Steve,

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

An anecdotal story for you...

About four years ago (subsequently laid off :-( ) I was working for a
company that was making a virtual (that is, used digital signal processing)
RF/Microwave measurement system. One of our customers was a Major
communications satellite builder. They were complaining that our system was
not working right for measuring noise figure. I knew it was, of course,
since I had written the software and thoroughly tested it :-). I used two
different manual methods, one of which is exactly what you are doing (except
for using acalibrated noise source) and got the same answer as my software.
Turns out that they (the Major satellite builder) didn't know how to
use their nosie figure meter!

Jim
N8EE

"Steve Kavanagh" wrote in message
om...
I've been playing with trying to make rough noise figure measurements
on the cheap and have a couple of questions:

(1) Are there any issues with the following setup and procedure for
making relative noise figure measurements (e.g. comparing two
receivers) ?

____________ __________ ________
| | | | | |
|Uncalibrated| | Step | |Receiver| Audio Out
| Noise |----|Attenuator|------| Under |-------*----o (0 dB)
| Source | | | | Test | _|_
|____________| |__________| |________| | |
| | 2.7k
|_|
|
*----o (-3 dB)
_|_
| |
| | 6.65k
|_|
_|_
///
Procedu
(a) For receiver 1 connect a high impedance AC voltmeter to the audio
output marked "0 dB". Record voltage with noise source off.
(b) Turn noise source on and measure AC voltage at "-3 dB" output.
Adjust step attenuator to get same voltage as in step (a). Record
step attenuator setting.
(c) Repeat steps (a) and (b) for receiver 2.
(d) The difference in noise figure between the two receivers is the
same as the difference in attenuator settings recorded in (b) and (c).
For example if the attenuation for receiver 1 is 10 dB and for
receiver 2 is 12 dB, then receiver two has a noise figure which is 2
dB less than that of receiver 1.

Assuming this is OK we move on to question 2:

(2) To avoid the expense of a calibrated noise source, I wonder if the
repeatability from unit to unit of simple low noise amplifier circuits
(perhaps a MAR-6 ?) is good enough to allow one to be used as a noise
figure standard, at least for fairly rough measurements at HF and VHF.
I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
Has anyone measured the NF performance of simple MMIC amps at HF & VHF
? Or looked into noise figure repeatability ?

73,
Steve VE3SMA

"Jim" wrote in message ...

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

73,
Steve VE3SMA

Steve Kavanagh wrote:
"Jim" wrote in message ...

It has been a couple of years since I have done a noise figure measurement
so my brain may be a bit rusty, but I don't see anything wrong with this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the 'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

You might find Terry Ritter's work on getting a good noise source to
be of at least a bit (ahem!) of interest:

http://www.ciphersbyritter.com/NEWS5/FMRNG.HTM

There are lots of other hits in Google for a search on
'calibrated "pseudo random" noise'
(without the outer single quotes).

It's hard to generate good noise, and at least as hard to find it.

--
Mike Andrews

Tired old sysadmin

Mike Andrews wrote:

[...]

You might find Terry Ritter's work on getting a good noise source to
be of at least a bit (ahem!) of interest:

http://www.ciphersbyritter.com/NEWS5/FMRNG.HTM

There are lots of other hits in Google for a search on
'calibrated "pseudo random" noise'
(without the outer single quotes).

It's hard to generate good noise, and at least as hard to find it.

--
Mike Andrews

Tired old sysadmin

Also take a look at Terry's analysis of various other noise sources:

http://www.ciphersbyritter.com/NOISE/NOISRC.HTM

Good noise is hard to find

Mike Monett

"Steve Kavanagh" wrote in message
om...
"Jim" wrote in message
...

It has been a couple of years since I have done a noise figure
measurement
so my brain may be a bit rusty, but I don't see anything wrong with
this.

Thanks, Jim. I thought I had it right but did want a check from
someone with more experience in this field.

In fact, if you had a calibrated noise source, this is one of the
'official'
methods of measuring noise figure.

Ahhh...and therein lies the real problem for most hams. It just
occurred to me that there might be another solution to this (at least
at HF), which could be to generate noise at an accurately calibrated
level (as accurate as the power supply voltage) using a pseudo-random
digital signal.

73,
Steve VE3SMA

Sorry about the double post of my response. I had a major problem with my
hard drive here and had to reload Windows XP from scratch. Evidently
Outlook Express (my news reader) burped the first time I used it.


Anyway....

As other's have said, there are ways to build a calibrated noise source.
I've even seen zener diodes and just plain old switching diodes used. Just
do a web search. You may even be able to find a commercial one at a flea
market, but I would question its quality.

What frequency are you using? One thing to keep in mind is that any loss or
mismatch can affect your measurement. We (when I was doing the software)
were operating at up to 32 GHz. At that frequency microscopic burs on the
sub miniature coax connectors caused all kinds of headaches!.

Jim
N8EE

"Jim" wrote in message ...

As other's have said, there are ways to build a calibrated noise source.
I've even seen zener diodes and just plain old switching diodes used.

I use a 1N21 (in reverse breakdown) as an uncalibrated source up to
about 5 GHz. A friend uses a 1N23 to 10 GHz. But how can it be
calibrated without using professional test gear ? I don't know.

What frequency are you using?

Anywhere from HF to 24 GHz is of interest ! I have used the relative
noise figure measurement scheme between 3.5 & 903 MHz so far.

One thing to keep in mind is that any loss or
mismatch can affect your measurement.

Indeed...I assume that the use of a reasonable minimum attenuation in
the step attenuator will minimize the effects of receiver input
mismatch on the measurement accuracy (assuming the attenuator is
itself well matched). I rather doubt that I would be able to homebrew
an accurate measurement system at 24 GHz (or even 10 GHz). But
construction tolerance issues should not be a problem at HF or (with
care) at VHF.

73,
Steve VE3SMA

I am hoping that accuracies of +/- 1 to 2 dB might be achievable.
If you are satisfied with +/- 1 to 2 dB variation, the link I gave to
the Mar MMIC 35dB "calibrated" Noise generator should be
accurate enough.
That why I "forgot" the approximatly statement.

You can not compare noise power with a calibrated sine wave generator.
That's right, not directly.
You can calculate the RMS for a sine wave.
Also calculate the RMS for noise power, but its another formule.
Use the same impedances and frequency's.


If you know the amount of (milli) Ampere's through the noise diode and
know the impedance of the load, you can make power calculations like
we do with DC.
With the calculation method you don't need to have a calibrated noise head.
Someone else discussed the method, let me give the formule.
The formule is in Chris Bowick's book RF Circuit Design.
Its about Shot Noise. (Not thermal)
In^2 = 2qIdcB

In^2 = the mean square noise current
q = the electron charge (1.6 x 10e-19 coulombs)
Idc = the direct current in Ampere's
B = the bandwith in Hertz

Onces you have the mean square current, calculate the power in the load
resistor.
Did not made calculations with it, since I have the opportunity to measure
noise
right away. Will do it in future to check the formule.

The MMIC I used.... I was afraid there would come a question about it.
It's in a factory designed preamp of Japanese origin, has a forgotten
product
number, I mean National, not sure of that.
Searched hours for datasheets, because I was convinced the NF of the device
was less at 144Mhz, the manual stated approximatly 2dB 2Ghz.
EME use 144Mhz, with the knowledge of bipolar transistors that have an
increase of NF with frequency, 144Mhz would have a NF of about 1 - 1.5dB.
The manual said nothing about a lower NF at lower frequency's.
So bought it with the deal, not good money back.
Found the datasheets, the NF was 2.8dB flat from almost DC to 2Ghz.
From there comes the idea of flat NF respons of MMIC's.
I brought the device back to store and got money back. 150$.
More research learnt me that similar devices with less NF in order of 0.7dB
were avaiable for 20$, OK no box around it, no blinky leds.
I know only that the MMIC is obsolete today, forgot the type number.

A single mosfet, BF981 does 1.7dB at 100Mhz and cost about 1$.
I love homebrewing.
With that device you could also calibrate your noise generator.
It matched the input impedance of the 50 Ohm receiver.
Many EME amateurs use it in their preamps.
If they all use the same scheme it would be accurate within a variation
of say +/- 0.1dB ?

Since I can measure noise power, how to calibrate whitout a calibrated
noise head is not a hot item for me anymore.
Just practice.
Before that, I read several books, spent hours on the internet, collected
many
data.
Its only to share thoughts, nothing science, professional or years of
experience,
and yes I can be wrong.
The goal was maybe you can do something with it, for me, maybe there is a
clever
way to determine the NF without calibrated noise head, maybe even whitout
math.

The JT44 software program, I just finished the interface from receiver to
PC.
Details at the original PSK31 homepage.
Used a "Jensen" audio trafo, to prevent ground loops.
A lot to do... no practical data at this moment.

Have read Terry Ritter's stuff.
He said, no white noise at all. Just pink. See the graph's.
Read in some university books, noise is of pink behavior at the lowest
frequency's.
The graph showed the audio band.
White noise is flat (whitin 15 - 20Khz partitions) and at higher
frequency's, and random.
Whit respect to the measurements of course.

There is an increase of NF to higher frequency regions.
How came that MMIC to a flat respons ?

Sorry I was wrong about the noise figure at 100Mhz for the mosfet BF981.
It has to be 0.7dB instead of 1.7dB!

It can still better with the BF998, to get an impress of the scheme try the
following link http://lea.hamradio.si/~s53ww/4xbf998/4xbf998.htm

I think it can be reproducible whitin small variations.