Fluke 730A DC Transfer Standard

[W6EL]

This is quite impressive.

I wonder if those little pops you saw on one channel could be from a fresh capacitor. Maybe it will need a day of burn-in to stop?

I like to reform my caps before use. Sometimes you will see some odd stuff if you monitor the microamps during a reform. I imagine such a thing could show up in your data, especially at these levels.

Or, something else. Who knows.

[dietert1]

Looking at the diagram one could say a single reference is about 4 uVpp and the mean about 2 uVpp - as expected from statistics. To me it appears as if the Channel 3 reference may be ok and its visible noise was a measurement problem, as the mean behaves normal. The red curve with its low noise indicates there is room for improvement. Still a nice sup-ppm reference.

Regards, Dieter

[branadic]

To me it appears as if the Channel 3 reference may be ok and its visible noise was a measurement problem, as the mean behaves normal.

What you mean by that? While I agree in theory on the observation, the setup was left untouched between the measurements, but only the buttons on the front panel were pushed from arithmetic mean to channel 1 ... 4 and to arithmetic mean again. The cable was connected to the output binding post prior the very first measurement with some waiting time and left as is.

-branadic-

[dietert1]

For example there could be a bad contact in the switch. Or the averaging output of each reference module is separate from the output used for the switch. I seem to remember this from the schematics of the Valhalla unit you showd before.

Regards, Dieter

[branadic]

I think there is a simple test to find out if the switches are dodgy. Instead of selecting each reference by the switch, I could set the unit to arithmetic mean and set all other references to open via the voltage output switch, except the one I want to measure. This way I can see the contribution of the selected reference to the mean.

-branadic-

[alm]

I think there is a simple test to find out if the switches are dodgy. Instead of selecting each reference by the switch, I could set the unit to arithmetic mean and set all other references to open via the voltage output switch, except the one I want to measure. This way I can see the contribution of the selected reference to the mean.

Or how about using the Centronics-type connector at the back which has each individual reference on a separate pin?

[branadic]

Easy answer, there is no Centronics connector at the back of this unit
I wonder if the connector at the second unit is a factory modification. 

-branadic-

[branadic]

I've repeated the measurement with the front push switch in the arithmetic mean position and by setting the reference to be measured to the 10V position using the rotary switch, while the other three references were set to open. As we can see, the result looks identical to the former one. This way we can rule out the reference selection push switch to be the reason for the large noise on channel 3. We can also see signs of popcorn noise on this reference board.

-branadic-

[branadic]

To further rule out switches or voltage regulators, I rearranged references in the unit, that is reference 4 went to position 2, 2 to 3 and 3 to 4. I then repeated the measurement (after some short adjustment that was necessary as output voltage changed on all of them, most likely due to the supply voltage).
So I'm now sure it's the reference board and not a switch contact or similar.

-branadic-

[branadic]

While working on a replacement reference board I found that the schematic for Rev. A given in the manual has some mistakes at the card edge connector, so attached a corrected version of the schematic.

-branadic-

[branadic]

I received some card edge connectors, which helps to pull the reference boards and to perform some t.c. measurements on them. Attached a very first result of reference board 4, which is the one with the large noise. I used a limited temperature range of 10 ... 23 ... 36 °C. I will go on with one after the other, so we get an idea of how they perform.

-branadic-

[branadic]

Meanwhile, I've measured t.c. on reference board 3 and board 2 is currently running. What I found interesting is, that board 3 with lower noise also has a larger second order component in its t.c. curve. Is that just a coincidence or some correlation that someone else has observed in general and can confirm?
I think we will know more once I've measured all four boards of F730A Rev. A showing different noise levels.

I've also ordered a set of Sub-C NiMH cells to build a battery pack for F730A Rev. B first.

-branadic-

[Dr. Frank]

While working on a replacement reference board I found that the schematic for Rev. A given in the manual has some mistakes at the card edge connector, so attached a corrected version of the schematic.

-branadic-

This schematic still has some faults inside, especially concerning the trim pot R10, its value and its location. This pot should be located between  R11 and R12 and should have on the order of 50 Ohm, or should be drawn as two separate resistors, as you have correctly drawn it in your schematic
Frank
You have

[branadic]

Frank, R10 isn't a trim pot, but a wirewound resistor network on a single mica card.

-branadic-

[branadic]

What I found interesting is, that board 3 with lower noise also has a larger alpha in its parabolic t.c. curve. Is that just a coincidence or some correlation that someone else has observed in general and can confirm?

So while the evidence gets stronger during the current temperature sweep, I've searched the web and found at least one source that seems to confirm my current observation

ZENER-DIODE NOISE GENERATORS

Zener diodes operated in the breakdown region have been investigated as transfer noise standards in the r.f. region. A close correlation has been found between the variation in excess noise ratio (e.n.r., dB) and the temperature coefficient of Zener diodes.

while this is an interesting read too
Noise as a Diagnostic Tool for Quality and Reliability of Electronic Devices

-branadic-

[Kleinstein]

There is correlation between the noise, TC and zener voltage.  Lower voltage zeners (in the relevant 5-7 V range) tend to have less noise and a more negative TC. The noise however also depends on the construction. In combination there is also a correlation noise and linear TC. On the other side the high voltage zeners can have a lower differential resistance and are thus less critical with the current stability.

With some of the zener reference there are reports that Zeners (and AFAIR also resistors) with more popcorn type noise also showed more long term drift than the less noisy ones. This somewhat makes sense as the popcorn type noise can be seen as current paths that switch between active and inactive. Beside the random changes causing noise there could as well be more permananet activation / deactivation of the same paths.
So there is some incentive to look for the low noise ones, not just for the noise, but also to hopefully avoid the rather drifty ones.

The zener references here show a good trim of the linear TC (selecting the right resistors in the ref. amplifier) and only a difference in the 2nd oder TC and thus the curvature. The curvature may still correlate with the noise, but that is likely more indirectly, e.g. via different voltages as a common cause for both. With just one sample of a slightly more noisy ref. this are not really much data. It could be as simple as a lower ref. current because of a different voltage to start with.
With a relatively high linear TC (at least for parts) there is a chance that temperature fluctuations cause low frequency noise.

[dietert1]

How are these TC measurements done? What is the temperature parameter?
As far as i understood, each reference board is used with a cable extender, i.e. outside of the 730A instrument and inside some temperature chamber. So the nice temperature adjustment for 20 .. 26 °C might be subject to additional heating when running inside the instrument, with all covers closed.

Regards, Dieter

[branadic]

The reference boards are pulled from the F730A and connected to the meter as well as a linear lab power supply set to 13.2 V (that is what the regulators in the F730A are set to) using a card edge connector. The board is sitting inside my thermal chamber (incubator controlled by an Arroyo 5305). The temperature you are looking at is the temperature of the chamber, with the temperature sensor of the controller being close to the DUT. By the way, inside the instrument there is plenty of space between the cards.

Attached is an update of the diagram with the result of reference card 2 included.

-branadic-

[branadic]

Attached is the t.c. plot with all four reference being measured. To me it still looks like the noise really correlates with the opening of the parabolic t.c. curve as reference board 1 showed the lowest noise of 'em all followed by ref 2, 3 and eventually 4.

-branadic-

[dietert1]

Yes, if we assume that inside the instrument the temperature will be 5 or 10 °C higher, then references 3 and 4 will be operating in a region with much higher slope than reference 1. One could try to adjust their zero TC temperatures a little higher.

Regards, Dieter

[Dr. Frank]

Hello,
it would be of interest, whether branadic measured the temperature inside or outside the package (I mostly measure the inner temperature on such experiments.)
For the LTZ1000 reference, the temperature difference between inside and outside, with a thermal isolation around the circuit, is at about 6..7°C, if I remember correctly, and all my ovens run at around 50°C.
The A version consumes about 18mA @ 12V supply, whereas the non A version consumes 22mA, i.e. these are 220 and 260mW.

As branadic wrote to me, one module / reference of the 730A draws <6mA @ 13.2V, i.e. 80mW or 1/3 of the LTZ case.
So I expect a rise of the inner temperature of only 1..2 °C.

I guess, that Fluke has taken that into consideration, and set the zero T.C. point to the intended nominal room temperature.
This is evidently 25°C, as the 730As T.C. is specified (symmetrically) 0.5ppm/°C from 20 .. 30°C.

Frank

[branadic]

Hi,

it would be of interest, whether branadic measured the temperature inside or outside the package (I mostly measure the inner temperature on such experiments.)

My thermal chamber is way too small to fit the whole 19" rack into it, so

The reference boards are pulled from the F730A and connected to the meter as well as a linear lab power supply set to 13.2 V (that is what the regulators in the F730A are set to) using a card edge connector. The board is sitting inside my thermal chamber (incubator controlled by an Arroyo 5305). The temperature you are looking at is the temperature of the chamber, with the temperature sensor of the controller being close to the DUT. By the way, inside the instrument there is plenty of space between the cards.

As the Rev A unit also suffers from not having a connector on the rear panel I have no temperature sensor inside the rack. It would require me drilling a hole into the unit, which I haven't done just yet.

-branadic-

[branadic]

In the meantime I received some battery cells and am in the process of welding/building battery packs for both units. I also received some switch assemblies and was able to repair the power button on Rev. A unit.
I also added a connector at the back and an internal 10k thermistor (GA10K3A), to monitor internal temperature in the future.
Furthermore, the kind previous owner of the Rev. A unit sent me more reference boards so I can select the ones with lowest noise to complete the unit into a fully working and usable device. More noise and t.c. measurements to be performed soon.

-branadic-

[branadic]

From the reference boards I received I selected 4 out of 8 that showed lowest noise for F730A Rev. A. I had to add a small 3Ω resistor to one of the reference boards to get back into adjustment range for the 10 V.
After some adjustments of the 10 V on all channels I captured 1 hour stability data for the averaged output and it looks rather promissing, given the unit in this configuration is just running a few hours and needs some time to stabilize. Note: This measurement was made with the unit powered from mains, as some battery cells are backordered, so I couldn't complete the fourth battery pack for both units.

Next step is to check and if necessary adjust the 1 V, 1.018 V and 1.019 V as preparation for a volt comparison with Frank.

-branadic-

[branadic]

More cells arrived and I could finish the battery packs for F730A Rev.A. I used NiCd batteries for this one.

-branadic-