LCR Quadtech/Genrad/IET Digibridge 1689 - use different calibration resistors

[timeandfrequency]

As I understand the service manual specification, it says that the Q value should be know with 25ppm of the real value. So that a Q value of 5ppm could be measured from -20ppm to 30ppm, and not 25ppm of the 5ppm (meaning a range from 4.999875ppm to 5.000125ppm).

It is quite sure that your assumption is correct : 25 ppm is actually the allowed uncertainity on the Q value.
Because asking for a 25 ppm precision on a Q value of 5 ppm compares to a moonshot

Using resistor decades
At first glance, I would also favor @manganin 's point of view.
But on the other side, 1 kHz is still a very low frequency. And the Q values you measured tend to confirm that there's not much reactance.

Generally speaking, your two sets of measured values are really close.
Using the test equipement you mentionned, as far it is calibrated, plainly fullfils the Genrad SM requirement (20 ppm precision for R and 25 ppm max uncertainity for Q)


And many thanks to @gnif (one of the forum administrators) for his quick help, because I could not post a section of my previous message (the forum app raised an error).
He found the reason why my message was rejected and is currently working on a fix.

[rodpp]

I did the calibration of the Digibridge 1689.

First the frequency calibration using a signal generator and an universal counter. The Digibridge stores the constant (6ppm in my case) but the test frequency doesn't change after this "adjustment". It seems that it uses the constant in some compensation calculation, not affecting the electronic circuit.

The frequency correction K factor adjustment doens't work in my unit, probably because it uses an old ROM version. I have two service manuals, and only the newer one has this adjustment (item 5.9.. Maybe in the future I'll update the ROM to a newer version to check this.

And finally I adjusted the ranges using the decade boxes, multimeter and bridge.

Everything seems to be working good.

Thenk you all!

[RolandK]

For calibration, read the 1693 instruction, too. The procedure is ah ... very interesting.

If your 1689 still has the calibration only check your calibration resistors with mean value and highest averaging (measure Q in ppm, write down both values) . If you realize one of the calibration resistors in the meter has drifted out of tolerance, only recalibrate this range with the R-Value from the Ohmmeter and the Q-Value from the check measurement. The Q value does not drift significant, as it is based on the construction of the internal reference, the pcb layout and the like.
Do NOT try to calibrate if avoidable. It will not get better and to reach the same level is very hard work.

Calibration:

FIRST: Start with the frequency calibration.

I used the Q factors from a genrad calibration certificate, until i can measure it better (see Q values below).

i just use 4 bare Vishay Precision resistors, there is no need to have exact the manual values, in fact in the 1689 the reference resistors are used in series, so the sum values are better. I just measured them with my best ohmmeter. Therefore the parts tolerance doesn`t matter. But I use the Vishay because of their very low temperature coefficient. Standard resistors with 100 ppm/°C are not my liking there. So cheaper vishays with e.g 0.1% tolerance are fine.

R= 25 Ohm Q = 11 ppm. Y169025R0000T9L is a four wire shunt resistor upto 8W. For the desktop variant i put a piece of paper between the legs, so each of the 4 legs makes only contact to one electrode, so a perfect Kelvin connection. The inner contacs are bend outwards, so they clip the paper with the 2 outer ones (see picture)
R= 400 Ohm Q = 0 ppm Y0007400R000V0L
R= 6K Q = 2 ppm  in use: 1k + 5k soldered together, so that i can use all 3 values (if i need a 1k or a 5k) Y14531K00000T9L and Y14535K00000V9L
R= 100k Q = -258 ppm Check this value with your lowest D Foil capacitor, eg. a X2 type, should be arround 100 ppm. D of capacitors is never negative.

Of course make all measurements with correct open short calibration. For checking the open short must be done for each frequency extra. (eg the hp 4274a / 4275a does it for all frequencies at once and no special code, just 2 buttons)

I checked it against a genrad 1404 100 pF vacuum capacitor, which has D < 10 ppm according to the article in genrad experimenter Aug 1963 page 8.
The procedure for it was really annoying, as i use different cable sets and had to make open and zero calibration when changing between the 100k and the 100pF. I was tweeking the Q of the 100k resistor until the D of the capacitor was at 10 ppm.
This is useful to measure low D foil capacitors.

On picture: pins of 6 k resistor in 60° positions: 1k - 5k - 6k, all 3 values are usable
25 Ohm kelvin resistor pins bent to paperclip for kelvin connection in desktop 1689
short consists of a piece of 1,5 mm² wire flatened with a vice with flat surface. This is ok for 4 wire connection.