[volt-nuts] 3458A Questions

[Frank Stellmach]

Hi Joe.

I've got a unit for about 3k€, would be lesss in $, I think, as we are 
living in a 2nd hand diaspora, here in good old Europe.

I equipped the unit with 4x 32kByte SRAMs, which I still had on stock, 
following good tradition:
I really needed this bargain Opt. 01 20yrs. ago when I did real-time 
digitizing with a 20MHz AT PC, using assembler code for averaging the 
wave form.
Nowadays, I think, it's not necessary any more, if you've got a GPIB 
card, and are able to download the data directly to your PC.

Opt 02, well, it's only necessary, because HP runs the LTZ on 90°C, 
instead of 45..60°C.
I assume, the 3458A had been designed mainly for military use, but 
metrology amb. temp. range is much lower, so the stabilization temp. 
could be much lower under comtrolled environment.

Timely drift - at continous operation only - increases by a fyctor of 2 
every 10°C.

So I simply pimped my unit by adding a 150k (?) ww. resistor in the LTZ 
circuitry, reducing heating temp. to about 55°C, which should reduce 
thermal, annual drift to about 1ppm/yr.
The 02 option is for sure very delicate. I wonder, if FLUKE did similar 
measures as me, when they offered their own, more stable version, i.e 
their HFL 3458.


Interior temperature is about 40..50°C, depending on the ambient, which 
should be more than 10°C below the ref. temp.

I recently moved my analogue x-nut equipment to our cellar, where we 
have constantly 21..22°C,slightly depending on season, into an open rack 
(from ikea), which reduces the interior temperature during operation to 
38°C max.

This is the crucial thing about the 3458A: You need a very constant 
ambient temperature to get stable - comparative - measurements for DCV, 
but especially for Ohms.
DCV reference is heated, so its calibration is not very susceptible 
against amb. temp.

I compare the 3458A to two homeground external LTZ, and a Fluke 5442A, 
and the differnece is less than 1ppm for all 4 standards, over a period 
of 1 year or more.

But Ohms ref. definitely is dependant on amb. temp. - so the 3458A is 
not suitable as a "standard", unless the calibration lab specifies its 
own amb. temp, and monitors the 3458As' internal temp. during 
calibration, which you have to reproduce as close as possible.

According to the specs, and to my own measurements, the internal VISHAY 
ref. resistor has a quite huge TC of about 0.5 .. 1ppm/K.
So you alternatively need a stable 10k external reference. (I will 
report about my Z201 experiment soon).

Speaking about the hardware, I have an 18 years old unit, regarding the 
analogue part, and identified by the ser. no.
The digital/interface PCB is about 10 years old, obviously already 
changed, due to NVRAM discharge, which I have to save and replace in the 
coming years.

So its not easy to decide from external, if the unit is worth its money.
"AGILENT" imprint in favorable, also a newer date code, for getting 
replacable batteries, which is hard to find, and to finance.

So good luck, but just do it.

Frank