R*B)
51 ohms is sqrt (4 * 1.38E-23 * 300 * 51) = 0.9 nV/sqrt(Hz) 100 ohms
is ~1.4 nV/sqrt(Hz)
What kind of op amp? what's the current noise vs the voltage noise? -
you might low voltage noise, but high current noise, and that current
noise across the input impedance can turn into surprisingly high voltage
noise at the output.
The REF_buffer creates a virtual ground, the Audio_LNA amplifies into
the differential audio output .
Why did the lower noise opamp not make a difference?
Also the setup is acting like a nice microphone. Tapping the housing
is clearly audible. Which component may be causing the microphony?
My guess would be a parasitic capacitance between circuit and housing.
changing the distance changes the C.
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