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Andy G4JNT
Until recent years we can safely say all received noise on the uWave bands was purely Gaussian, unlike at HF where its can be moe spiky in nature.
With some of our QRN now coming from Cell Phones and other transmissions, is it still the case the noise can be considered Gaussian?
What about VHF / UHF ?
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An interesting question, Andy, and quite complex to answer. What
do we mean by 'noise'?
Gaussian noise is the effect of random (stochastic) processes.
The 'noise' generated by radio systems tends to be cyclic, albeit
sometimes in a rather complex manner, but is still non-stochastic,
particularly after filtering, and is thus not strictly Gaussian!
Professional transmitters - and that includes mobile phone
systems - are designed to very high standards with regard to noise
outputs, and usually have extensive filtering to allow co-siting
of transmitters and receivers. A lot of the 'noise' from other
services detected in amateur systems seems to me to be a function
of reciprocal mixing, and even-order intermod taking place in our
receivers. IMD2 ... is rarely considered in amateur receiver
design.
73
Chris G4DGU
Andy G4JNT wrote:
toggle quoted messageShow quoted text
Until
recent years we can safely say all received noise on the uWave
bands was purely Gaussian, unlike at HF where its can be moe
spiky in nature.
With
some of our QRN now coming from Cell Phones and other
transmissions, is it still the case the noise can
be considered Gaussian?
What
about VHF / UHF ?
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Andy G4JNT
And to add to that, multicarrier systems like OFDM obey all (I think; certainly 'very nearly all') the tests for a Gaussian noise waveform. Even a comb of a large number pure sinusoids does.
The question really came about after reading an article in the latest QEX about noise monitoring and WSPR, where they use two methods of measuring noise, both of which I've adopted in the past. The RMS of the time domain signal, measured during the quiet period of no WSPR transmissions. And the FFT techniques where the bins are put into order of power and noise deemed to be the level in the bin at the 30% point ( I use the lower quartile plus 5dB - thoughts on this differ but the delta is minimal)
In that article they mention that often the two measurement methods give significantly different results, and at other times are very close. This is put-down to the non-Gaussian nature of noise at HF. Hence my thoughts at higher frequencies.
On Wed, 16 Sep 2020 at 12:20, Chris Bartram G4DGU < chris@...> wrote:
An interesting question, Andy, and quite complex to answer. What
do we mean by 'noise'?
Gaussian noise is the effect of random (stochastic) processes.
The 'noise' generated by radio systems tends to be cyclic, albeit
sometimes in a rather complex manner, but is still non-stochastic,
particularly after filtering, and is thus not strictly Gaussian!
Professional transmitters - and that includes mobile phone
systems - are designed to very high standards with regard to noise
outputs, and usually have extensive filtering to allow co-siting
of transmitters and receivers. A lot of the 'noise' from other
services detected in amateur systems seems to me to be a function
of reciprocal mixing, and even-order intermod taking place in our
receivers. IMD2 ... is rarely considered in amateur receiver
design.
73
Chris G4DGU
Andy G4JNT wrote:
Until
recent years we can safely say all received noise on the uWave
bands was purely Gaussian, unlike at HF where its can be moe
spiky in nature.
With
some of our QRN now coming from Cell Phones and other
transmissions, is it still the case the noise can
be considered Gaussian?
What
about VHF / UHF ?
|
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|
Hello again, Andy,
I can certainly see that off-air HF noise could include artefacts
of the non-linear nature of the propagation medium - I'm old
enough to remember the 'Luxembourg Effect' on MW! IMD2 does
nowadays seem to be taken more-or-less seriously, particularly
since the advent of direct sampling receivers. But much VHF and
up equipment still seems to ignore it.
The use of FFT techniques could easily lead to measurement
inaccuracies/uncertainties in a situation where the 'noise'
spectrum was non-stochastic. I may be a bit of a dinosaur, but I
still use a zero-bias diode detector with proven square-law
response, a thermal wattmeter, or alternatively a wideband true
RMS voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are available surplus
at quite low prices, and with a simple downconverter can be used
as a back-end for noise ratio measurements. However that's getting
a bit off-topic.
Chris G4DGU
toggle quoted messageShow quoted text
The
question really came about after reading an
article in the latest QEX about noise monitoring
and WSPR, where they use two methods of
measuring noise, both of which I've adopted in
the past. The RMS of the time domain signal,
measured during the quiet period of no WSPR
transmissions. And the FFT techniques where
the bins are put into order of power and noise
deemed to be the level in the bin at the 30%
point ( I use the lower quartile plus 5dB -
thoughts on this differ but the delta is
minimal)
In
that article they mention that often the two
measurement methods give significantly different
results, and at other times are very close.
This is put-down to the non-Gaussian nature of
noise at HF. Hence my thoughts at higher
frequencies.
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Andy G4JNT
We're talking X-purposes. I'm interested in bandwidths captured by A/D converters, either in a soundcard for narrowish bands or a DS-SDR for wider . That way it's trivial to obtain an RMS value in the time domain; just square and sum every sample in a defined block length, then root the answer. Result, exact RMS of that set of samples. The QEX article does it in blocks of 50ms, then chooses the lowest sum to ensure the highest probability of getting a block with no spurious signals
For the FFT technique, do an FFT on a block, length suited to the frequency resolution of interest and convert the complex number in each bin to a power with Pythagorus. Then do the ordering and statistics on the result. That way any signals present get shoved up to the top of the ordered set and can be thrown away. Practical measuring kit: A thermal wattmeter will give you true RMS (well, power really) but averaging and time depends on the thermal mass of the detector and any electronic averaging put in later. I have a skepticism about "true RMS" meters. They are pretty universally designed for 50Hz mains, so I suspect their accuracy at audio frequencies until proven otherwise - they probably are OK, but they use an approximation / feedback method to measure RMS, relying on averaging in a capacitor to set a block / time constant for the RMS. Anyone recall the old Analog Devices AD590? That's always the trouble with RMS measurement - when is a time period part of the RMS and when is it a varying quantity? And if a Cap is used in the averaging, where and how is the transition. (And why is a DC component never included in RMS measurement in any real kit) Diode detectors may well give you a nice relative value for Sun and Moon peaking purposes, but they drift with temperature, DC offset etc. Also, if they're calibrated using a carrier then used with noise, there is at least 3:1 ratio for 99% of the noise peak to RMS. The detector has to remain properly square law to well over 10dB more dynamic range than the thing you're measuring. OK for Moon noise, but when it comes to sun noise with a large dish and good LNA, you end up needing over 25dB of perfect SQL linearity to make a really accurate measurement. I remember that problem playing with the FRARS EME system. The diode detector was a bit suspicious on sun noise measurements. I believe they've moved to SDR-IQ continuum measurement now As you may gather - I've become quite a geek when it comes to measuring noise and S/N accurately - it's not an easy subject.
On Wed, 16 Sep 2020 at 14:06, Chris Bartram G4DGU < chris@...> wrote:
Hello again, Andy,
I can certainly see that off-air HF noise could include artefacts
of the non-linear nature of the propagation medium - I'm old
enough to remember the 'Luxembourg Effect' on MW! IMD2 does
nowadays seem to be taken more-or-less seriously, particularly
since the advent of direct sampling receivers. But much VHF and
up equipment still seems to ignore it.
The use of FFT techniques could easily lead to measurement
inaccuracies/uncertainties in a situation where the 'noise'
spectrum was non-stochastic. I may be a bit of a dinosaur, but I
still use a zero-bias diode detector with proven square-law
response, a thermal wattmeter, or alternatively a wideband true
RMS voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are available surplus
at quite low prices, and with a simple downconverter can be used
as a back-end for noise ratio measurements. However that's getting
a bit off-topic.
Chris G4DGU
The
question really came about after reading an
article in the latest QEX about noise monitoring
and WSPR, where they use two methods of
measuring noise, both of which I've adopted in
the past. The RMS of the time domain signal,
measured during the quiet period of no WSPR
transmissions. And the FFT techniques where
the bins are put into order of power and noise
deemed to be the level in the bin at the 30%
point ( I use the lower quartile plus 5dB -
thoughts on this differ but the delta is
minimal)
In
that article they mention that often the two
measurement methods give significantly different
results, and at other times are very close.
This is put-down to the non-Gaussian nature of
noise at HF. Hence my thoughts at higher
frequencies.
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G4RFR EME on 10GHz uses a 10GHz HP Spec Ann , on the 618MHz i.f. output of an Octagon LNB ; SDR14 running Spectraview , on a 28MHz down-converted , Continuum Mode and a G4JNT 144MHz diode detector noise meter system ( 2MHz filter bandwidth) .All downconversion stages use GPS referenced LO s .Typical Moon surface noise 1.8dB , Sun at 15.5dB .Echoes with 200W at feed "readable SSB" throughout the Lunar cycle . Hope to be QRV again soon .
73 John G0API
We're talking X-purposes. I'm interested in bandwidths captured by A/D converters, either in a soundcard for narrowish bands or a DS-SDR for wider . That way it's trivial to obtain an RMS value in the time domain; just square and sum every sample in a defined block length, then root the answer. Result, exact RMS of that set of samples. The QEX article does it in blocks of 50ms, then chooses the lowest sum to ensure the highest probability of getting a block with no spurious signals
For the FFT technique, do an FFT on a block, length suited to the frequency resolution of interest and convert the complex number in each bin to a power with Pythagorus. Then do the ordering and statistics on the result. That way any signals present get shoved up to the top of the ordered set and can be thrown away. Practical measuring kit: A thermal wattmeter will give you true RMS (well, power really) but averaging and time depends on the thermal mass of the detector and any electronic averaging put in later. I have a skepticism about "true RMS" meters. They are pretty universally designed for 50Hz mains, so I suspect their accuracy at audio frequencies until proven otherwise - they probably are OK, but they use an approximation / feedback method to measure RMS, relying on averaging in a capacitor to set a block / time constant for the RMS. Anyone recall the old Analog Devices AD590? That's always the trouble with RMS measurement - when is a time period part of the RMS and when is it a varying quantity? And if a Cap is used in the averaging, where and how is the transition. (And why is a DC component never included in RMS measurement in any real kit) Diode detectors may well give you a nice relative value for Sun and Moon peaking purposes, but they drift with temperature, DC offset etc. Also, if they're calibrated using a carrier then used with noise, there is at least 3:1 ratio for 99% of the noise peak to RMS. The detector has to remain properly square law to well over 10dB more dynamic range than the thing you're measuring. OK for Moon noise, but when it comes to sun noise with a large dish and good LNA, you end up needing over 25dB of perfect SQL linearity to make a really accurate measurement. I remember that problem playing with the FRARS EME system. The diode detector was a bit suspicious on sun noise measurements. I believe they've moved to SDR-IQ continuum measurement now As you may gather - I've become quite a geek when it comes to measuring noise and S/N accurately - it's not an easy subject. On Wed, 16 Sep 2020 at 14:06, Chris Bartram G4DGU < chris@...> wrote:
Hello again, Andy,
I can certainly see that off-air HF noise could include artefacts
of the non-linear nature of the propagation medium - I'm old
enough to remember the 'Luxembourg Effect' on MW! IMD2 does
nowadays seem to be taken more-or-less seriously, particularly
since the advent of direct sampling receivers. But much VHF and
up equipment still seems to ignore it.
The use of FFT techniques could easily lead to measurement
inaccuracies/uncertainties in a situation where the 'noise'
spectrum was non-stochastic. I may be a bit of a dinosaur, but I
still use a zero-bias diode detector with proven square-law
response, a thermal wattmeter, or alternatively a wideband true
RMS voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are available surplus
at quite low prices, and with a simple downconverter can be used
as a back-end for noise ratio measurements. However that's getting
a bit off-topic.
Chris G4DGU
The
question really came about after reading an
article in the latest QEX about noise monitoring
and WSPR, where they use two methods of
measuring noise, both of which I've adopted in
the past. The RMS of the time domain signal,
measured during the quiet period of no WSPR
transmissions. And the FFT techniques where
the bins are put into order of power and noise
deemed to be the level in the bin at the 30%
point ( I use the lower quartile plus 5dB -
thoughts on this differ but the delta is
minimal)
In
that article they mention that often the two
measurement methods give significantly different
results, and at other times are very close.
This is put-down to the non-Gaussian nature of
noise at HF. Hence my thoughts at higher
frequencies.
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Hello again, Andy,
Yes we are talking cross purposes!
Firstly, I don't disagree with your comments about doing things
with an FFT. The maths is maths, and for some kinds of measurement
it is absolutely the right approach. But it is, as usual in
engineering, a matter of horses for courses.
However, I do disagree with your comments about RMS voltage
measurement instruments. The units I quoted are not 'true RMS'
multimeters' in the sense that it is used by the manufacturers of
general purpose multimeters. I've been caught by their limited
bandwidths in the past. The Racal Dana 9300 is a 20MHz BW RMS RF
voltmeter, while the 9303 has a bandwidth of 2GHz. I have one of
each on my shelves.
I did comment that I was talking about noise power measurements,
but perhaps I should have included the word relative in that.
I agree that simple uncompensated diode detectors can, nay will,
drift when making level measurements, but their simplicity and
wide bandwidth have obvious attractions, particularly for amateur
microwave use. Temperature compensation isn't exactly rocket
science, either. Extending the measurement range is easily
achieved using calibrated attenuators. My approach for noise level
difference measurements is to use a calibrated 1dB step
attenuator to set the larger noise voltage incident on the diode
to within about 3dB of the smaller, and to read the delta from the
detector. And yes, I have measured Sun noise on my old 10GHz dish
in excess of 20dB using that technique, albeit on a day when the
Sun was rather active!
Do I remember the AD590? I certainly remember using an AD device
in a couple of work projects in the late '80s/early '90s which
involved the measurement of transmission losses and noise levels
at VLF of 2-wire balanced transmission lines lying on the ground
with a spacing of 4ft 8 1/2inches ... That may have used a thermal
converter IIRC. I also looked at some chips at that time which
used analogue multiplication, but they weren't really suitable.
I think I'd agree with you wholeheartedly that noise measurement
is something of an art, and getting good accuracy requires
attention to detail.
Chris G4DGU
toggle quoted messageShow quoted text
On 16/09/2020 15:54, Andy G4JNT wrote:
We're
talking X-purposes. I'm interested in bandwidths captured
by A/D converters, either in a soundcard for narrowish bands
or a DS-SDR for wider . That way it's trivial to obtain an
RMS value in the time domain; just square and sum every
sample in a defined block length, then root the answer.
Result, exact RMS of that set of samples. The QEX article
does it in blocks of 50ms, then chooses the lowest sum to
ensure the highest probability of getting a block
with no spurious signals
For
the FFT technique, do an FFT on a block, length suited to
the frequency resolution of interest and convert the complex
number in each bin to a power with Pythagorus. Then do the
ordering and statistics on the result. That way any
signals present get shoved up to the top of the ordered set
and can be thrown away.
Practical
measuring kit:
A
thermal wattmeter will give you true RMS (well, power
really) but averaging and time depends on the thermal mass
of the detector and any electronic averaging put in later.
I have a skepticism about "true RMS" meters. They are
pretty universally designed for 50Hz mains, so I suspect
their accuracy at audio frequencies until proven otherwise -
they probably are OK, but they use an approximation /
feedback method to measure RMS, relying on averaging in a
capacitor to set a block / time constant for the RMS.
Anyone recall the old Analog Devices AD590?
That's
always the trouble with RMS
measurement - when is a time period
part of the RMS and when is it a
varying quantity? And if a Cap is
used in the averaging, where and how
is the transition. (And why is a DC
component never included in RMS
measurement in any real kit)
Diode
detectors may well give you a nice
relative value for Sun and Moon
peaking purposes, but they drift with
temperature, DC offset etc. Also, if
they're calibrated using a carrier
then used with noise, there is at
least 3:1 ratio for 99% of the noise
peak to RMS. The detector has to
remain properly square law to well
over 10dB more dynamic range than the
thing you're measuring. OK for Moon
noise, but when it comes to sun noise
with a large dish and good LNA, you
end up needing over 25dB of perfect
SQL linearity to make a really
accurate measurement. I remember that
problem playing with the
FRARS EME system. The diode detector
was a bit suspicious on sun noise
measurements. I believe they've
moved to SDR-IQ continuum measurement
now
As
you may gather - I've become quite a
geek when it comes to measuring noise
and S/N accurately - it's not an easy
subject.
On Wed, 16 Sep 2020 at
14:06, Chris Bartram G4DGU < chris@...>
wrote:
Hello again, Andy,
I can certainly see that off-air HF noise could include
artefacts of the non-linear nature of the propagation
medium - I'm old enough to remember the 'Luxembourg
Effect' on MW! IMD2 does nowadays seem to be taken
more-or-less seriously, particularly since the advent
of direct sampling receivers. But much VHF and up
equipment still seems to ignore it.
The use of FFT techniques could easily lead to
measurement inaccuracies/uncertainties in a situation
where the 'noise' spectrum was non-stochastic. I may be
a bit of a dinosaur, but I still use a zero-bias diode
detector with proven square-law response, a thermal
wattmeter, or alternatively a wideband true RMS
voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are
available surplus at quite low prices, and with a simple
downconverter can be used as a back-end for noise ratio
measurements. However that's getting a bit off-topic.
Chris G4DGU
The
question really came about
after reading an article in the latest
QEX about noise monitoring and WSPR,
where they use two methods of
measuring noise, both of which I've
adopted in the past. The RMS of the
time domain signal, measured during
the quiet period of no WSPR
transmissions. And the FFT techniques
where the bins are put into order
of power and noise deemed to be the
level in the bin at the 30% point ( I
use the lower quartile plus 5dB -
thoughts on this differ but the delta
is minimal)
In
that article they mention that often
the two measurement methods give
significantly different results, and
at other times are very close. This
is put-down to the non-Gaussian nature
of noise at HF. Hence my thoughts
at higher frequencies.
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