Jim Lux wrote in
:

....
Indeed, this would be a very challenging measurement, because you also
have to take into account the match of that load, and if it's just a

Jim, an important point, and more generally on the mismatch issue...

It seems that many measuring sun noise rise prefer to measure the rise by
attenuator substitution.

Though it seems a simple and sound method of measurement, the effects of
mismatch need to be considered, not only on the power delivered to the
receiver chain, but also the noise figure of the device with the changing
input or output loads. The effects are not necessarily easy to quantify,
which makes the apparently simple method a bit of a trap.

Owen

On Mon, 27 Aug 2007 23:02:06 GMT, Owen Duffy wrote:

Indeed, this would be a very challenging measurement, because you also
have to take into account the match of that load, and if it's just a

Jim, an important point, and more generally on the mismatch issue...

Hi All,

Given the notoriety that follows discussion about the Real component
of the transmitter's source Z....

Let's see, How do I measure thee, let me count the ways. Anyone want
to venture a guess on the value of the Real component of the receivers
load Z? (And then we game this into the S-9 50µV across those myriad
resistors.)

73's
Richard Clark, KB7QHC

Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:

Dave Oldridge wrote in
9:

Near as I could measure it, the NF of the receiver after my mod was
1.2db. I had to resort to boiling and freezing water and a tiny dummy
load to measure it at all.

I haven't tried hot/cold tests using ice and boiling water, I didn't
think it was practical.

Only just and you need a good 4-digit or better AC voltmeter to do it at
all. I wasn't after accuracy, just a ball-park estimate and I know I got
it fairly close because the receiver did show a marked increase in noise
when any decent antenna was connected.

You finally measured a receiver noise temperature of 50K with hot and
cold loads of 270 and 370.

That means a Y factor of 1.059dB.

If Y were just 0.1dB greater, NF would be 0.78dB, 0.1dB lower and, NF
would be 1.66dB.

Yep...the most I'd be willing to commit to with that measurement would be
that it was below around 2.5 and PROBABLY fairly close to my measurement.
I measured the voltages alternately 25 times and took a mean to try to
smooth out the errors.

With this configuration the sensitivity of NF to changes in Y are
extreme, 0.4dB change in NF per 0.1dB change in Y around that point.

If you made the Y measurements using the audio output of a narrow band
receiver, it is very hard to make high resolution measurements (eg to
0.01dB resolution) with say, a multimeter.

It is. You need a good AC voltmeter with decent digital accuracy and
resolution and you have to average a bunch of readings.

I have done these tests with a liquid nitrogen cooled load and room
temperature load, and that gives more practical Y ratios, 3.7dB for a
1.2dBNF, and the sensitivity in NF is 0.08dB per 0.1dB change in Y.
This
still demands high resolution measurement of noise power.

Yes, anything less than 4 digits is just about useless.

--
Dave Oldridge+
ICQ 1800667

Dave Oldridge wrote in
9:

Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:
....
If you made the Y measurements using the audio output of a narrow
band receiver, it is very hard to make high resolution measurements
(eg to 0.01dB resolution) with say, a multimeter.

It is. You need a good AC voltmeter with decent digital accuracy and
resolution and you have to average a bunch of readings.

I put some notes together on a perspective of the noise measurement
(sampling) process and its statistical uncertainty, they are at
http://www.vk1od.net/fsm/nmu.htm .

It is my experience that a digital voltmeter probably samples for
something around 100ms, and with a 2kHz wide noise bandwidth, you might
expect an uncertainty of near 0.5dB at the 90% confidence level. Just
watch the readings bounce around.

Sure, if you average 100 of those measurements (actually, you should get
the root of the sum of the squares... because it is power you average,
not voltage), you might reduce that uncertainty to around 0.05dB... but
it is not a very practical manual method, if recording accuracy (ie
writing down the wrong number) doesn't get you, environmental drift will.

Owen

Dave Oldridge wrote:
Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:


Dave Oldridge wrote in
. 159:


Near as I could measure it, the NF of the receiver after my mod was
1.2db. I had to resort to boiling and freezing water and a tiny dummy
load to measure it at all.

snip


This

still demands high resolution measurement of noise power.


Yes, anything less than 4 digits is just about useless.

That would be necessary but not sufficient.

I suspect that other aspects of the measurement introduce greater
uncertainty than the voltmeter. For instance, do you know the
reflection coefficient of the load to 4 digits? (that would be knowing
Z to about 0.1 ohm, at the RF frequency of interest), and is it stable
with temperature to that level?

For instance, a high quality load from Maury Microwave (a 2610F ) is
specified to have a VSWR of 1.005 from DC-1GHz, which is a reflection
coefficient of 0.0025. But that's only at 25C.

An Agilent metrology grade cal kit with N connectors specifies
rho0.00398 for the lowband loads, but only within 1 degree of the
specified temperature.
See, for example:
http://cp.literature.agilent.com/lit...5054-90049.pdf

A good thinfilm resistor might have a tempco of 5 ppm, with metal film
being around 50 ppm, and thick film more in the 200 ppm area. For a 100
degree change, that's a 500 ppm (for the thin film) or a reflection
coefficient change of 0.00025. Clearly one doesn't want to use any old
resistor for the calibration load here.


Measuring RF power to an accuracy of 1% is challenging. Your system is
measuring a change in noise power of 100K out of 300K, roughly, so
you've got a 30% change in noise power into the system.

The Y-factor method essentially plots two points (one at 273K another at
373K, if you're using ice and boiling water), and then calculates the
intercept at 0K. Since zeroK is about 3 times farther away than the
measurement's width, errors in the measurement are roughly tripled at
the intercept, and then doubled because you're using two measurements,
so an error of 1% in the power measurement leads to about 5% error in
the NF (if you're around 100K) (and this also applies if you have
consistent errors.. say both power measurements are 1% high, the NF will
come back as 105K instead of 100K).

A 5% measurement uncertainty for power (0.2dB) gets you about 25-30%
uncertainty in NF.

The best way to improve the accuracy is to push the low temperature
lower (e.g. with dry ice (195K) or LN2 (77K)), but that, of course,
aggravates the change in reflection coefficient of your load with
temperature.

Jim Lux wrote in news:46D463CF.1080309
@jpl.nasa.gov:

Dave Oldridge wrote:
Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:


Dave Oldridge wrote in
.159:


Near as I could measure it, the NF of the receiver after my mod was
1.2db. I had to resort to boiling and freezing water and a tiny
dummy
load to measure it at all.

snip


This

still demands high resolution measurement of noise power.


Yes, anything less than 4 digits is just about useless.

That would be necessary but not sufficient.
....

Just following through on the '4 digit' issue...

I have done two series of 250 measurements of audio noise voltage from a
SSB receiver using two different digital multimeters, the 9932 is a
modern digital multimeter that is NOT true RMS responding, and the 506 is
a modern digital multimeter that is true RMS responding with bandwidth
adequate to cover the receiver output response.

From observation with a stopwatch, I estimate that the 9932 updates 3
times per second, and the 506 updates 2 times per second. The integration
times are probably .33 and .5 seconds respectively.

I have measured the receiver equivalent noise bandwidth and it is 1600Hz.

95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for the
506. These observations reconcile well with my Chi-squared based estimate
of the uncertainty that I referred to in an earlier post.

As for the number of digits, they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV resolution,
so the representational error is 0.04dB. The error due to the number of
digits in this downscale three digit application is insignificant
compared to the sampling error of 0.4dB and 0.3dB.

Graphically, the distributions are shown at
http://www.vk1od.net/nfm/temp.gif .

Different meters with different integration times, and different
receivers with different noise bandwidth will result in different
outcomes, but I argue that the uncertainty is predictable.

Owen

On Thu, 30 Aug 2007 10:11:22 GMT, Owen Duffy wrote:

they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV resolution,

Hi Owen,

The convention for decades has been to describe them as 3½ Digits, or
2000 count, not 4 digit unless they could represent 9999. Adding
digits does not generally add precision, resolution, monotonicity, or
accuracy. However, as it costs money to add a digit, the underlying
circuitry could usually support "some" of these attributes. Better
instruments perform rounding after the last digit.

73's
Richard Clark, KB7QHC

Owen Duffy wrote:
Jim Lux wrote in news:46D463CF.1080309
@jpl.nasa.gov:


Dave Oldridge wrote:

Owen Duffy wrote in news:Xns999955EE72868nonenowhere@
61.9.191.5:



Dave Oldridge wrote in
5.159:



Near as I could measure it, the NF of the receiver after my mod was
1.2db. I had to resort to boiling and freezing water and a tiny

dummy

load to measure it at all.

snip

This


still demands high resolution measurement of noise power.


Yes, anything less than 4 digits is just about useless.

That would be necessary but not sufficient.

...

Just following through on the '4 digit' issue...

I have done two series of 250 measurements of audio noise voltage from a
SSB receiver using two different digital multimeters, the 9932 is a
modern digital multimeter that is NOT true RMS responding, and the 506 is
a modern digital multimeter that is true RMS responding with bandwidth
adequate to cover the receiver output response.

From observation with a stopwatch, I estimate that the 9932 updates 3
times per second, and the 506 updates 2 times per second. The integration
times are probably .33 and .5 seconds respectively.

I have measured the receiver equivalent noise bandwidth and it is 1600Hz.

95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for the
506. These observations reconcile well with my Chi-squared based estimate
of the uncertainty that I referred to in an earlier post.

As for the number of digits, they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV resolution,
so the representational error is 0.04dB.


Gotta be a bit careful there, because quantization error has a uniform
distribution, so the variance is 1/12 of the span. This is different
than the (presumably) normally distributed actual measurands.

When giving an uncertainty (sampling error), one should also say whether
it's a one sigma, two sigma, or 3 sigma number. *Standard uncertainty*
is 1 sigma... *expanded uncertainty*, often given as a +/- number is
usually the 95% percent confidence interval, which, for normal
distributions, is 2 sigma


given your statistics above, you would be giving the expanded
uncertainty as 0.41dB

By the way, unless your device actually directly measures dB (e.g. it
has a log detector) or the errors are inherently ratios, it's probably
better to give the value in a linear scale (milliwatts?) with the
uncertainty in the same units. That gets you around the "ratio" problem
where log(1+delta) -log(1-delta)

http://physics.nist.gov/cuu/Uncertainty/index.html has the simple
explanation, and the technical note (TN1297) , and references to the ISO
Guide to Expression of Uncertainty in Measurment (aka the GUM)


The error due to the number of
digits in this downscale three digit application is insignificant
compared to the sampling error of 0.4dB and 0.3dB.

Graphically, the distributions are shown at
http://www.vk1od.net/nfm/temp.gif .

Different meters with different integration times, and different
receivers with different noise bandwidth will result in different
outcomes, but I argue that the uncertainty is predictable.

Indeed, it is.


Owen

Jim Lux wrote in
:

Owen Duffy wrote:

...

Just following through on the '4 digit' issue...

I have done two series of 250 measurements of audio noise voltage
from a SSB receiver using two different digital multimeters, the 9932
is a modern digital multimeter that is NOT true RMS responding, and
the 506 is a modern digital multimeter that is true RMS responding
with bandwidth adequate to cover the receiver output response.

From observation with a stopwatch, I estimate that the 9932 updates 3
times per second, and the 506 updates 2 times per second. The
integration times are probably .33 and .5 seconds respectively.

I have measured the receiver equivalent noise bandwidth and it is
1600Hz.

95% of 250 readings were within 0.41dB for the 9932 and 0.31dB for
the 506. These observations reconcile well with my Chi-squared based
estimate of the uncertainty that I referred to in an earlier post.

As for the number of digits, they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV
resolution, so the representational error is 0.04dB.


Gotta be a bit careful there, because quantization error has a uniform
distribution, so the variance is 1/12 of the span. This is different
than the (presumably) normally distributed actual measurands.

Ok, point taken. I think more correctly, the maximum error would be 20
*log(1+1/200/2) or 0.0217dB.

The expected error due to representation in three digits does not account
for the variation in measurements.


When giving an uncertainty (sampling error), one should also say
whether it's a one sigma, two sigma, or 3 sigma number. *Standard
uncertainty* is 1 sigma... *expanded uncertainty*, often given as a
+/- number is usually the 95% percent confidence interval, which, for
normal distributions, is 2 sigma

Whilst it might be reasonable to assume that the combined error in
measurement of a high S/N sine wave voltage might be normally
distributed, and that might also be true of measurement of noise voltage
in some circumstances, I propose that measurement of noise power in
narrow bandwidth with short integration times is distributed as Chi-
squared and the number of samples becomes relevant in determining the
number of degrees of freedom for the distribution. For this reason, I
have talked about a confidence level rather than sigma (which is more
applicable to normally distributed data).

Just for interest, in the case of the 9932 measurement set:

Average=0.201, sigma=0.0046, 1sigma based uncertainty estimate=0.20dB,
2sigma based uncertainty estimate=0.41dB, 3sigma based uncertainty
estimate=0.62dB.



given your statistics above, you would be giving the expanded
uncertainty as 0.41dB

I stated it as 95% of obs within 0.41, I should have said 95% of obs
within +/-0.41, I was explicit about the implied confidence, the 95%
doesn't equate to either the 1sigma or 3sigma confidence, it is very
close to the 2sigma confidence (95.45%), and it is at the high confidence
end of the scale.


By the way, unless your device actually directly measures dB (e.g. it
has a log detector) or the errors are inherently ratios, it's probably
better to give the value in a linear scale (milliwatts?) with the
uncertainty in the same units. That gets you around the "ratio"
problem where log(1+delta) -log(1-delta)

I understand what you mean in your last sentence.

I did record the voltage, and converted the values to dB for analysis.
The interval was calculated by taking the average of the 2.5 percentile
and 97.5 percentile, which is an approximation, but as such small values
is pretty close.

I have converted results to dB to make it easier to see the relevance of
the error or uncertainty, but in so doing, another (small in this case)
error is introduced.


http://physics.nist.gov/cuu/Uncertainty/index.html has the simple
explanation, and the technical note (TN1297) , and references to the
ISO Guide to Expression of Uncertainty in Measurment (aka the GUM)

In terms of the above, I am proposing that measurements of narrowband
noise with short integration time is not strictly normally distributed,
and an estimate of its uncertainty to a given confidence level can be
obtained from the Chi-square distribution.

One could not estimate the results of the test from knowledge of the
instrument accuracy (inherent and representational error) alone.

I think the experiment supports the proposition that digital multimeters
with typically short integration times do not deliver high resolution
measurement of narrow band (eg SSB telephony) noise.

The error due to the number of
digits in this downscale three digit application is insignificant
compared to the sampling error of 0.4dB and 0.3dB.

Graphically, the distributions are shown at
http://www.vk1od.net/nfm/temp.gif .

Different meters with different integration times, and different
receivers with different noise bandwidth will result in different
outcomes, but I argue that the uncertainty is predictable.

Indeed, it is.

Thanks, appreciate the comments.

Owen

Richard Clark wrote in
:

On Thu, 30 Aug 2007 10:11:22 GMT, Owen Duffy wrote:

they are both 4 digit multimeters which
doesn't mean a lot. They were used to measure 200mV with 1mV
resolution,

Hi Owen,

The convention for decades has been to describe them as 3½ Digits, or
2000 count, not 4 digit unless they could represent 9999. Adding
digits does not generally add precision, resolution, monotonicity, or
accuracy. However, as it costs money to add a digit, the underlying
circuitry could usually support "some" of these attributes. Better
instruments perform rounding after the last digit.

Hi Richard,

It is interesting in marketing hype that reference is made to 2 digit and
3 digit instruments, which implies a log based metric (10*log
(MaxReading)) when you assume a 'full count', and the same hype refers to
the upper digit if it can only have values of 0 or 1 as half a digit,
whereas it probably has a weight of log(0.5) or 0.3... so in utility
terms, a 2 1/2 digit instrument is really a 2.3 digit instrument.

In my case, I was making the measurements straddling 200mV, so I needed a
bit of headroom for outliers, say 1dB or 225mV fsd, so it was effectively
2.35 digit instrument if you followed that argument.

Nevertheless, the error introduced by the resolution issue and instrument
accuracy does not explain the experimental results... something else is
happening, and one needs to look beyond the instrument itself to form a
realistic view of measurement uncertainty when measuring narrowband
noise.

Owen