Thought about replacing the voltage divider R4/R5 for the tempreature with a PWM divider. Hoping to get at least some advantages from that.
- Cheaper?
- Adjustable Tempreature
- Higher stability?
What do you think?
I think that such questions should always be started from the (engineering) requirements and from design goals, not backwards from an anticipated solution... as this would be better and professionell engineering practice. Consequently, all the replies to your three questions so far did not give any concrete answer.
So, to answer your questions concretely: I think that a PWM would be a very bad approach in all of your categories.
At first stability: what stability requirements do you mean, like short term (noise), temperature, long term drift, or susceptibility to E.M.C. , pressure, humidity, vibration?
These 2 resistors theoretically contribute only by 1/75 .. 1/100 of their own stability figures, so the LTZ chip itself has paramount influence on mostly all these parameters; that has been measured / demonstrated in this lengthy thread many times. Practically, the residual T.C. of up to 0.3ppm/K, despite e.g. T.C. matching of the resistors, is not yet understood here, and is obviously not relevant, as the overall T.C. can easily be trimmed, down to <0.02ppm/K.
As the oven regulation and in turn the reference voltage is very sensitive to external noise, keyword voltage dips, for a PWM solution you'd need a very elaborate filtering, which would probably yield high cost and in the end worse noise figures than the passive divider solution.
Concerning cost, these Vishay hermetically sealed, oil filled, luxury resistors are not really necessary; much cheaper ones will do the job as well. These only have to fulfill certain minimum technical requirements regarding e.g. T.C. and timely drift.
Please, first make yourself aware of the BOM cost of these cheaper solutions, they are in the ballpark of < 20€ for two b.m.f. resistors, or even much less for other technologies, like e.g. TiN has presented on the discussion about the clone reference of the 34470A.
Concerning oven set point change, that question makes no sense at all to me.
Instead, what would be the purpose/application, or your design goal for that feature?
If you would start from the requirement "timely stability", you would know, that the lower the temperature, the better this parameter.
50°C oven temperature gives about -0.8ppm/year, each 10°C more will double that number. Therefore, simply chose a fixed, lowest possible oven temperature.
Changing the oven temperature will change the reference voltage by about +50ppm/K, which would violate the overall stability requirement as such.
If your feature goal would be to have an easy and cheap(er) possibility to make thermal cycling like in the Pickering patent, well that's also already done in the Fluke 7000 devices, so no need to re-invent the wheel by another complicated solution.
From all these aspects, I can not imagine at all, what would be the benefit of a PWM solution, other than over-engineering an already solved problem
, just like this other voodoo stuff here, as PCB meander cuts, fancy resistors, ceramic and multi layer PCBs and so on.
Frank