[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[ghf at hoffmann-hochfrequenz.de]

Am 2022-07-10 9:11, schrieb Erik Kaashoek via time-nuts:

> @Bob,
> You mentioned "dual supplies with high voltage" for the first gain
> opamp. How much impact would dual voltage bring as its a pain to
> implement.

I think is was Rubiola who wrote that he exploded a costly microwave 
mixer
with a LT1028 that had lost one of its power rails.

If you go AC coupling, don't forget that the input capacitor must not be
selected for f-3dB but that it must be much bigger to short the thermal 
noise
of the bias network to pV levels through the low impedance source.
Otherwise you see a noise rise towards 0 Hz that is MUCH steeper than 
1/f.

Scott Wurzer (designer of ad797) saw that immediately on my 20 * ada4898
220pV/rtHz amplifier. Wish he was more explicit. It took me some time to 
get it. :-)
I had 10K/100u foil, ended up with 10k/4700uF wet tantalum, which opens
another can of worms.

I have converted to FETs now. A few pVrtHz more, but much less noise 
current.
In cross correlation setups, the noise current of both amplifiers 
produces
a common voltage drop in the (common) source resistance, and that does 
not
average away. (May apply only to voltage measurements from a single 
source.)

Gerhard