Re: [PSES] Spectrum analyzer and noise floor

[Ed Price]

Not wanting to sound like too much of a dinosaur, but back in the 1960's and
early 70's, emission data was acquired by slowly tuning a receiver across
the frequency range while listening to headphones and watching a meter. You
would record the frequency and amplitude at several frequencies per octave,
or more if you were feeling charitable, and also record any uniquely
"interesting" emission peaks. When you were finished acquisition, you might
have several pages of tabular data (perhaps 75-100 frequency / amplitude
data pairs).

 

Then, you could go back to your desk and break out your correction data, a
set of graphs for each of the parameters of cable loss, gain flatness,
bandwidth variation, pre-amplifier gain, antenna factors and maybe filter
attenuation (nothing was flat in those days). Typically, you might have 5 of
these variables, so that meant writing down (5 x 75) 375 numbers on your
data sheet. Then, you added up the raw amplitude and all factors for each
frequency and entered that corrected amplitude. Next, you used the limit of
the standard to visually interpolate and enter a limit value for each
measurement frequency. And lastly, you compared the amplitude value to the
limit value, and entered the over-limit values if necessary. And, if this
was a very formal test, you often had to break out that old pre-printed
multi-cycle logarithmic graph paper and plot the corrected data and the
limit.

 

Data correction took as long, or longer, than data acquisition. I dread to
think of the error budget!

 

The first automation that I encountered was a box using a variable speed
motor and a flexible speedometer cable connected to the tuning knob of a
receiver. You would set a "reasonable" scanning speed and connect an analog
plotter to plot detector amplitude versus scan progress (or the Volts/MHz
receiver output if you had a very advanced receiver). In this case, you
would make a master plot with a derived limit (as Ken Javor described), then
make photocopies of that master for the working measurement plots.

 

We have come a very long way in 40 years.

 

Ed Price
WB6WSN
Chula Vista, CA USA



 

From: Ken Javor [mailto:ken.ja...@emccompliance.com] 
Sent: Tuesday, February 23, 2016 5:13 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Spectrum analyzer and noise floor

 

I agree.  A modern machine adjusts the raw data or noise floor for any and
all transducer/amplifier/attenuator factors, and if the transducer factor is
not flat, neither will be the adjusted noise floor.

The only reason we do things this way is an "embarrassment of riches" in
processing power.  Absent a digital controller, the sane way to take data
would be to adjust the limit for the transducer factors and arrive at an
adjusted limit in terms of dBuV, or dBm.  Consider the number of
computations involved in adjusting a thousand data points across a screen
for the transducer factors, vs. a simple flat or log-linear limit and
transducer factors that only need be reentered at the next frequency at
which they have changed by some set amount from the last frequency, such as
1 dB, or 0.5 dB. The adjusted limit represents orders of magnitude less
computation, plus one can reverse engineer an adjusted limit if one knows
the transducers in use, whereas adding factors to a signal above noise
renders this impossible.

Ken Javor
Phone: (256) 650-5261



  _  

From: "McDiarmid, Ralph" <ralph.mcdiar...@schneider-electric.com>
Reply-To: "ralph.mcdiar...@schneider-electric.com"
<ralph.mcdiar...@schneider-electric.com>
Date: Tue, 23 Feb 2016 16:42:47 -0800
To: <EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: Re: [PSES] Spectrum analyzer and noise floor

I would like to explain to a colleague why the noise floor on a SA does not
look flat as it sweeps across a given frequency range after antenna factors,
cable factors, external gain and external attenuation are programmed into
its display function.  

I think it breaks down to these fundamental points: 

1. the SA receiver has noise in its attenuator, mixer and filter circuits
(say -80 dBm, and maybe flat within a limited  frequency range) 
2. the external amplifier has some noise too, but its gain lowers the noise
floor created by #1  (also flat within a limited frequency range) 
3. the cables have losses which are frequency dependant, and those can be
entered as loss factors into the SA  (shapes the noise floor a little and
those losses raise the noise floor) 
4. the antenna has a gain which is frequency dependant with several dB of
hills and valleys across its usable frequency range (that really shapes the
noise floor more than 1, 2 or 3 above) 
5. noise floor shape caused by #4 is the mirror image of the antenna factor
vs frequency 

Is that a decent summary? 
. 
Ralph McDiarmid
Compliance Engineering
Residential/Commercial
Solar Business
Schneider Electric D  +1 (604) 422 2622 x62622 

E  ralph.mcdiar...@s

Re: [PSES] Spectrum analyzer and noise floor

[Ed Price]

Ralph:

 

That's a workable explanation for the variation from flatness. Those who
spend a lot of their time acquiring emission data get very accustomed to the
shape of the corrected noise floor, and any variation from expected is a
reason to stop the test and examine everything that is going on in the
signal and the data processing paths.

 

However, there is a problem with your statement #2. A pre-amplifier is
chosen primarily for it having a first stage which has a Noise Figure lower
than that of the input stage of your spectrum analyzer. You could well have
a pre-amp with a small-signal gain of 35 dB and a noise figure of 1 dB, but
if you put that pre-amp onto a spectrum analyzer that has a noise floor of
-15 dBuV, you will not be able to see emissions of -50dBuV at the input of
that pre-amp. Regardless of pre-amp gain, the Minimum Discernable Signal
through the pre-amp and SA system will be limited by the pre-amp's 1st stage
Noise Figure.

 

You can prove this, without any math, just by getting a signal generator
that puts out say 60 dBuV. Now add another 60 or so dB attenuation on the
signal generator output, the idea to be able to apply a very smal signal to
your SA. Now, put the pre-amp into the circuit and see what happens to the
SA amplitude. Then, begin reducing the signal generator amplitude while
noting the SA response. This will show you where the Minimum Discernable
Signal fades into the pre-amp Noise Figure.

 

Perhaps Ken or Ghery can give us a more elegant mathematical description of
this effect.

 

Ed Price
WB6WSN
Chula Vista, CA USA



 

From: McDiarmid, Ralph [mailto:ralph.mcdiar...@schneider-electric.com] 
Sent: Tuesday, February 23, 2016 4:43 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Spectrum analyzer and noise floor

 

I would like to explain to a colleague why the noise floor on a SA does not
look flat as it sweeps across a given frequency range after antenna factors,
cable factors, external gain and external attenuation are programmed into
its display function.   

I think it breaks down to these fundamental points: 

1. the SA receiver has noise in its attenuator, mixer and filter circuits
(say -80 dBm, and maybe flat within a limited  frequency range) 
2. the external amplifier has some noise too, but its gain lowers the noise
floor created by #1  (also flat within a limited frequency range) 
3. the cables have losses which are frequency dependant, and those can be
entered as loss factors into the SA  (shapes the noise floor a little and
those losses raise the noise floor) 
4. the antenna has a gain which is frequency dependant with several dB of
hills and valleys across its usable frequency range (that really shapes the
noise floor more than 1, 2 or 3 above) 
5. noise floor shape caused by #4 is the mirror image of the antenna factor
vs frequency 

Is that a decent summary? 
. 


Ralph McDiarmid
Compliance Engineering
Residential/Commercial
Solar Business
Schneider Electric 

D  +1 (604) 422 2622 x62622
E   <mailto:ralph.mcdiar...@schneider-electric.com>
ralph.mcdiar...@schneider-electric.com 

3700 Gilmore Way
Burnaby
BC
Canada

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Re: [PSES] Spectrum analyzer and noise floor

[alfred1520list]

I am just now gotten very interest on this subject. I think you mention a key 
factor: antenna factor. The SA noise floor unit is dBm. After you have added 
the antenna factor the unit is dBuV/m. Everything else being perfect, zero 
loss, 0 dBi antenna gain across frequency, etc., the antanna factor is still be 
highly frequency depedent. I think: AF = 20log(MHz) - 29.8. Therefore the flat 
SA noise floor in dBm will be traslated into a curve in dBuV/m.

Regards,
Alfred

On February 23, 2016 4:42:47 PM PST, "McDiarmid, Ralph" 
 wrote:
>I would like to explain to a colleague why the noise floor on a SA does
>
>not look flat as it sweeps across a given frequency range after antenna
>
>factors, cable factors, external gain and external attenuation are 
>programmed into its display function. 
>
>I think it breaks down to these fundamental points:
>
>1. the SA receiver has noise in its attenuator, mixer and filter
>circuits 
>(say -80 dBm, and maybe flat within a limited  frequency range)
>2. the external amplifier has some noise too, but its gain lowers the 
>noise floor created by #1  (also flat within a limited frequency range)
>3. the cables have losses which are frequency dependant, and those can
>be 
>entered as loss factors into the SA  (shapes the noise floor a little
>and 
>those losses raise the noise floor)
>4. the antenna has a gain which is frequency dependant with several dB
>of 
>hills and valleys across its usable frequency range (that really shapes
>
>the noise floor more than 1, 2 or 3 above)
>5. noise floor shape caused by #4 is the mirror image of the antenna 
>factor vs frequency
>
>Is that a decent summary?
>.
>Ralph McDiarmid
>Compliance Engineering
>Residential/Commercial
>Solar Business
>Schneider Electric
>D  +1 (604) 422 2622 x62622
>E  ralph.mcdiar...@schneider-electric.com
>3700 Gilmore Way
>Burnaby
>BC
>Canada
>
>-
>
>This message is from the IEEE Product Safety Engineering Society
>emc-pstc discussion list. To post a message to the list, send your
>e-mail to 
>
>All emc-pstc postings are archived and searchable on the web at:
>http://www.ieee-pses.org/emc-pstc.html
>
>Attachments are not permitted but the IEEE PSES Online Communities site
>at http://product-compliance.oc.ieee.org/ can be used for graphics (in
>well-used formats), large files, etc.
>
>Website:  http://www.ieee-pses.org/
>Instructions:  http://www.ieee-pses.org/list.html (including how to
>unsubscribe)
>List rules: http://www.ieee-pses.org/listrules.html
>
>For help, send mail to the list administrators:
>Scott Douglas 
>Mike Cantwell 
>
>For policy questions, send mail to:
>Jim Bacher:  
>David Heald: 

-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.

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Re: [PSES] Spectrum analyzer and noise floor

[Brent DeWitt]

Generally pretty correct Ralph.  The only "picky" point I would have is
about #2.  The pre-amp really doesn't lower the noise floor created by #1.
It raises the signal level further above it.

 

Best of luck,

 

Brent DeWitt

Bose Corporation

Framingham, MA

 

From: McDiarmid, Ralph [mailto:ralph.mcdiar...@schneider-electric.com] 
Sent: Tuesday, February 23, 2016 7:43 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Spectrum analyzer and noise floor

 

I would like to explain to a colleague why the noise floor on a SA does not
look flat as it sweeps across a given frequency range after antenna factors,
cable factors, external gain and external attenuation are programmed into
its display function.   

I think it breaks down to these fundamental points: 

1. the SA receiver has noise in its attenuator, mixer and filter circuits
(say -80 dBm, and maybe flat within a limited  frequency range) 
2. the external amplifier has some noise too, but its gain lowers the noise
floor created by #1  (also flat within a limited frequency range) 
3. the cables have losses which are frequency dependant, and those can be
entered as loss factors into the SA  (shapes the noise floor a little and
those losses raise the noise floor) 
4. the antenna has a gain which is frequency dependant with several dB of
hills and valleys across its usable frequency range (that really shapes the
noise floor more than 1, 2 or 3 above) 
5. noise floor shape caused by #4 is the mirror image of the antenna factor
vs frequency 

Is that a decent summary? 
. 


Ralph McDiarmid
Compliance Engineering
Residential/Commercial
Solar Business
Schneider Electric 

D  +1 (604) 422 2622 x62622
E   <mailto:ralph.mcdiar...@schneider-electric.com>
ralph.mcdiar...@schneider-electric.com 

3700 Gilmore Way
Burnaby
BC
Canada

-


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Re: [PSES] Spectrum analyzer and noise floor

[Ken Javor]

I agree.  A modern machine adjusts the raw data or noise floor for any and
all transducer/amplifier/attenuator factors, and if the transducer factor is
not flat, neither will be the adjusted noise floor.

The only reason we do things this way is an ³embarrassment of riches² in
processing power.  Absent a digital controller, the sane way to take data
would be to adjust the limit for the transducer factors and arrive at an
adjusted limit in terms of dBuV, or dBm.  Consider the number of
computations involved in adjusting a thousand data points across a screen
for the transducer factors, vs. a simple flat or log-linear limit and
transducer factors that only need be reentered at the next frequency at
which they have changed by some set amount from the last frequency, such as
1 dB, or 0.5 dB. The adjusted limit represents orders of magnitude less
computation, plus one can reverse engineer an adjusted limit if one knows
the transducers in use, whereas adding factors to a signal above noise
renders this impossible.

Ken Javor
Phone: (256) 650-5261



From: "McDiarmid, Ralph" <ralph.mcdiar...@schneider-electric.com>
Reply-To: "ralph.mcdiar...@schneider-electric.com"
<ralph.mcdiar...@schneider-electric.com>
Date: Tue, 23 Feb 2016 16:42:47 -0800
To: <EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: Re: [PSES] Spectrum analyzer and noise floor

I would like to explain to a colleague why the noise floor on a SA does not
look flat as it sweeps across a given frequency range after antenna factors,
cable factors, external gain and external attenuation are programmed into
its display function.

I think it breaks down to these fundamental points:

1. the SA receiver has noise in its attenuator, mixer and filter circuits
(say -80 dBm, and maybe flat within a limited  frequency range)
2. the external amplifier has some noise too, but its gain lowers the noise
floor created by #1  (also flat within a limited frequency range)
3. the cables have losses which are frequency dependant, and those can be
entered as loss factors into the SA  (shapes the noise floor a little and
those losses raise the noise floor)
4. the antenna has a gain which is frequency dependant with several dB of
hills and valleys across its usable frequency range (that really shapes the
noise floor more than 1, 2 or 3 above)
5. noise floor shape caused by #4 is the mirror image of the antenna factor
vs frequency 

Is that a decent summary?
. 
Ralph McDiarmid
Compliance Engineering
Residential/Commercial
Solar Business
Schneider Electric D  +1 (604) 422 2622 x62622
E  ralph.mcdiar...@schneider-electric.com
<mailto:ralph.mcdiar...@schneider-electric.com> 3700 Gilmore Way
Burnaby
BC
Canada 
-


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Attachments are not permitted but the IEEE PSES Online Communities site at
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well-used formats), large files, etc.

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Re: [PSES] Spectrum analyzer and noise floor

[McDiarmid, Ralph]

I would like to explain to a colleague why the noise floor on a SA does 
not look flat as it sweeps across a given frequency range after antenna 
factors, cable factors, external gain and external attenuation are 
programmed into its display function. 

I think it breaks down to these fundamental points:

1. the SA receiver has noise in its attenuator, mixer and filter circuits 
(say -80 dBm, and maybe flat within a limited  frequency range)
2. the external amplifier has some noise too, but its gain lowers the 
noise floor created by #1  (also flat within a limited frequency range)
3. the cables have losses which are frequency dependant, and those can be 
entered as loss factors into the SA  (shapes the noise floor a little and 
those losses raise the noise floor)
4. the antenna has a gain which is frequency dependant with several dB of 
hills and valleys across its usable frequency range (that really shapes 
the noise floor more than 1, 2 or 3 above)
5. noise floor shape caused by #4 is the mirror image of the antenna 
factor vs frequency

Is that a decent summary?
.
Ralph McDiarmid
Compliance Engineering
Residential/Commercial
Solar Business
Schneider Electric
D  +1 (604) 422 2622 x62622
E  ralph.mcdiar...@schneider-electric.com
3700 Gilmore Way
Burnaby
BC
Canada

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