Voyager10
Well-known member
You're right. I was having doubts about the accuracy of using a 68M resistor, so I did an LTSpice noise simulation:
Green is 68M, blue is 1G, both in parallel with a 68pF capacitor.
The explanation is that noise voltage increases as the square root of resistance, but the attenuation due to the capacitor increases proportional to resistance. Total noise is the same, as per theory, but as you say it's shifted towards frequencies which will be filtered out.
It's worth re-measuring with R1 = 1G (and possibly 500M, as most versions of the circuit have 2 x 1G around this point).
Green is 68M, blue is 1G, both in parallel with a 68pF capacitor.
The explanation is that noise voltage increases as the square root of resistance, but the attenuation due to the capacitor increases proportional to resistance. Total noise is the same, as per theory, but as you say it's shifted towards frequencies which will be filtered out.
It's worth re-measuring with R1 = 1G (and possibly 500M, as most versions of the circuit have 2 x 1G around this point).