"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