On 7/12/16 3:44 AM, Martyn Smith wrote:
Hello,
I have a customer who is measuring the phase noise of my 10 MHz ultra-low phase
noise frequency standard.
He is seeing spurious signals at line frequencies (50 and 100 Hz as we are in
Europe) at a level around -130 dBc.
My opinion is that it's impossible to get much better than that. Even running
on batteries make little difference, since the equipment is in a test rack with
AC signals everywhere.
Even the £50k R&S test set he is using only quotes a spurious spec of -90 dBc.
What experience does anyone have here?
It's impossible to get rid of that. You'd need a magnetically shielded
enclosure, because most of the line frequency is in the form of the
magnetic fields from the building wiring, transformers, etc.. It's
almost impossible to build your oscillator and measuring system without
any loop area, into which those magnetic fields couple.
Let's assume you're at 0dBm which is 0.223 Vrms (It's easier for me to
work in voltage, although for magnetic field, you should be working in
current).
So, 130 dB down is 3E-7 times less, or about 74 nV. It doesn't take a
very big field to induce a few nV in a circuit, even if it's all low
impedance stuff.
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