[volt-nuts] HP-3458A questions

[Randy Evans]

Frank,

Thanks for the data.  The 100K seems acceptable since I have a Fluke 732A
 to recalibrate the 3458A with after the change and the 3458A is not in a
rack.

Randy

On Tue, Jun 2, 2015 at 2:50 PM, Frank Stellmach <frank.stellmach at freenet.de>
wrote:

> Hi Randy,
>
> in advance, this change only improves the stability of the instrument,
> when the 3458A is powered.
> The LTZ1000 will not age (nearly), when the instrument is switched off.
> Therefore, it makes sense mainly, if your instrument is running for an
> extended period of time.
>
> (Reference: Spreadbury: "Ultra Zener..")
>
> If you change the oven temperature, you will loose the 10V calibration, as
> the LTZ1000A will change its reference voltage.
>
>
> Although my instrument is strictly switched off, when not doing
> measurements, I reduced the oven temp. by connecting a 100k precision
> wirewound (3ppm/K) in parallel to the 15k BMF resistor. That gives 13k in
> total, yielding about 65°C like in the LT datasheet.
> An alternative placement is already foreseen on the PCB; in the CLIP, it's
> component designator is X411.
> I'll try to link in a photo of my change.
>
> lymex on the bbs35.hot forum obviously was more agressive, and paralled
> 75k of Vitrohm Thin Film, giving 12.5k for about 55°C.
>
> I would recommend my solution above the one of lymex, although his one
> gives theoretically more stability.
> Also, the lower stability of TF reistors may affect the stability (T, t)
> of the 15k BMF.
>
> With 100k/65°C, the oven runs about 30°C lower.
> Therefore, the ambient temperature has to be limited to max. 35°C, the fan
> has ALWAYS to be kept clean, giving a maximum temperature rise of 15°C, and
> the instrument may not be put in a rack (which would give additional
> 10..20°C temperature rise). Under these conditions, (35°C ambient + 15°C
> 3458A inner heating + 10°C self heating of LTZ1000A + 5°C regulation
> margin), the oven will always be stable.
>
> These 75k / 55°C would be possible, only if you limit the ambient
> temperature further, to <= 25°C, and precisely monitor the 3458A inner
> temperature (TEMP?).
>
> Although my instrument is kept at 20..23°C throughout the year, i wouldn't
> feel comfortable with 55°C.
> The only possibility would be the exchange of the reference to the non-A
> version, as the self heating effect may vanish!
>
> Frank
>
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