[volt-nuts] HP-3458A questions

Frank Stellmach frank.stellmach at freenet.de
Tue Jun 2 17:50:10 EDT 2015

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!

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