Steve,

There is another method of measuring noise figure that does not need a noise
source.

The reason that it is not used as often is that it is harder to measure low
noise figures accurately. Since there are still a few layers of rust on my
brain I will not geive the details here, since i do not want to say anything
that is too far off.

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated properly.
The noise output is a combination of the DUT noise and the 'ambient' noise
from the input termination, times the gain of the receiver. Working
backwards you can then determine the DUT noise. As you can probably guess,
the reason it doesn't work so good on small noise figures is that any errors
in the measurement can easily outnumber the actual noise figure you are
trying to measure! And I have actually had to deal with this problem. That
is where half my gray hairs came from (the other half from being laid off).

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Jim
N8EE

"Jim" wrote in message ...

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated
properly

....any errors
in the measurement can easily outnumber the actual noise figure

Thanks for reminding me of that one, Jim. But I see your point about
errors. For example, in measuring the gain one needs a standard. One
of the few pieces of real test gear I have is a bolometer-type RF
power meter which can measure about -13 dBm accurately. If the
measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
thermal noise is -114 dBm. So I need about 100 dB gain for a very low
noise figure DUT. To measure that I might need five 20 dB attenuators
as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
there's +/-2.5 dB error (well, I suppose I could cross my fingers and
RSS the numbers). Or I have a diode-type power meter that will
measure lower power, which leads to issues of how the detector
responds to noise. And then there's the problem of knowing the noise
bandwidth precisely...

is where half my gray hairs came from (the other half from being laid off).

I think about half of mine come from the latter factor too !

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

Definitely. Though, with my method the input never gets more than 3
dB above the receiver noise floor and in most cases a well designed
receiver will have no AGC response at that level. But with a preamp
in front it usually will activate the AGC, so AGC has to be switched
off when comparing preamps - which of course is impossible to do in
most ham rigs !

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Hence the interest in NF repeatability of MMIC amps, since they are
hard to build wrong, are well matched over a wide bandwidth and don't
require tuning for best noise figure. The known DUT can also be the
standard itself (in association with a receiver of only roughly known
NF) to avoid issues of errors in calibrating the noise source ENR.

73,
Steve VE3SMA

On 14 Oct 2004 16:44:35 -0700, (Steve
Kavanagh) wrote:

I haven't really been following this thread but if you are looking for
noise diodes and associated stuff, you should take a look at:

http://www.noisecom.com/

Seems to me that at one time they would calibrate a homebrew source
for a modest fee. That may have been a ham that worked there paving
the way; not sure.




|"Jim" wrote in message ...
|
| This other method involves measuring the gain of the device under test and
| then measuring the noise power output with the input terminated
| properly
|
| ....any errors
| in the measurement can easily outnumber the actual noise figure
|
|Thanks for reminding me of that one, Jim. But I see your point about
|errors. For example, in measuring the gain one needs a standard. One
|of the few pieces of real test gear I have is a bolometer-type RF
|power meter which can measure about -13 dBm accurately. If the
|measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
|thermal noise is -114 dBm. So I need about 100 dB gain for a very low
|noise figure DUT. To measure that I might need five 20 dB attenuators
|as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
|there's +/-2.5 dB error (well, I suppose I could cross my fingers and
|RSS the numbers). Or I have a diode-type power meter that will
|measure lower power, which leads to issues of how the detector
|responds to noise. And then there's the problem of knowing the noise
|bandwidth precisely...
|
| is where half my gray hairs came from (the other half from being laid off).
|
|I think about half of mine come from the latter factor too !
|
| If you are measuring an entire receiver there are a few things you have to
| be careful with. The receiver must be a linear receiver (no FM, AM diode
| detector, etc.---basically just SSB). There should be a filter to pick just
| one sideband. Turn the AGC off. Make sure you measure the gain in the
| linear region, which also applies to a simple amplifier.
|
|Definitely. Though, with my method the input never gets more than 3
|dB above the receiver noise floor and in most cases a well designed
|receiver will have no AGC response at that level. But with a preamp
|in front it usually will activate the AGC, so AGC has to be switched
|off when comparing preamps - which of course is impossible to do in
|most ham rigs !
|
| If you have a DUT with a known noise figure, I think that this would be one
| way of calibrating a homebrew noise source.
|
|Hence the interest in NF repeatability of MMIC amps, since they are
|hard to build wrong, are well matched over a wide bandwidth and don't
|require tuning for best noise figure. The known DUT can also be the
|standard itself (in association with a receiver of only roughly known
|NF) to avoid issues of errors in calibrating the noise source ENR.
|
|73,
|Steve VE3SMA

Wes Stewart wrote:

I haven't really been following this thread but if you are looking for
noise diodes and associated stuff, you should take a look at:

http://www.noisecom.com/

Seems to me that at one time they would calibrate a homebrew source for
a modest fee. That may have been a ham that worked there paving the
way; not sure.

In connection with the QST article by Bill Sabin, W0IYH, NoiseCom had a
program of single-quantity sales to amateurs, and offered a low-cost
calibration service. I'm not sure whether that program still operates,
but some years ago NoiseCom were very helpful in supplying a free sample
outside of the USA.

If I were building a noise source again, I wouldn't put NoiseCom to any
trouble. Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

ENR flatness of the whole noise source depends on your construction. The
other important factor is that the output impedance must not change
significantly between the 'on' and 'off' states of the diode. To achieve
both of these, the best technique is to build the noise source using
SMD/microstrip technique including an attenuator of about 7dB made from
0805 SMD resistors; and then buy a high-class 20dB attenuator (N or SMA)
which need not be expensive at a hamfest. This attenuator then becomes a
permanent part of the noise source - not to be taken off and used for
something else!

This level of attenuation will give you an ENR of about 5-6dB, which is
what you need to measure typical modern low-noise amplifiers.

On the DC side, it is a very good idea to include a constant current
source for long-term stability of ENR. Most devices will give a peak of
ENR at a few mA, so you need to adjust the current to the top of this
peak where the variability is least. (There is much more noise at
currents of a few hundred uA, but you don't want to go there - the noise
output there is far too sensitive to the DC current, temperature, color
of carpet, phase of moon etc.)

It is VERY important to design for the industry-standard power supply of
+28V DC, so that your noise source is a simple plug-in replacement for
any professional source... because that is how you're going to get your
ENR calibration.

All of these ideas come from an article by DJ9BV in DUBUS magazine which
described a very high-class noise source, good up to 10GHz. Mine uses
simpler and much less precise construction, so the ENR begins to wobble
above a few GHz due to resonances.

The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

As for calibration, the best way is to take your noise head to a
microwave meet where there is professional NF measuring equipment, and
take a cal from the HP346A there. Alternatively, get to know someone -
anywhere - who has access to these facilities, and can do it for you one
lunchtime.

Other useful background information is in an Agilent App Note, at:
http://literature.agilent.com/litweb/pdf/5952-3706E.pdf

If you can still find a copy of 'The VHF/UHF/DX Book' (out of print)
there's a lot of information in there too.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK" wrote in message ...

Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

That's an interesting suggestion...but presumably the 35 dB ENR can't
be counted upon to the level needed for measurements without
calibration.

The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

Thanks, Ian. I found the DUBUS articles...though I haven't read them
through yet. They look very interesting. Though perhaps not *exactly*
what I was after, as they use a real microwave noise diode, whereas my
thrust is more in the direction of what can be done with more common
parts and without any need for calibration of individual noise
standards. I recognize that this probably won't be possible in the
microwave range but it seems likely (to me, anyway) that reasonable
accuracy (not good enough for EME preamps !) can probably be attained
up to VHF, even with these constraints.

73,
Steve, VE3SMA

Steve Kavanagh wrote:
"Ian White, G3SEK" wrote in message
...

Instead, I would use the base-emitter junction of a
small-signal UHF/microwave transistor. It seems that just about any
junction that goes into avalanche breakdown at a reverse voltage of 5-7V
will give an Excess Noise Ratio of about 35dB. With a modern SMD
package, the device parasitics will be lower than the wire-ended diodes
that NC were offering, so the ENR will be flatter with frequency into
the microwave region.

That's an interesting suggestion...but presumably the 35 dB ENR can't
be counted upon to the level needed for measurements without
calibration.

That's correct; it is only a design guide to the amount of attenuation
that will be needed to give an ENR that's in the right ballpark. But
then you need to know what the actual value *is* - and for that, you
still need a calibration.


The DJ9BV articles (in both English and German) are on the DUBUS
website, at:
http://www.marsport.demon.co.uk/archive.htm
Look around 1990, and there are a few follow-ups in later years.
(Unfortunately both archive sites are down right now, but do keep trying
- these articles are *exactly* what you've been looking for!)

Thanks, Ian. I found the DUBUS articles...though I haven't read them
through yet. They look very interesting. Though perhaps not *exactly*
what I was after, as they use a real microwave noise diode, whereas my
thrust is more in the direction of what can be done with more common
parts and without any need for calibration of individual noise
standards.

My point was that you don't need a real microwave noise diode - any
small, cheap UHF/microwave bipolar transistor will give almost the same
performance up to several GHz.

I recognize that this probably won't be possible in the
microwave range but it seems likely (to me, anyway) that reasonable
accuracy (not good enough for EME preamps !) can probably be attained
up to VHF, even with these constraints.

You can optimize NF using an uncalibrated source, but with this kind of
source you cannot can make worthwhile quantitative measurements without
an external calibration.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

You also have to know the noise bandwidth of the system to use this
method. If the response is dominated by a single, fairly steep-sided
filter, this is easy. But otherwise (such as if both IF filtering and AF
amplifier response shape the overall response), some calculation and/or
measurement is required. I've gotten what I believe are reasonable
results on HF and AF amplifiers using this method. But I haven't tried
it on a really low NF system, so would heed Jim's caution.

Roy Lewallen, W7EL

Jim wrote:

Steve,

There is another method of measuring noise figure that does not need a noise
source.

The reason that it is not used as often is that it is harder to measure low
noise figures accurately. Since there are still a few layers of rust on my
brain I will not geive the details here, since i do not want to say anything
that is too far off.

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated properly.
The noise output is a combination of the DUT noise and the 'ambient' noise
from the input termination, times the gain of the receiver. Working
backwards you can then determine the DUT noise. As you can probably guess,
the reason it doesn't work so good on small noise figures is that any errors
in the measurement can easily outnumber the actual noise figure you are
trying to measure! And I have actually had to deal with this problem. That
is where half my gray hairs came from (the other half from being laid off).

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Jim
N8EE