Shunt capacitance of 1206 SMD resistors; Jeroen Belleman - December 2010

[joeqsmith]

For fun, I soldered a 100k onto the coplanar fixture.  When looking at the data from CERN, their 100k seems to be a flyer (Capture1).    I then normalized both data sets to the ideal 50fF.   

[Joel_Dunsmore]

From a time in the past, I found a very good model of SMT 1206 resistor is the resistance in series with a 1 or 1.5 nH inductor, all shunted by a .2 or .3 pF capacitor.  This model seems to work well up to about 3-6 GHz.

[joeqsmith]

From a time in the past, I found a very good model of SMT 1206 resistor is the resistance in series with a 1 or 1.5 nH inductor, all shunted by a .2 or .3 pF capacitor.  This model seems to work well up to about 3-6 GHz.

I would guess that the problem is not your model but my home made cal standards and test fixture.  Or, I screwed up the math.  Lots of possible mistakes.  Feel free to repeat the test using your own math. 

[joeqsmith]

I was curious about using even higher value 1206 resistors.

[joeqsmith]

I was also wondering how the 0603 package would compare to the 1206.   To speed the collection, I went back to the coaxial quick change fixture.   They seem very similar.

[T3sl4co1l]

Neat!

0805 should be different, having a different aspect ratio.

TIm

[joeqsmith]

Just how much fun can you possibly have with a few resistors anyway?    

[T3sl4co1l]

Just how much fun can you possibly have with a few resistors anyway?   

I used to think not much, but you're beginning to make me question that conclusion...

Or you're just mental.

Tim

[David Hess]

Just how much fun can you possibly have with a few resistors anyway?   

In precision and high impedance circuits, plenty!

[joeqsmith]

Even with low impedances and standard parts,  I have kept myself amused.   Certainly seems a bit on the spectrum.... 

I was hoping Joel would have added his own twist to the experiment or at least commented on G0HZU's response about the 8753D upswing that Jeroen observed.    For some reason, I envision Joel having a home lab with every VNA HP ever made sitting in it or at least every flavor of the 8753.       

[Joel_Dunsmore]

I was hoping Joel would have added his own twist to the experiment or at least commented on G0HZU's response about the 8753D upswing that Jeroen observed.    For some reason, I envision Joel having a home lab with every VNA HP ever made sitting in it or at least every flavor of the 8753.   

  I have no home to go to, just always at work.

No, I'm not so much of a hoarder (I was until the big fires in 2017 and lost a lot of gear in my lab to smoke damage).  Now I'm down to only 4 PNAs, 2 PXI VNAs, a fieldfox and a streamline. and a quite old 8753D (A.K.A worlds best network analyzer).

On the up-turn of the data at low frequencies in the measurements: I would suppose it might be mismatch due to the source match of the bridge starting to run out of steam. In the 8753, the bias network and blocking cap started to degrade the match pretty seriously at low frequencies, similarly with the PNA.  And the impedance of the DUT measured is quite close to the edge of the Smith chart for hi-Z resistors, so source-match error is pretty significant.  Oh, and my earlier comment was mistaken; I looked again and I see more on the order of 0.02-.05 pF for these SMT resistors. (0.2 pF was for a through-hole part I wrote about back in the day).  BUT, the real thing about SMT parts is how they are positioned on a line.  We did a big study pre 2000 for SMT libraries in ADS, and found that series and shunt mounting made a big difference in the model (most of this was for capacitors but holds for resistors as well).  So to model your part properly you need to put in into a configuration that matches your use-case. For example for a resistor used in a Pi pad (shunt R, series R, shunt R) you need to build a shunt verison of your resistor model and a series version of your model, and very importantly, consider the pad configuration. For FR4 dual layer board at about 1.5 mm, the 50 ohm line will be wide enough for the resistor pad, but if you are using 4 or higher layer boards with RF lines on the inner layers, then the whole structure of the final-pad layout will have some strong affect on the model, and even more so if you use the resistor shunt to ground through a via.  Even the pad around the via can make a difference, odd as it sounds.   For example, two 100 ohm resistors in parallel will give a better match than one 50 ohm to ground, but the shunt inductance (which might limit the impedance at high frequency) is not 1/2 for two in parallel, due to coupling between the two resistors. So 2 resistors in a T-shape to ground is different than 2 resistors hung in parallel of the end of the line. It all becomes very 3-D ish.

[joeqsmith]

I was hoping Joel would have added his own twist to the experiment or at least commented on G0HZU's response about the 8753D upswing that Jeroen observed.    For some reason, I envision Joel having a home lab with every VNA HP ever made sitting in it or at least every flavor of the 8753.   

  I have no home to go to, just always at work.

No, I'm not so much of a hoarder (I was until the big fires in 2017 and lost a lot of gear in my lab to smoke damage).  Now I'm down to only 4 PNAs, 2 PXI VNAs, a fieldfox and a streamline. and a quite old 8753D (A.K.A worlds best network analyzer).
....

  I knew it!  I can only imagine what it was like before your having to clean house.   

Your mention of the large SMT library study sounds like what member niconiconi was asking.   

https://www.eevblog.com/forum/rf-microwave/can-a-siggensadirectional-coupler-be-used-as-a-poor-mans-scalar-analyzer/msg3605548/#msg3605548

I was playing around with a few of the free EM simulation tools but didn't get very far with it.  It's an interesting subject. 

[Joel_Dunsmore]

I knew it!  I can only imagine what it was like before your having to clean house.   

Before the remodeling for the big fire recovery, I as assigned a double cube and 2 benches, it was one of the perks of being a Keysight Fellow (well no, it wasn't, but that was what the lab manager would tell other engineers when they asked why I got the big space; truth was he didn't want to deal with the trouble of making me get rid of all my stuff).
After the fire there was a strict 1-cube rule, which since the cube next to me was not so occupied I laid claim to it and yes, the one next to that as well. So my equipment empire is being re-established.

Your mention of the large SMT library study sounds like what member niconiconi was asking.   

https://www.eevblog.com/forum/rf-microwave/can-a-siggensadirectional-coupler-be-used-as-a-poor-mans-scalar-analyzer/msg3605548/#msg3605548

I was playing around with a few of the free EM simulation tools but didn't get very far with it.  It's an interesting subject.

In fact I was reading that and posted a link to an SMT bridge from years ago. I'll need to repost to also comment on the original question of measuring the influence of footprints and other parasitic effects. Once you get above 1-2 GHz, you have to have rather precise phase to get a good estimate of reactive aspects such as excess shunt capacitance or series inductance. I don't think a scalar network analyzer would provide the needed information.

[joeqsmith]

So no double cubes, but nothing about only one cube per customer.  Cube etiquette.   

Does Keysight have internal auctions/giveaways for the engineers to help dispose of the fire damages and no longer used equipment? 


It does seem like they would need a VNA to make that measurement.  I was interested in seeing if I could simulate what they had proposed and then test it using actual hardware.   I suspect it would take more than my home hobby budget to pull it off.