This is actually less than the Johnson noise due to the 150 ohm resistor so it is a little optimistic.
Which means that there is a measurement error somewhere, and you can't trust your figure.Jakob E.
Which means that there is a measurement error somewhere, and you can't trust your figure.
I used a voltage and a scope to adjust the trimpot to -40 dB. I used a scope to measure the level at the input of the attenuator and the level at the output of the DUT. I then turned off the stimulus, removed the trimpot so that the input was terminated by just the 149R resistor and recorded the N+D reported by the QA400. Here are the results:input of attenuator: 7.49 Vppattenuation: -40.0 dBoutput of DUT: 7.71 VppN+D: 95.3 dBVSo thats +0.25 dB - 40.0 = 39.75 dBSo EIN is -(95.3 + 39.75) dB = -135.05 dB
What I am ultimately trying to do here is to devise a repeatable test of noise performance in general. Even though the SNR value is clearly not correct, it is repeatable. So precision seems good, accuracy not so much.
The gain is 39.75dB so EIN is -92.19 - 39.75 dBu = -131.94dBu. This is still 1dB better than the theoretical noise in a 150 ohm resistor so something else must be wrong