[volt-nuts] [Bulk] Re: 3458A reference boards on ebay

[John Phillips]

There is a bottom cover under the board.

On Tue, Jan 27, 2015 at 9:01 PM, J. L. Trantham <jltran at att.net> wrote:

> When these boards are installed in a 3458A, the LTZ1000 is covered by a
> small, white, 'hat' that mounts with screws that pass through the two holes
> on opposite sides of the LTZ1000.  I've never removed the board to see if
> there is anything on the bottom of the board that would cover the bottom of
> the LTZ1000.
>
> Does anyone know what this 'hat' is constructed of or the
> HP/Agilent/Keysight part number of this item is?  Is it available from
> Keysight?
>
> In the 3458A the reference board is under a cover that covers about half
> of the top of the meter but not 'insulated' beyond that, except for the
> 'hat' referred to above.
>
> It would appear desirable to find what that 'hat' is and see if something
> could be fabricated to 'insulate' the LTZ1000 for a 'do it yourself'
> standalone reference.
>
> Would appreciate hearing anything about how the reference board is
> 'insulated' in the 3458A.
>
> Thanks.
>
> Joe
>
>
> -----Original Message-----
> From: volt-nuts [mailto:volt-nuts-bounces at febo.com] On Behalf Of Orin Eman
> Sent: Tuesday, January 27, 2015 11:45 AM
> To: Discussion of precise voltage measurement
> Subject: [Bulk] Re: [volt-nuts] 3458A reference boards on ebay
>
> Dave,
>
> I'll be putting it in an enclosure to use as a reference.  It will
> probably be a smallish diecast aluminium box.  It's what I did with my
> Geller SVR-T and it compared very well with my Fluke 731B.  I'll likely use
> Pomona 3770 binding posts for output (I used them to repair the 731B).
> I'll also use the protection circuits from the Fluke 732 on the output -
> big reverse diode, gas discharge tube and TVS as I recall (I plan to add
> these protection circuits to the 731B).
>
> I'll likely do a PCB for it, though the local equivalent of veroboard
> would likely be OK; the socket spacing is 0.9" and the pin spacing 0.1".
> My meters are also 6 1/2 digit (3456A and 34461A) so I'm not too concerned
> with thermal EMFs with one count being 10uV on the 10V range.  However, I
> will try to avoid temperature gradients and keep the outputs close
> together.  I'll use star wiring for the 18V and ground as in the following:
>
> http://www.maxmcarter.com/vref/
>
> It is debatable whether the -15V supply is needed - I'll test with and
> without.  I'd add a few tantalum capacitors to the above design - on the
> regulator outputs and on the 3320 resistor, along with protection diodes
> for the LM317.
>
> There is also a HUGE thread about the LTZ1000 on EEVBlog - I'll be
> reviewing that for ideas:
>
> http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/
>
> I actually have a couple of LTZ1000As here that I was going to DIY, but
> never got around to getting the precision resisters .  I think there is a
> source in England mentioned in the EEVBlog thread.
>
> Orin.
>
> On Tue, Jan 27, 2015 at 2:24 AM, Dr. David Kirkby (Kirkby Microwave Ltd) <
> drkirkby at kirkbymicrowave.co.uk> wrote:
>
> > On 27 January 2015 at 09:16, Orin Eman <orin.eman at gmail.com> wrote:
> > > There is a seller letting a slow trickle of them out there... they
> > > seem
> > to
> > > be going for about $165.  The current batch of two is around $100,
> > > but
> > will
> > > probably be bid up.
> > >
> > > I just received one that I won.  Seems to be working fine after a
> > > quick breadboard lashup.  I'll be making an enclosure for it next.
> > >
> > > A little expensive IMO, but given the trouble getting the precision
> > > resistors to DIY with the LTZ1000A, probably worth it.
> > >
> > > Orin
> >
> > Orin,
> >
> > What do you intend doing with it?
> >
> > I was thinking about getting one of those, with a view to putting it
> > in a box with a couple of terminals to have something to compare with
> > my 6.5 digit 3457A.  But what put me off is a lack of knowledge in
> > knowing how to convert a 3458A reference board into a boxed unit with
> > a known output voltage at the terminals. How would I avoid / control
> > thermal EMFs?
> >
> > I'd be interested to hear what your plans are for it, and how you
> > intend tackling those issues.
> >
> > Do you know what the difference in the reference is between a standard
> > 3458A (8 ppm) and the high stability option 002 (4 ppm) model? I'm
> > guessing the chips for the option 002 might be the top performing
> > ones. I wonder if there's any way to tell from your board if it came
> > from a standard 3458A or a 3458A with option 002.
> >
> > Dave
> > _______________________________________________
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-- 

*John Phillips*