The discrepancy may be due to the small sample size. I believe you
want to use the "student's-T" distribution, which has more dispersion
to account for less information. If you do the experiment with more
samples - say at least thirty - the distribution should approach
normal. This information was stored somewhere in my brain and just
popped into my head when I looked at the post - I haven't looked at
this sort of thing in many years, so my recollection may not be exactly right.

Ed

Ed, I'll try to understand better the student's distribution,
should it give numbers not that far from a normal distribution?

If it can help, the standard deviation calculated by 7061 tend
to stabilize to a value very near to the one showed before,
also with more samples.
I used 13 samples because it was easy to check the values by hand.
It converges in the first few samples and stabilizes to about 2.4E-4
also if the samples are hundreds (the first test I made was with more
than 700 samples).
Visually all the measurements seem to have about the same noise that the
13 samples have, few uV up and down,
My problem is that the manual says that the 7061 calculates the
standard deviation the normal way.
But the values calculated by the instrument seem a pair of orders
of magnitude too high.

So probability says that there must be something stupid I'm doing wrong :)

Fabio.

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Hello all good folks:

A little bit off topic, but this defunct company with a heyday from ~
1950s-1980s made very good bench linear PSU, including a line called
'precision power supplies', something more labeled precision voltage
sources today.  However, the precision series was capable of 0.5-4A output.

A user could dial a VDC set precisely to uV levels on most voltage
ranges, and could hold its dial setting to single digit uV for ~ >= 8
hours, but to 100uVs over a month.

I'm interested in anyone who can give their impression of the history
of this company, their experience with their products, or have
advertisements or scans of their catalog of equipment during their prime.

From what I've gathered, they were in business making PSU from 1950s
through 1996, and were known for performance, quality, and low
cost.  The precision units up to the early 1970s were all discrete.
They declared bankruptcy by 1996, reorganized in CT [ they were in
upstate NY] as Technipower, became insolvent by mid-2000s and finally
sold and integrated their wares with Unipowerco.com.

HP made a short running line of precision power supplies in the early
1970s that met or exceeded Power Designs capabilities.  They were
also more costly.  Today, I can't find anyone who manufactures a
bench PSU with these specs today.

Ed,

I'm just a hobby user, and even that is a lot more part-time than I'd
like.  Even in retirement I don't seem to have time...

Anyway, I have four Power Designs supplies purchased on eBay; two TP340
triple output supplies, a 3650-S single output supply, and a 2005A
precision supply.  (The latter purchased as much as anything because I
thought the two dials on the front to select the voltage on the front
were cool and it didn't cost much.)  I'm satisfied I got good value for
my money.

I can't say much about their quality as far as noise/ripple/accuracy
goes but they have worked fine for me as bench supplies.  One of these
days maybe I'll check closer.....  I don't think I paid a whole lot for
any of mine relative to some of the better known brands.  They're
relatively heavy and don't seem to have been designed with compactness
as first priority.

I had to replace two switches on one because they didn't work, but they
were readily available toggle switches and one of those was a broken
switch handle.  Not a big surprise when you don't try for pristine
condition to save some money.  The 2005A has two pretty complex
multi-position rotary switches with open contacts that didn't reliably
give the same voltage on the same settings.  I used contact cleaner and
got rid of at least most of that, but I don't know how accurate or
repeatable it actually is since I didn't do any systematic testing.  If
their other precision supplies use the same type of switches, I'd say
that is a potential trouble area and probably would call for a good
voltmeter to confirm you're really getting what you think you're getting
on the output.  Other than that I haven't had to make any repairs.

They all seem to have pretty straight-forward construction so I'm
thinking (maybe hoping is a better word) they shouldn't be too hard to
maintain as long as something major like a transformer or those rotary
switches doesn't go out.  I can see the 2005A has a "Reference Amplifier
Oven" in a cylindrical can that is probably unobtainium, but mine have
no processor modules, PLDs, displays or custom switch panels like a lot
of the HP or Tektronix gear I've bought on eBay.  Not sure if that is
because of their vintage or a function of being built by a smaller
company that couldn't invest in lots of custom parts.

I'm not sure if I managed to get manuals for all of mine or not - I know
I have some.  I moved a year ago and still have boxes of stuff I've
never unpacked because I haven't got a workshop built yet.  Getting
close finally!!!

Hope this helps.

Alan

Marvin,

Oops!  Sorry I called you Ed.

Brain malfunction resulting in getting my e-mail replies mixed up.

Alan