Circulator excessive contribution to noise figure

[circo]

I have a problem in that a circulator appears to be adding noise into a system above the measured value of the circulator's insertion loss. I do not think that the problem is related to compression under power. The circulator is designed to handle the power specified for the system.

It has been suggested that "spin waves" may be the reason for the increase in NF. Does anyone have experience of this effect and how to reduce it?

Many thanks

 

[biff44]

I do not think there is a generic problem with circulators and noise. I did a project once with a 1.2 dB NF (isolator in front of LNA) and the isolator's insertion loss matched its NF contribution within 0.1 dB.

It is possible that you have a bad circulator.

It is more likely that whatever you are hooking to the 3rd port of the circulator (a transmitter for instance) is spewing out broadband noise and upsetting the receiver's noise figure. Happens all the time.

 

[madengr]

The ciculator may handle the power but it will heat up. The puck inside may be allot hotter then the whole unit. Did you measure the insertion loss cold ? Is the noise increase more than could be explained by a hot circulator. Measure NF vs time to see if there is a heat correlation and also vary the PA power to see if it is heat or as biff44 suggested.

Circulators have a nasty habit of increasing loss as power is increased, which leads to hotter temps, leading to more loss... kind of positive feedback but then it stabilizes.

 

[VSWR]

Is the transmitter on when you detect increased receiver NF? Does the NF decreases if you switch off the transmitter? If so, the NF increase can be caused by transmitter phase noise.