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

[J. L. Trantham]

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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