Fw: Re: Fw: Re: [Test-Equipment] Spectrum Analyzer BW measurement
Gary Schafer
[email protected]
Sat, 31 Jan 2004 22:41:06 -0500
Most analyzers in the auto position adjust the sweep speed for optimum
as you change resolution or sweep width. In the manual mode my old HP
141 has a light that comes on when you have the sweep speed too fast for
the band width and filter in use. If you try and increase the sweep
speed you will start to see the signal level drop. It will also distort
the pass band display as you go further. I suspect that there is a lot
of phase shift going on as well as filter ringing as Dave pointed out
earlier. This in addition to the signal not being in the pass band long
enough to extract the full energy in the signal.
Just like Dave was saying earlier about looking at a wide digital signal
that is wider that the filter you are using. You will not get the full
power of the full signal in the band pass at the same time. So you will
not extract all the power that is in the signal.
73
Gary K4FMX
WolfBob wrote:
> 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
>>>>
>>>>
>>>>
>>
>
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