From: Gary, WB9JPS on Fri, Sep 29 2006 7:26 pm

I built a noise source as described by Bill Sabin, W0IYH in QST (1995)
and EMRFD. Works fine, but now it's time for calibration. Chatting with
Bill, he reinforces the notion that the best way to go is comparison
with another calibrated source, or a direct measurement with a
calibrated noise figure test set. NoiseCom used to provide this service
to hams for $50, but now they claim they have dropped that offer (it's
about $600, now). So my question is, is there anyone in the SF Bay
Area (I live in Livermore) who might be able to help? If someone owns a
source or test set, perhaps we could get together. I'm open to other
suggestions, too.

FYI, this is primarily for testing HF rcvr performance. I'm building an
R2-style DC rcvr, and it's turned into a nice research and education
project, so I go a bit overboard sometimes, but it sure is fun. I'm
also using an IQPro DDS VFO. I wrote a couple reports on its
performance that are up on the AA0ZZ website.

If you can't get a calibrated comparison noise source, here's
some tips on workshop calibration techniques on random noise:

Random noise voltage or current can ONLY be calibrated by
a TRUE Root Mean Square measuring instrument. By "true" I
mean a thermionic type such as an RF power meter (thermistor,
bolometer, etc. sensor). Few voltmeters on the market have
TRUE RMS measuring capability; those that do are specified
as such and rather on the expensive side.

A very low-level random noise source can seldom be measured
directly by an available-market voltmeter or power meter.* To
calibrate it with such instruments requires amplification. A
receiver can provide that - provided - it is carefully checked
for linearity. By linearity I mean one that does not clip or
saturate the RF or IF or detector. A wideband TRUE RMS volt-
meter can measure the last IF output and do a comparison
between IF signals from a signal generator and the random
noise source into the front end. If the front end noise is
significant, the total noise input-equivalent will be the
good old square root of the sum of the squares of the noise
sources; knowing the true RMS values of both and just one
will let you find the true RMS value of input noise voltage
of the unknown. [kind of nuisance grunge work but then
most metrological work calibration is of that nature :-(]

* Jim Williams of Linear Semiconductor had a nice article in
EDN a few years back, pushing a Linear Semi true RMS IC. In
that was a list of available "RMS implied" voltmeters with
their sensor types stated. The cheapo averaging circuits in
common multimeters could be LOW on true RMS voltage readings
down to half of readings (despite their stated accuracy
specs). The Linear Semi IC apparently didn't sell well and
was taken off of active production. Perhaps only National
Semiconductor has a rough equivalent?


This is my first post to this group. There seem to be a lot of talented
folks lurking out there... Thanks in advance for any help.

Living in southern California I don't get up to mid-state
often. Difficult to drop over with a comparison source.
:-) However, a bunch of us lurking in here have been
"around the horn" a few times and we can supply a few
items of information to help. [or maybe to add to the
"random noise" level? :-) ]