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.stellm...@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 > > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.