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Why is the Noise Figure (NF) measurement of a Combiner 3dB higher than t= he expected value?
This is neither a bug nor a problem. You would notice this behavior if y= ou have two sources pumping noise into the combiner.
The Noise Figure/ Noise Temperature is something of a fiction. One measu= res the output noise spectral density of a system then this is divided by t= he device's gain to get an equivalent input noise power spectral density. T= his noise is added at input of the now noiseless system to get the observed= output noise. Assume that there are two identical noise sources pumping no= ise into the combiner. Since the output noise is combined, there is twice t= he noise power at the output than each input noise source. Noise Figure/Noi= se Temperature measurement is defined as all the noise coming from the one = input source, thus the input noise is doubled so there is a 3.01 dB increas= e in NF.
This is exactly the same as the situation with mixers, where there are e= ffectively two inputs, the RF and the image. When the output noise is due o= nly to the RF input, it results in the single-side-band noise figure. When = the output noise is due to both, the RF and the image, it produces the doub= le-side-band noise figure, which is 3.01 dB higher.
The default setting for the "TYPE" of the cascaded noise figure measurem= ent is "Ideal, Circuit and Channel". This can cause the noise figure throug= h a branched path; a splitter and combiner to be higher than expected. Chan= ging the "TYPE" to "Operating Point, Circuit and Channel" gives the d= esired results.