Originally posted May 21, 2013
I agree with the Grinch on this one
Noise doesn’t undermine everything we do, though sometimes it feels that way. And while adding a little noise can improve some measurements (the subject of a future post or two) it usually makes our jobs harder. Noise is a limiting factor in the performance of many systems and frequently limits both the accuracy and the speed of measurements.
In signal measurements noise acts to increase measurement variance and also adds undesired power to individual measurements. Thus noise can make real-world measurements slower by requiring some kind of averaging to reduce variance to acceptable levels. I’ll talk about averaging techniques in another post but here I’ll focus on the effects of undesired noise power on spectrum measurements.
First let’s look at two spectrum measurements of a low power multitone signal with amplitude that decreases 3 dB from tone to tone with increasing frequency:
Two spectrum measurements of a low power 7-tone signal. Amplitude decreases 3 dB for each tone and the scale is 3 dB/div. The two measurements show the effects of an approximately 12 dB difference in signal level vs. noise floor.
Notice the difference in the displayed power of each tone. The tone at the left appears to have the same power in each measured trace. However as the higher frequency tones decrease in power (3 dB/div. and 3 dB/tone) the difference between the measurements grows larger.
The effect of the analyzer noise floor (or the broadband noise of the signal or a combination of the two) is most clearly shown by the second tone from the right. The power of the tone is approximately the same as the noise floor of the analyzer’s yellow trace and the result reads 3 dB high. This makes perfect sense since the noise power in the analyzer’s resolution bandwidth filter at this frequency is the same as the power of the tone and the result is a reading 3 dB higher than it would otherwise be.
Of course this power addition affects more than just tones or other discrete signals. It also affects measurements of noise or signal/noise and is just as applicable to phase noise. The figure below summarizes the situation.
Expanded view of measurement of a CW signal near an analyzer’s noise floor. The analyzer’s own noise affects measurements of both the signal level and signal/noise.
Measurements close to the noise floor of the analyzer need to be made and interpreted carefully, since power addition in the analyzer’s IF will affect signal/noise measurements along with measurements of discrete signals.
For me, the result of making measurements and drawings such as these is a resolve to be careful whenever I am measuring anywhere close to an analyzer system or noise floor. For accurate measurements I’d define “close” as within about 20 dB if I’m looking for maximum accuracy. I remind myself that the analyzer is always measuring the total signal in its IF section and figure out the implications from that starting point.
One final point: This addition of noise power in the analyzer IF can work both ways, and if the analyzer’s noise floor is accurately known, its power can be subtracted from measurements to make them significantly more accurate. Agilent’s technique to accomplish this automatically on the PXA signal analyzer is called noise floor extension (NFE) and it’s discussed in the application note “Using Noise Floor Extension in the PXA signal analyzer.” An excellent general reference for understanding noise in signal measurements is application note 1303 “Spectrum and Signal Analyzer Measurements and Noise.”
* From the animated TV special “How the Grinch Stole Christmas”