LCR measurements at high DC bias - Agilent 16065A DC bias fixture teardown

[nexus]

Hi all,

Recently picked up a bit of kit that I had been on the lookout for. This is an Agilent 16065A DC bias LCR test fixture. It allows you to apply up to a 200 volt DC bias to a DUT while safely connecting it to any 4 terminal pair LCR meter. As you can imagine, quite useful if you would like to characterize parts like MLCCs.

I never saw any pictures or examples of one being used in the wild, so this was quite fun to pick it up and have a look inside. Also, made some measurements up to 200 volts and got pretty darn close to the curves shown in the part datasheet.

Lots of pics and details here: https://neonkev.com/2022/01/22/lcr-measurements-at-high-dc-bias-voltages-how-and-why-agilent-16065a-dc-bias-lcr-fixture-restoration-and-measurements/

Enjoy!

[inse]

What a complex add-on fixture.
Some time ago I made an adaptor for the same purpose (comparison of different MLCC ceramic types) for our good old 4284a.
For sure without all the fancy interlock and discharge details, it consisted of four components on perf board iirc.
Two large foil capacitors for DC decoupling of the instrument and two resistors for AC decoupling of the source.
And two springy pieces of silver wire to hold the chip capacitors.
Did the job surprisingly well :-)

[Conrad Hoffman]

We have one of those Keysight LCR meters at work. Nice piece of kit until you find out how much they want for a calibration. 

[nexus]

Yes, good old HP designed over engineering. The fixture is quite well made, sturdy sheet metal, all the switches and hinges mate perfectly. I suppose you do get something for your money besides a few capacitors haha!

That sounds like an interesting project. If you have any pics or schematics that would be great! The schematic for the 16065A is in the service manual, I attached a pic.

I'm not certain on why film capacitors seem to be preferable... is it a matter of quality factor or is it just for overall capacitance and voltage rating?

[nexus]

I just checked out the calibration price on the Keysight online service portal... $507.00 USD for a standard cal. Is that relatively high considering the cost of the instrument?

[inse]

I am going to have a look if this testboard is still laying around, it has only been made for this one purpose years ago.
Foil capacitors are preferred for decoupling as they don't have bias issues.

[Conrad Hoffman]

Hmmm... maybe we have a different version. We got a quote of over $3k each, and we have three of the instruments to do! I think ours is E4890A, with all the options, about $16k when we bought them. OTOH, maybe Keysight wised up on their pricing as this was some years ago.

[nexus]

Keysight has several different tiers of calibration, from the standard cal to a cal with "guardbanding + uncertainties" if you really need the highest level of traceability and accuracy spec. Depends on the line of work you are in and the liability you incur by selling a specific type of product (medical, avionics, etc.)

Also important to remember that a calibration is only valid between two points of time. A cal today does not mean my measurement is in spec until the next future calibration confirms it was.

[jonpaul]

Some LCR have built in bias or provide for ext like the Wayne-Kerr.

More useful than V bias is a bias DC current in testing inductors.

That is a much more difficult adapter to design.

Generally, iron or ferrite inductors will have a large variation in L with DC bias I
Resistors have a voltage coefficient at high voltage, especially carbon based.
Capacitors voltage coefficient are seldom an issue as the high coefficient caps are used in bypass or other non critical applications.

Kind Regards,

Jon

[TERRA Operative]

Nice, I've been keeping an eye out for one of these for a while, but nothing has popped up locally. I'll have to see if I have a spare test fixture of some type here and maybe convert it to a DIY one of these.

I have a 4276A with a DIY +/- 40V DC Bias Option that works well, but higher voltages would be really handy for testing higher voltage electrolytics.

[inse]

Unfortunately, my DC Bias test rig could not be found anymore, maybe I have discarded it myself.
But thanks to a well-filed documentation I can show two pictures.
With this simple, ugly test setup, I characterized different ceramic capacitor types.
They have been measured for capacity vs. temperature and DC bias from chilly -40C to scorching 125C and up to 300V.
As mentioned before, the foil capacitors block the DC voltage from the instrument and the resistors minimize the influence of parasitics in the power supply.
The measurement error due to the foil capacitors has been removed in the excel analysis file.

[nexus]

Nice! Looks like a handy fixture to have for some quick dc bias measurements. When you calculated measurement error, did you do it by performing LCR measurements directly on a cap first, then comparing the same cap when measured using fixture at same dc bias level (0V for control, or max. safe DC bias voltage for meter directly)?

[nexus]

More useful than V bias is a bias DC current in testing inductors.

That is a much more difficult adapter to design.

Generally, iron or ferrite inductors will have a large variation in L with DC bias I

That is an interesting point Jon. I never thought that one would be able to inject DC current while making an accurate AC LCR measurements.

I did some research and indeed found that there are special DC current power supplies meant for doing DCI bias on a DUT while attached to an LCR meter. I found some really good info on Voltech's website where they explain challenges of making a DCI biased LCR measurement: https://www.voltech.com/products/dc1000a-25a-dc-bias-supply/how-it-works/

It makes sense that the special DC supply in this case should be designed to minimize sinking current from the LCR meter AC source, since that is how the LCR impedance measurement is calculated in an auto-balancing bridge LCR meter like the E4980A/AL.

I found a Voltech DC1000 on eBay going for auction today and bid sniped it! I will be sure to open it and have a peek, with some pics. It may be an older box, so I might recap the electrolytics and look for any RIFA paper cap madness that is waiting to explode!

[inse]

Iirc, with this setup I confirmed that there was no DC bias issue with the MKT's by placing another one as DUT first.
 I don't remember all details, but I assume I also confirmed the functionality by a comparison between direct measurement and test setup measurement.
The purpose was no precision measurement of all possible properties, but to provide dC/dU/dT graphs not published by different suppliers in a comparable way.
Big plus: this rig could be put into the oven!

[nexus]

Opened up the DC1000 to have a look. Details are here: https://www.eevblog.com/forum/testgear/voltech-dc1000-peek-inside/

[nexus]

Made some DCI biased LCR measurements! More info at the above thread.