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.

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.

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.

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.

Owen

Dear Owen:
Others too may remember the use of a noise source comprising a gas tube
crossways in a piece of waveguide (with 50 ohm probes) to estimate noise
figure in the VT days. One would fire the gas tube and it was estimated to
have a very large, "known" noise temp. Boiling water, while a known temp.,
would not have been hot enough.

Ice water was critical in the use of the HILLMS receiver used to measure
absolute flux. The antenna was a very long horn antenna resting on the side
of a deep gully. In those days, a horn antenna was one of the few antennas
with a predictable gain. The receiver switched at a low frequency between
the antenna and a load kept in ice water. The gain of the receiver was
stabilized with a huge amount of negative feedback. Once a day, the source
would pass through the antenna's beam and a strip chart recorder would
indicate the difference between ice water and the source's temp.

Today, with much lower NF, and much more EM pollution, different techniques
might be used.
73 Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Owen Duffy" wrote in message


Dave Oldridge wrote in



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.

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.

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.

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.

Owen

Owen Duffy wrote:
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.

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.

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.

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.

Owen

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
resistor that you're plunging into hot and cold, its resistance will
almost certainly change. At microwave frequencies, a more common
technique for radiometers is to use a flat plate absorber that has been
characterized for changes in absorption over temperature.

One might want to take a look at how NIST does this kind of thing.
Here's the slides from a talk by Jim Randa
http://www.boulder.nist.gov/div818/8...t%20Crs_06.pdf

he's a noise measurement guru at NIST.. check out the website:

http://www.boulder.nist.gov/div818/81801/Noise/



I've had a precision noise source (used to do Y factor measurements on a
precision 13.402 GHz receiver) measured in their lab over a week. The
measurement uncertainty (for a system with waveguide connections) was in
the few Kelvins range (out of a noise power of 7000K or so), and the
connect/reconnect uncertainty dominates. I doubt one could get this kind
of performance with a coaxial connector (the uncertainty in the mismatch).


By the way, a good noise diode source is probably a better standard for
the hot side than heating a resistor. They're very, very stable over
time, once calibrated, and if properly designed, have a very stable
match as the noise is turned on and off. (that's what we were using in
the above system, a temperature controlled Noise/Com style source).

http://www.boulder.nist.gov/div818/8...ability_IM.pdf
http://www.boulder.nist.gov/div818/8...ility_CPEM.pdf

describes the performance


Another useful link might be:
http://www.boulder.nist.gov/div818/8...97_Amps_IM.pdf
D.F. Wait, J. Randa, "Amplifier Noise Measurements at NIST", IEEE Trans
on Inst. and Meas., v.46, n.2, Apr 1997

They give measurement uncertainties of 0.04dB on a 0.5 dB NF for 2-4 GHz..

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