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Products => Test Equipment => Topic started by: electr_peter on April 08, 2017, 08:22:42 pm

Title: Trace average calc on SA or FFT
Post by: electr_peter on April 08, 2017, 08:22:42 pm

Some DSOs have a function for trace averaging which is implemented as arithmetic average of several acquisitions. Spectrum power display on SA or FFT also can be displaced as an average of several acquisitions. As spectrum is mostly displayed in dBm units (logarithm of power), "trace average" can mean arithmetic value of dBm units or arithmetic average of actual power. The latter is the case in my understanding. However, manuals mention "trace display average", which is not 100% clear.

How is "trace averaging" implemented on SA or FFT? Is it arithmetic average of dBm units or arithmetic average of actual power?

For example, say one trace on SA is at -10 dBm, other is at -50 dBm level. Then arithmetic average of two traces is -30dBm while actual power average is -13 dBm.

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display average = { -10dBm + -50dBm } / 2 = -30dBm
power average = 10 * log10[ { 10^(-10 dBm/10) + 10^(-50 dBm/10) } / 2] ~ -13dBm

Title: Re: Trace average calc on SA or FFT
Post by: nctnico on April 08, 2017, 08:40:20 pm

In my Advantest R3477 I can select between power average (RMS), video average and voltage averag.

Title: Re: Trace average calc on SA or FFT
Post by: Bud on April 08, 2017, 08:47:33 pm

To me trace average implies whatever trace you have on the display gets averaged. Not the data the trace was derived from.

Title: Re: Trace average calc on SA or FFT
Post by: MrW0lf on April 08, 2017, 10:13:30 pm

(https://www.eevblog.com/forum/testgear/trace-average-calc-on-sa-or-fft/?action=dlattach;attach=306812)

Title: Re: Trace average calc on SA or FFT
Post by: electr_peter on April 09, 2017, 07:38:02 pm

Thanks for the input.
From responses it seems that the terms "trace average" or "average" with respect to spectrum measurements can have several definitions (mostly meaning real power average of some sort) in SAs or DSOs. Manual of specific instrumentation should be consulted carefully if in doubt.

Title: Re: Trace average calc on SA or FFT
Post by: w2aew on April 10, 2017, 01:19:35 pm

Note that averaging the log power values is mathematically incorrect, but is included in most spectrum analyzers for historical purposes (it was there on older analyzers, and a lot of specifications for component and system testing were written around using this). 

Title: Re: Trace average calc on SA or FFT
Post by: bson on April 10, 2017, 06:41:13 pm

Trace averaging is nice to reduce visible noise and get a better defined trace while eyeballing.  But obviously, linear voltage averaging is more useful for measurement of e.g. a peak value even if displayed in log.