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How come my noise figure decreases thru a cascade?

Question asked by rulonv Employee on Jul 31, 2003
Decreasing or negative noise figure is typically related to:

1. Reflected noise power across the channel
2. Not enough simulation noise points across the channel
3. Parallel noise paths

The equation for Cascaded Noise Figure measurement in SPECTRASYS is:
    CNF[n] = CNP[n] - CNP[0] - CGAIN[n] (dB), where n = stage number and CNP is the Channel Noise Power
and the Cascade Gain measurement is:
    CGAIN[n] = DCP[n] - DCP[0] (dB), where n = stage number and DCP is the Desired Channel Power
Cascade Gain is therefore a function of all forward traveling power in the channel which is subject to VSWR effects.

Remember, that SPECTRASYS keeps track of forward traveling noise power (not the total noise power).  Consequently, the forward travelling noise power will be reduced by the amount of reflected noise power and this will appear as a decrease in cascaded noise figure.

Furthermore, VSWR will generally be worse across wide channel bandwidths than narrow ones ... especially when filters are present in the simulation.  Narrowing the channel bandwidth will generally reduce the VSWR across the channel thus decreasing the reflected noise power and increasing the cascaded noise figure.

- An indication of VSWR is when Gain and Cascaded Gain values seem unexpected

Another issue usually is that the Channel Measurement Bandwidth is much wider than the channel signals.  This is ok but extra noise points may need to be added to improve the accuracy of the Channel Noise Power measurement.  SPECTRASYS interpolates between all noise and signal data points.  If there is a lot of amplitude ripple in the circuit sufficient noise points must be added for each signal to properly account for these variations.  If the noise spectrum looks very stick-figure-ish then extra noise points may need to be added.

Adding extra noise points can be done on the 'Calculate' tab of the 'System Analysis' dialog box.

If cascaded noise figure is being examined through a hybrid combining network the cascaded noise figure will appear to artificially peak at the internal nodes to the hybrid network.  This occurs because cascaded gain used is only for the current path and not all parallel paths used in the hybrid network.

[SPECTRASYS, Cascaded Noise Figure, NF, CNF, Negative]