Here are my tests:
All according to component specification except for temperature setpoint where i have a ratio of 12.7 with slightly lower resistance for each of the resistors.
Sweep from 10 degrees Celsius to 52 degrees. Start at 10 degrees, step of 3 degrees, 18 min for each step.
Two graphs below
LTZ no 2.2K : Keep pretty much within 10 uV except for dip to 10V where heater was collapsed. I have fans in my TEC cooler box and no cover on the LTZ, so makes sense. I think this is a very good result.
LTZ with 2.2K: TC turns into a disaster.
Pretty strange.
Hello,
I cannot see a "disaster" with the 2k2 assembled. As Andreas already wrote:
In both cases, the oven regulation does not work any more, I estimate from 40°C onwards, these are the
dips decreases of -3 and -5ppm in the middle of
both diagrams.
Usually the reference voltage will go up in this case, not down, due to its +50ppm/K unregulated T.C. so that's kinda strange.
The noise is very big with about 1ppm peak-peak, might arise from low NPLC and lack of filtering. And I see several negative dips of many ppm, which definitely arise from external noise.
I simply copied Andreas design, removing some unnecessary angst capacitors, but this circuit, assembled into a tuner box is very immune to external noise, I've never again seen such dips. So maybe you are lacking this external shield, and you might have some EMC stinkers in your lab.
The 2k2 is meant for starting the reference regulation towards positive and to relieve the OpAmp from driving current, as Kleinstein already stated.
Some AZ types show much lower open loop gain, when they are loaded, or even have higher Offsets (not sure about latter). Another effect is less heat dissipation inside the OpAmp, which might reduce offsets for non chopper types.
Its value depends on the supply voltage, zener voltage, the zener current and a bit on the oven setting temperature, as this also changes the zener current.
The reference circuit w/o OpAmps and oven supply consumes less than 5mA in most cases. I have chosen 48..55°C, 12V supply, so I have used 1k5, if I remember right, leaving about 1.5mA for the OpAmp. 2k2 is good for 15V
max.. If you further increase the supply voltage, like in the 3458A, you must increase this resistor also, because the regulation will fail otherwise.
Changes in this resistor or change of external temperature won't give rise to any increase of the zener current, neither does it influence the reference circuit at all, as long as the oven works correctly.
The external temperature rise will decrease the heating power on the other hand.
I think Dieter Teuchert himself does not understand the function and influence of this resistor, and of the design modifications we have done.
But I also did not expect that from him, I have to retort.
A few other engineering hints:
This circuit was also designed for lowest cost, i.e. using a single sided PCB, lesser parts, less expensive PWW resistors. Another advantage is that all solder joints are on the bottom side, intended to have them all on the same temperature if the PCB is assembled horizontally, and if an appropriate heat isolation is used. The idea was to reduce or compensate any e.m.f. by creating an isothermal layer that way, as this is one of the greatest source of error in this circuit, see hints in the LTZ datasheet.
Meanwhile I replaced the LTC1052 by the AD4522, which is much less noisy.
Frank
Edit 14.8.:
I added an equivalent diagram, T
env. and V
ref for my LTZ #7.
This has an 'A' chip, requiring at least 12°C lower room than oven temperature.
It's set to about 52°C T
oven, 12k/1k resistors for 13:1 ratio, and you can clearly see in both diagrams, that the oven fails at about 40°C, with +50ppm/K.
The decrease of V
ref to the right, between 8400 .. 10200sec arises from too fast cooling down, i.e. that's probably caused by component temperature imbalance.
Compared to your measurement, you can also see how low the noise of my measurement is, typ. 0.15ppm
pp, and that there are no dips originating from the reference.