[volt-nuts] HP 3456A AC cal

[M K]

On 12/09/2014 08:36, John Phillips wrote:
>  From what I remember: The 3455A has a good AC front end but is a real pain
> to calibrate with a lot of adjustment (read flexibility). I think HP gave
> up a lot of accuracy for a simpler calibration. The 3457A seems to do AC a
> lot better than the 3456A. Nether meter come close to doing what the 3458A
> does.
>
> On Fri, Sep 12, 2014 at 12:04 AM, M K <m1k3k1 at hotmail.com> wrote:
>
>> On 12/09/2014 00:30, Richard Moore wrote:
>>
>>> I'm trying to get my head around the AC cal procedure for the HP 3456A --
>>> there is no full-scale cal, just some extremely finicky tweaks for the
>>> bottom ends of three of the four ranges. I really don't understand how this
>>> is supposed to work, especially since it is very accurate on all DC ranges
>>> (up to 500V -- above that some heating of the divider R seems to occur and
>>> it loses accuracy as the voltage increases). But on AC, the tweaks don't
>>> result in anything like an accurate level reading compare to my 3458A -- at
>>> 10VAC it is off by hundreds of ppm. Anybody shed any light on this? Is
>>> there a way to get full scale cal points in AC mode? Any mods to fix this
>>> -- lovely meter otherwise; I much prefer it to the 34401A I used to have,
>>> except for the overall bigness....
>>>
>>>   Hi Richard,
>> Does the meter have a DC to RMS converter chip in it, because if it does
>> you have to account for the self noise and offset of the gilbert cell at
>> the heart of the converter.
>>
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>
>
I downloaded the service maual from the BAMA archive, it is an absolute 
value circuit followed by a logging anp, not quite the AD634 gilbert 
cell, but still prone to the same offset and noise creating a 
non-linearity at the bottom of each range.