Fw: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement

Sat, 31 Jan 2004 16:46:58 -0800

I don't understand this. The amplitude displayed on the display is 
related to the AVERAGE energy in the spectrum analyzers bandwidth. Not 
the peak or RMS (or power although they often calibrate it in power). If 
the signal is narrow band and embedded in wide band noise then a change 
in the bandwidth of the analyzer will not change the signal amplitude 
but the noise level will change in direct proportion to the bandwidth. 
This principle is used in receiver design and in most all 
instrumentation devices where it is desireable to have the bandwidth of 
the measurement system match the bandwidth of the thing being measured. 
If you have a complex waveform to be measured you can employ a conjugate 
matched filter, like for a pulse, the optimum bandwidth is a sinx/x 
filter envelope (kinda hard to build).

WBob

Dave Emery wrote:
  	Another issue you have not commented on is the different
> behavior of narrow filters versus wide with noise like signals versus
> signals with narrow band energy (relative to the filter bandwidth).   On
> a purely noise like signal (many modern digital signals) a narrow filter
> will capture proportionately less of the signal energy which is
> distributed more or less uniformly across a wide bandwidth relative to
> the filter than a wider filter will.   This makes the amplitude of a
> noise like signal much less as measured in a narrow bandwidth filter
> compared to a wide band one that includes more or even most of the signal
> energy inside its bandpass.
> 
> 	But if the signal contains mostly strong narrow band spectral
> lines (a carrier, sidebands from modulating tones etc) which are
> narrower than or close in width to the resolution bandwidth in use those
> will show up with close to the same amplitude indication in a narrow
> bandwidth as they would in a  significantly wider resolution bandwidth.
> 
> 	What this means in practice is that wide band digital noise like
> digital signals when seen with resolution bandwidths less than the
> bandwidth of the signal are much shorter peaks than unmodulated carriers
> or narrow band signals that actually contain less total energy. This can
> be quite deceptive, if one assumes the amplitude of the peak is
> proportional to total signal power.   And especially so if the sweep
> width and other parameters are set such that it is not obvious the
> wide band signals are actually wider than the resolution bandwidth in use
> (or the narrow band signals narrower).
> 
> 
> 
> 
> 
>>  
>>--------- Forwarded message ----------
>>From: Dave Emery <[email protected]>
>>To: [email protected]
>>Date: Sat, 31 Jan 2004 01:21:40 -0500
>>Subject: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement
>>Message-ID: <[email protected]>
>>References: <[email protected]>
>>
>>On Fri, Jan 30, 2004 at 07:15:44PM -0600, [email protected] wrote:
>>
>>>Thanks, Dave. That answers some of the questions as to why 3Khz.
>>> 
>>>
>>>>       Your RC remote control spec seems have used the later form -
>>>>specifying how many db down from the carrier level the noise power
>>>>reading in a 3 khz RBW centered 20 khz from the carrier is supposed to
>>>>be.   This obviously is trivial to measure correctly if you have a
>>>>spectrum analyzer with a 3 khz bandwidth, but a little more complex if
>>>>you have one (as you do with only 500 hz and 5 khz bandwidth).
>>>
>>>Right, and what is the "little more complex" proceedure ? Other than my
>>
>>>"rough estimate" indicated in a previous note.
>>
>>        Depends a little bit on what you are expecting the energy
>>to look like.   Basicly you need to synthesize a 3 khz bandwidth
>>from your 500 hz data by summing a list of 500 hz samples multiplied
>>by a weighting factor (window).   The crudest is a rectangular window
>>of 6 samples.   A fancier window is a gaussian shape matching a typical
>>3 khz bw filter with more than six samples and unequal weights.
>>
>>        You have to convert dbs to noise power in each sample, multiply
>>that by the weighting factor and sum the samples , then convert that
>>back to dbs.   An antilog and log operation.
>>
>>        To get a curve, you slide the window over one sample and do
>>it again for the resulting set of samples.
>>
>>
>>
>>>There are actually two specifications. One is the FCC spec for R/C
>>>transmitters
>>>which specs BWs at -25dB, -45dB, -55dB, and peak detect (based on the
>>>+/-10Khz BW spec at -55dB a RBW/BW ratio of a few percent is implied). 
>>>Of the 5 RBWs I have, 500Hz RBW fits best and I have no problems with 
>>>those measurements. 
>>
>>        Yup, should work fine.
>>
>>>The other is an AMA spec for -55dB for (apparently +/-) 20Khz using 
>>>3Khz RBW. That's the one I have problems with ....correlation to my 
>>>measurements using 5Khz RBW or correlation to the FCC spec. 
>>>
>>
>>        As I say, this can for most signals be synthesized from
>>the sum of weighted narrower band measurements.
>>
>>        It actually gets a bit trickier if you want real precision
>>and the signal has signal components that correllate with each other,
>>but to a first approximation my algorithm will work.
>>
>>
>>
>>>73 Kees K5BCQ
>>>_______________________________________________
>>>Test-Equipment mailing list
>>>[email protected]
>>>http://mailman.qth.net/mailman/listinfo/test-equipment
>>
>>-- 
>>   Dave Emery N1PRE,  [email protected]  DIE Consulting, Weston, Mass
>>02493
> 
> 