All this 1/f noise talk got me to thinking about some of my noise
measurement and generation projects over the years. One project that I
want to revisit is one for making "nearly-true" random white noise all
the way down to DC, for comparing and testing things in the very low
frequency 1/f realm. I have several different noise generator
build/repair projects in the works that I haven't even looked at in
years. One in particular is something I had pictured a while back, using
this Scientific Atlanta 4647 IF tester thing that I got long ago, but
could not find any info at all. I dug it out from under a pile and
looked it over. Today I did find a little info on it (a brochure), that
at least gives some idea of how it's supposed to work. I checked it out
and it seems to do about what it says.
The brochure is here:
https://www.atecorp.com/atecorp/media/pdfs/data-sheets/scientific-atlanta-4647_brochure.pdf
It's just a few pages, and every other so-called "manual" that shows on
the downloading sites seems to be that same thing or less.
I'm wondering if anyone is familiar with this or knows of any
documentation - the ideal of course, being a pdf service manual with
schematics. A paper manual (supposedly) popped up on ebay, but the
picture is of an HP binder with unknown contents. I'd prefer to find a
pdf somewhere rather than spend fifty bucks for something dubious, or
the hassle of figuring it out.
What's nice about this unit is that it has "calibrated" and settable
noise and carrier levels, and better yet, it seems to be in working
order as it is. It makes pretty good (15 dB crest factor) white noise in
a flat band about 50-90 MHz, that normally gets added to a 70 MHz IF
carrier, in various proportions and modes for testing. It also makes its
own 70 MHz carrier, if needed.
What I've been thinking is to simply mix the 70 MHz out with the
noise-only (mode) out, then LPF to say 20 MHz or less, and there you go
- a nice DC to whatever white noise source. I'll start with some
experiments with external stuff, then probably build it into the box as
another feature. The added items may include an amplifier to get the 70
MHz up to 7-10 dBm for mixer drive, and the LPF(s), maybe a DC-coupled
post amp, and some miscellaneous for routing and control. The main thing
is that the basic box already takes care of most of what's needed,
including space and power.
The available 50-90 MHz noise is quite stout, specified up to -73
dBm/Hz. I looked at it on the 8566B, and the noise envelope with 3 MHz
RBW can almost bury the 0 dBm carrier. Likewise on a scope, the nice 70
MHz sinewave disappears into a cloud of noise as the power goes up.
The flatness, crest factor, and leveling won't be as good as the
original signals, but I think still pretty decent. One complication is
that this thing is all 75 ohm, so I'll probably need to do some mods
here and there to make it 50.
Ed
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