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

Sat, 31 Jan 2004 18:46:08 -0800

While we are babbling on about spectrum analyzers, how about a tutorial 
on sweep speed. This is the most miss used and poorly understood part of 
the utilization of analyzers for measurements.

WBob

Dave Emery wrote:

> On Sat, Jan 31, 2004 at 04:46:58PM -0800, WolfBob wrote:
> 
>>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).
> 
> 
> 	For most spectrum analyzers what you are saying is roughly true.
> What is typically measured is the low pass filtered output of an
> envelope  (or sometimes quasi RMS) detector looking at the output of a
> log IF amplifier.   Some very modern instruments sample the IF signal
> and use DSP processing and can measure true RMS in some situations,
> however.
> 
> 	But what you are saying is entirely correct.  A narrow band
> signal will be more db over the noise floor with narrower resolution
> bandwidths.   In fact all other things being equal, the noise floor
> should drop 10 db for each factor of 10 decrease in bandwidth - if it
> does not something is likely wrong with the instrument.
> 
> 	But what I was talking about in my comments below is what one
> sees comparing the amplitude of a wide digital signal with a narrow
> carrier.   Given a resolution bandwidth wide enough to include almost
> all the energy of the digital signal, equivalent power signals (a
> narrowband carrier and a wide digital signal) will have equivalent
> height peaks on the display.     But use a narrower RBW, perhaps to have
> a lower displayed noise floor, and the equivalent power digital signal
> will be a much lower hump compared to the hot spike of the carrier.
> 
> 	Looking at the display without much thought it might be 
> natural to think the carrier is more powerful because it reads higher,
> but in fact given my example the signals have equal power.
> 
> 	What one can say is that the energy density in dbm/hz of the
> narrow signal is INDEED greater, but the total energy in both the
> signal is the same.
> 
> 	But how the pips of the two signals appear on an analyzer 
> relative to each other depends on the RBW used versus the bandwidth
> of the signal, which was my point.
> 
> 
>>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
>>>
>>>
> 