Noise Figure Measurements
 

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

Cheap calibrated noise source, 35dB Bandwith 100kHz - 500Mhz.
http://www.elecraft.com/manual/N-gen...%20rev%20C.pdf
Noise performance of Mar's MMIC's should be noted on datasheets.
I have them "somewhere", with a little luck Google finds it too.
You can measure and calibrate your own noise generator.
Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.
Remove resistor, and switch ON the unknown ENR, Z = 50 Ohm.
The only thing to do is, ATTenuate in steps until you are at first measured
output labeled zero dB.
Add simply the dB's of the ATT units, this should be the ENR dB's.
Please correct me if I am wrong.
Homebrew ATTenuator pads can be build low as 0.1dB accurate enough.

Sources of how to measure NF are radio amateurs who do EME.
Google should come up with data if you search at "low noise" "EME" etc.
PS: Google for VE3DNL Using Noise Generators to measure Noise Figure.
Greetings Bas.

"Steve Kavanagh" schreef in bericht
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

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

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

"bviel" wrote in message ...

Thanks for the suggestions. I'll have a look at the web references.
Just a couple of notes on things that won't work.

Noise performance of Mar's MMIC's should be noted on datasheets.

Yes, but they never show the range of noise figures, only a typical or
worst case numbers. Of course if the noise figure is very low (PHEMT)
then the variability is pretty small as long as the circuit is done
right.

Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.

This doesn't work, because the noise is dominated by the receiver
noise, not the resistor thermal noise (unless the receiver is very
very good !).

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

"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

"Steve Kavanagh" schreef in bericht
om...
"bviel" wrote in message
...

Thanks for the suggestions. I'll have a look at the web references.
Just a couple of notes on things that won't work.

Noise performance of Mar's MMIC's should be noted on datasheets.

Yes, but they never show the range of noise figures, only a typical or
worst case numbers. Of course if the noise figure is very low (PHEMT)
then the variability is pretty small as long as the circuit is done
right.
The noise figure of a MMIC is flat from DC to Ghz, the one I worked with.
Did not test the Mar's.

Terminate receiver with 50 Ohm resistor, measure output and label it
zero dB.

This doesn't work, because the noise is dominated by the receiver
noise, not the resistor thermal noise (unless the receiver is very
very good !).
The method is from JT44 EME software where you can measure noise figures.
The Help with the program should explain everything.
I myself am in the position to measure noise with my scope for a "second"
opinion.
The noise of a high frequency transistor b-e diode should deliver the right
sort of noise
also called white noise.
Or high frequency diode microwave type.
The transistor has because of his junction noise behavior the most real
white noise,
or general white noise.




73,
Steve VE3SMA