Explanation of the residual FM concept

[karote]

Hi,

If someone could explain the concept of residual FM? If I have very small RMS integrated phase error calculated from phase noise curve, does it automatically mean that also residual FM is small? Or is the requirement for low integrated phase noise somhow in contradiction to get low residual FM? Than you in advance.

 

[Lance_RFdude.com]

residual fm measurement

Hi,

If someone could explain the concept of residual FM? If I have very small RMS integrated phase error calculated from phase noise curve, does it automatically mean that also residual FM is small? Or is the requirement for low integrated phase noise somhow in contradiction to get low residual FM? Than you in advance.

the way I've seen residual FM used is very similar to an old two-way term "hum and noise". It is just residual FM modulation of a signal source/transmiter/LO/system.

"FM" noise increases much more rapidly with frequency offset if you have a "level" phase noise curve... because for FM to have the same single-sideband power density, it effectively has a much higher deviation. So where RMS phase error/noise increases linearly with area under the curve, the curve is multiplied (weighted) by the offset frequency.

You can see this effect in the graph on this web page: tools.rfdude.com/PLL/pll.html
the black curve is just the "FM" noise from the same source as the red curve (or at least I think so).


it is probably best to play with the calculations for a few cases to see the effects. when working on this stuff I found that you really had to be much more careful when picking loop bandwidth for FM systems just due to this weighting of noise (whereas the relationship, as I said, is much more linear with linear modulation such as QAM, etc.)

Lance

 

[biff44]

what is residual fm

Residual FM is a term that is used less today than it used to be. Residual FM and phase noise are related only that if you have low phase noise, you probably also have low residual FM.

But like said above, phase noise is a measurement made at many different offset frequencies. This is much more meaningful for modern communications systems. For example, if you have a low data rate system using phase modulation (qpsk for instance), you only care about the phase noise close to the carrier. If you take that same qpsk system and try to send 50 MBPS data on it, you really only care about phase noise far from the carrier (further than 10 KHz from the carrier).

Residual FM is basically an integrated snapshot of phase noise, so there is no information of the noise shape. It was a good measurement when systems used only FSK modulation.