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