Don said:
By a conservative estimate, the manmade and terrestrial noise at this
frequency will be something like 170 decibels ABOVE the ktb noise.
It does not matter in the least which device he uses for his input stage.
Don, after further studying, I believe that I can cancel out nearly all
the terrestrial noise with a second nearby coil since the 37Hz signal
would be at a local small area. The second coil, that would be away
from the 37 Hz signal, would be the terrestrial noise cancellation
coil. I would expect the terrestrial noise to cancel out fairly well
since the RF wavelength at 37 Hz is over 8 million meters.
Ken said:
For those who are very interested in low noise at lowish frequencies:
Take a look at the company called "interfet" in Texas. They make JFETs
with a noise of about 1/4 nV/sqrt(Hz).
Thanks Ken. I found a great pdf file on noise,
http://eesof.tm.agilent.com/docs/iccap2002/MDLGBOOK/7DEVICE_MODELING/6NOISE/NOISEdoc.pdf
In considering the JFETs as an amplifier, wouldn't I have to use
several resistors? It seems that resistors are a definite source of
noise, which would add to the 1/4 nV/sqrt(Hz) JFET.
You can also get that level by putting 16 LSK170s in parallel.
Yes, great idea. I was thinking about that yesterday. Although I
didn't know that it was also the sqrt() of total parallel devices.
Isn't paralleling equal to multiple sampling? Consider a computer
program that does averaging. Say we take 10 samples, sum up all 10
samples, and take average signal. I thought that 10 averages would
decrease random noise by a factor of 10. BTW, the program would be in
sync with the 37Hz signal so that averaging would not cancel it out
like the noise.
Does anyone have any input on the following idea? According to the
above pdf, Inductors are considered fairly noise free. Perhaps that
pertains to well made L's. I have not verified that. If that is true
then a transformer should also be fairly noise free-- say a nice
toroid. Why not amplify the signal to say a few hundred times with the
transformer and then use a differential amplifier? That's presuming
that I am able to cancel out enough of the terrestrial noise. For
example, lets say we have a 200pV 37Hz signal with no noticeable noise.
After the transformer, we get a 20nV 37Hz signal with no noticeable
noise. Now our amp should have no problem with the 20nV signal. Lets
say the amp adds 1nV of noise. So that's a 20:1 signal to noise ratio.
Without the transformer it would be a 0.2:1 signal to noise ratio.
Thanks for the input,
Paul