Hi,

I'm working on a project that needs the characterization of a SOLT calibration kit.

The kit represents a transition from 3.5mm SMA connector to a microstrip line of length 20mm.

The measurement for frequencies above 3GHz requires the modelization of the open fringing capacitance as a polynomial of order 3.

I perform the following steps:

1- 1-port calibration at the end of the test cable (85033D calibration kit).

2- Connect short standard (20mm 50ohm microstrip line ended by 2-parallel 50ohm resistors) and make port extension until 180 degrees phase shift.

3- Disconnect short standard and connect the open one.

4- Adjust the delay until the phase is negative (Negative phase across the whole band).

5- Extract the formatted RI data and trace the parallel capacitance values as a function of frequency.

6- Use mathematical tool to fit the graph in order to evaluate the coefficients of the capacitance polynomial.

Note that the open phase trace is neither flat nor linear and reaches -28 deg at the end of the band (6 GHz) if we want negative phase for the whole operating band.

My problem is that the values of the coefficients of the capacitance polynomial are big.

(C0=38.33E-15; C1=79676.1E-27; C2=-35687.91E-36; C3=4270.35E-45).

The VNA doesn't accept values more that 10000 or less than -10000.

Actually, I tried to divide the band into two sub-bands, but that same problem results.

Moreover, I tried to minimize the band in order to achieve flat phase (less than 1 deg variation) but also the same trouble arises.

Thanks for your help.

I'm working on a project that needs the characterization of a SOLT calibration kit.

The kit represents a transition from 3.5mm SMA connector to a microstrip line of length 20mm.

The measurement for frequencies above 3GHz requires the modelization of the open fringing capacitance as a polynomial of order 3.

I perform the following steps:

1- 1-port calibration at the end of the test cable (85033D calibration kit).

2- Connect short standard (20mm 50ohm microstrip line ended by 2-parallel 50ohm resistors) and make port extension until 180 degrees phase shift.

3- Disconnect short standard and connect the open one.

4- Adjust the delay until the phase is negative (Negative phase across the whole band).

5- Extract the formatted RI data and trace the parallel capacitance values as a function of frequency.

6- Use mathematical tool to fit the graph in order to evaluate the coefficients of the capacitance polynomial.

Note that the open phase trace is neither flat nor linear and reaches -28 deg at the end of the band (6 GHz) if we want negative phase for the whole operating band.

My problem is that the values of the coefficients of the capacitance polynomial are big.

(C0=38.33E-15; C1=79676.1E-27; C2=-35687.91E-36; C3=4270.35E-45).

The VNA doesn't accept values more that 10000 or less than -10000.

Actually, I tried to divide the band into two sub-bands, but that same problem results.

Moreover, I tried to minimize the band in order to achieve flat phase (less than 1 deg variation) but also the same trouble arises.

Thanks for your help.

> {quote:title=Frequency Lover wrote:}{quote}

> Hi,

>

> I'm working on a project that needs the characterization of a SOLT calibration kit.

This interests me too, as you might have seen from another thread:

"Can one have a negative offset and capacitance for an "open" cal standard?" You might want to look over that thread at

https://community.keysight.com/message/58675#comment-58675

> The kit represents a transition from 3.5mm SMA connector to a microstrip line of length 20mm.

My interest was characterisation at the open end of a connector, rather than the end of 20 mm of transmission line, which is your interest, but of course there are simularites in our wishes too.

If you have not read it, you really should look at Dr. Joel Dunmsore's book,

http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119979552.html

section 9.1,"Creating your own Cal Kits" pages 552-569. This is very much based on PCB structures like you have. It makes extensive use of the TDR option on a VNA, to characterise the end of the transmsion line using time-gating to remove the effect discontinutiy of the SMA/micorstrip transition.

Do you have the TDR option on your VNA? That is option 010 on the 3 VNAs I've owned (HP 8753A, HP 8720, Agilent N9923A), so *perhaps* it is option 010 on all HP/Agilent VNAs. If your VNA says option 010 on the rear, I'm almost certain it will have the TDR option, but it could have been added after the instrument was built, as it is only a software option. If your VNA is not too old, the TDR option can be purchased. What VNA do you have? FWIW, on my 8720D, if I push the "System" button on the front panel, I see as one of the softkeys the words "Transform menu".

> The measurement for frequencies abov"e 3GHz requires the modelization of the open fringing capacitance as a polynomial of order 3.

I think you will find it is a lot lower than 3 GHz where you need this.

> I perform the following steps:

>

> 1- 1-port calibration at the end of the test cable (85033D calibration kit).

> 2- Connect short standard (20mm 50ohm microstrip line ended by 2-parallel 50ohm resistors) and make port extension until 180 degrees phase shift.

It does not make sence to me to use 25 Ohms of reistance as a short. I would have thought a couple of zero Ohm resistors would be a more accurate representation of a low-inductance short.

How much port extension do you have? I would have expected as first approximation it would be the electrical length of the transmission line, which you can calculate that from the permittivity of the substrate, the thickness and width of the substrate. There is also the electrical lenght between the SMA connector's reference plane to where the SMA connects to the microstrip. The attached document, which is the MIL standard for many RF connectors, which is very poorly laid out, will show you where the reference plane is. Note male and female parts of the connectors are on wildly differing page numbers.

What are you using when you connect the open standard? I'm guessing you are using the coaxial one from your 85033D cal kit. I think you need to connect the microstrip sized open/short/load on the end of the transmission line on the PCB, not the coaxial ones from your 85033D cal kit.

> 4- Adjust the delay until the phase is negative (Negative phase across the whole band).

What phase? With your open, or 25 Ohms of resistance?

> Thanks for your help.

I hope I given you some help on this, *but I am no expert, and are still struggling with a similar problem to you.*

Dave

Edited by: drkirkby on Jan 4, 2013 10:45 AM

Edited by: drkirkby on Jan 4, 2013 11:06 AM