It has been a few years....

The noise density in the JFET has a strong 1/f characteristic. This means it
increases by 6 db each time you halve the measurement frequency. For
silicon, I recall the corner frequency being in the ~ 100 Hz.range. The unit
"nV/root-hertz" is the noise density, not the noise. You need to measure the
voltage within a finite bandwidth. From that, you can derive
the noise power, then the noise density. When measuring below the corner
frequency you have to account for this 1/f slope.

Thermal noise in a 50 ohm system at room temperature is about 0.9
nV/root-hertz. In a 1000 ohm system it is about 4 nV/root-hertz. This will
set the lower measurement limit, you test equipment will probably limit you
to worse than this. Using adequate measurement bandwidth and correct circuit
resistance (within the confines of the JFET parameter range) will provide
sufficient noise voltage to actually measure, but it will take some gain
(via a very low noise measurement amplifier) to produce something you can
hook to a measuring device.

There are lots of subtle way to make noise measurement errors. I recommend
that you use the test setup that is specified by the manufacturer to make
the measurement, otherwise your results will likely not correlate with their
numbers.

-- Tom, N5EG


wrote in message
...
I need to measure the "equivalent noise voltage", in units of
nV/square root( hz) of a JFETs drain-source at a given Vds & Ids and
F=10 hz. How do I do that? What's throwing me is the F=10 Hz. What do
I do with that? I thought to take the measurement, I would bias the
FET to the required Vds and Ids and measure the voltage across the
drain - source with an RMS meter. Then divide that by the sqrt of the
bandwidth of the RMS meter. I've Googled, but didn't find anything to
straighten me out. Any help?