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Adding Lterm(inductance) for load standard to calibration kit

Question asked by UsmanSadiq on Jul 14, 2017
Latest reply on Sep 7, 2017 by UsmanSadiq

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Hi,

I have a question regarding adding the calibration kit to the Agilent PNA. Generally with probes, for load standard there is an inductance Lterm. How can we add this to the definition of cal kit. Previously, I always used fixed load of 50 Ohm without defining Lterm as there is no way to define L term with a fixed load (shown in screen shot attached).

In the attached screen shot, it shows the delay term has been added based on Lterm.

Can anyone guide me what is the best practice to do it? Is it advisable to ignore Lterm?

Thanks

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Dr_joel

Jul 14, 2017 9:44 PM

Well, I will point out that this is thoroughly documented on page 559 of my book. but to save you a few bucks, the formula is L=Z*t, where Z is the offset impedance and T is the offset delay. In you case it models a load with the effective inductance of 1.5 pH; a very small number. compute it's reactance and decide if it is important or not (at 100 GHz it might be important, maybe not a 100 MHz).

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UsmanSadiq @ Dr_joel on Jul 17, 2017 12:30 AM

Hi Joel,

Thank you for your reply. However its not clear yet. How do I get offset impedance? How did you get 1.5 pH?

In my case the L term mentioned on calibration substrate for ACP 150 probe I am using is -1.7 pH.

Kindly mention your book name name as well.

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Dr_joel

@ UsmanSadiq on Jul 17, 2017 9:19 AM

Set the offset impedance to 500 ohms. Compute the T delay to get the inductance you want.

For 1.7 pH: T=L/Z = 1.7e-12/500=0.0034 psec. Which is just about what your picture shows. So that is the right L value.

This will have a reactance of Xl=2*pi*f*L. At 10 GHz, it has a reactance of .0002 ohms; it can be ignored. In fact it can likely be ignored all the way up to 100 GHz, if I did my math correctly.

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UsmanSadiq @ Dr_joel on Sep 4, 2017 7:09 AM

Hi @Dr_joel,

I have been trying to use the above mention method to calibrate the GS probes using Cali substrate from Cascade. However, I am getting very weird results for some reason I don't understand yet. I have always ignored Lterm for GSG case where it was 1.7 pH and corresponds to 0.1 ohm of reactance at 10 GHz and it always works which makes sense as per your suggestion. It is my very first encounter with GS probes which cannot be different that GSG though.

But for GS case since reactance becomes significant at 10 GHz i.e. 3 ohm for L termination of 47.2 pH (I am using delay of 0.0944 ps for it) and 2.58 ohm for L short (delay 0.086 ps) 41.2 pH . My question is that in Load and Short standard definition in Cal Kit should I use 50 ohm or 500 ohm as shown in the files attached. It may sound stupid but I just want to make sure that I am doing everything right. If this is right then there is some thing else I am doing wrong.

Can you guide me again on this?

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IMG_20170904_153559.jpg3.2 MB
IMG_20170904_153544.jpg3.3 MB

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Dr_joel

@ UsmanSadiq on Sep 6, 2017 5:20 PM

Sorry the late reply: for the fixed load use 500 ohms. for the short use the delay of the short with 50 ohms, and excess inductance directly. The short has both delay and inductance. The load has only delay, so it must be used to model the inductive term. I guess the short would be no delay and only inductance in the case of a probe.

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UsmanSadiq @ Dr_joel on Sep 7, 2017 5:03 AM

Thanks, Joel, your answers are always very helpful,

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6 Replies

Dr_joel Jul 14, 2017 9:44 PM
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Well, I will point out that this is thoroughly documented on page 559 of my book. but to save you a few bucks, the formula is L=Z*t, where Z is the offset impedance and T is the offset delay. In you case it models a load with the effective inductance of 1.5 pH; a very small number. compute it's reactance and decide if it is important or not (at 100 GHz it might be important, maybe not a 100 MHz).
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- UsmanSadiq @ Dr_joel on Jul 17, 2017 12:30 AM
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  Hi Joel,
  
  Thank you for your reply. However its not clear yet. How do I get offset impedance? How did you get 1.5 pH?
  
  In my case the L term mentioned on calibration substrate for ACP 150 probe I am using is -1.7 pH.
  
  Kindly mention your book name name as well.
  Like • Show 0 Likes0
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  - Dr_joel @ UsmanSadiq on Jul 17, 2017 9:19 AM
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    Set the offset impedance to 500 ohms. Compute the T delay to get the inductance you want.
    For 1.7 pH: T=L/Z = 1.7e-12/500=0.0034 psec. Which is just about what your picture shows. So that is the right L value.
    
    This will have a reactance of Xl=2*pi*f*L. At 10 GHz, it has a reactance of .0002 ohms; it can be ignored. In fact it can likely be ignored all the way up to 100 GHz, if I did my math correctly.
    Like • Show 0 Likes0
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    - UsmanSadiq @ Dr_joel on Sep 4, 2017 7:09 AM
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      Hi @Dr_joel,
      
      I have been trying to use the above mention method to calibrate the GS probes using Cali substrate from Cascade. However, I am getting very weird results for some reason I don't understand yet. I have always ignored Lterm for GSG case where it was 1.7 pH and corresponds to 0.1 ohm of reactance at 10 GHz and it always works which makes sense as per your suggestion. It is my very first encounter with GS probes which cannot be different that GSG though.
      
      But for GS case since reactance becomes significant at 10 GHz i.e. 3 ohm for L termination of 47.2 pH (I am using delay of 0.0944 ps for it) and 2.58 ohm for L short (delay 0.086 ps) 41.2 pH . My question is that in Load and Short standard definition in Cal Kit should I use 50 ohm or 500 ohm as shown in the files attached. It may sound stupid but I just want to make sure that I am doing everything right. If this is right then there is some thing else I am doing wrong.
      
      Can you guide me again on this?
      Attachments
      IMG_20170904_153559.jpg3.2 MB
      IMG_20170904_153544.jpg3.3 MB
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      - Dr_joel @ UsmanSadiq on Sep 6, 2017 5:20 PM
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        Sorry the late reply: for the fixed load use 500 ohms. for the short use the delay of the short with 50 ohms, and excess inductance directly. The short has both delay and inductance. The load has only delay, so it must be used to model the inductive term. I guess the short would be no delay and only inductance in the case of a probe.
        1 person found this helpful
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        UsmanSadiq @ Dr_joel on Sep 7, 2017 5:03 AM
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        Thanks, Joel, your answers are always very helpful,
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Adding Lterm(inductance) for load standard to calibration kit

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