[time-nuts] OXCO Spurious Output at Line Frequencies

jimlux jimlux at earthlink.net
Tue Jul 12 18:49:26 EDT 2016

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 

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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