为什么Keysight假设“开放”电容在3 GHz E4991B上固定？

我有一个1.8 GHz的HP 4291B射频阻抗/材料分析仪，但有点困惑的是看到开放标准的电容是82 fF，虽然它可以改为任何_constant_值。
换句话说，它*不*支持三阶多项式C（f）= C0 10 ^ -15 ^ + C1 10 ^ -27 ^ f + C2 10 ^ -36 ^ f ^ 2 ^ + C3 10 ^ -45
^ f ^ 3 ^（f in Hz，C in Farads）1.8 GHz 4291B已经过时，但我查看了更换产品的在线手册（E4991B）并再次看到这个假设开放标准的电容与
频率。
我试图理解为什么会这样，尽管较低频率的VNA支持3 ^ rd ^阶多项式。
我的4291B（P / N 04191-85302）没有一个非常基本的打开，它不能控制夹头的确切位置，因此使用85050B校准套件（85050-80010）打开更好的质量
），其中电容使用以下多项式计算：C（f）= 90.4799 10 ^ -15 ^ + 763.303 10 ^ -27 ^ f -63.8176 10 ^ -36 ^ f ^ 2 ^ + 6.4337 10 ^ -45 ^ f
^ 3 ^ 1 MHz时的电容为90.48 fF，1.8 GHz时的电容为91.6846 fF。
因此，在1 MHz和1.8 GHz之间存在1.2 fF的变化。
由于没有办法将电容作为多项式输入，我选择了75％的上限频率，假设电容在高频时更为关键。
我宁愿任意选择91.41 fF，这是1350 MHz的值。
我错过了什么？
这对阻抗分析仪而言比VNA要小得多吗？
DaveEdited：drkirkby于2016年1月6日下午2:03



     以下为原文

  I've got a 1.8 GHz HP 4291B RF Impedance/Material analyzer but was a bit puzzled to see that the capacitance of the open standard was 82 fF, although it can be changed to any _constant_ value. In other words, it does *not* support the 3rd order polynomial

C(f) = C0 10 ^-15^ + C1 10 ^-27^ f  + C2 10 ^-36^ f ^2^ + C3 10 ^-45^ f ^3^

(f in Hz, C in Farads)

The 1.8 GHz 4291B is obsolete, but I checked the online manual for the replacement product (E4991B) and see that again this assumes that the capacitance of the open standard is independent of frequency.  

I'm trying to understand why this can be so, despite lower frequency VNAs support a 3 ^rd^ order polynomial.  

I don't have one of the very basic opens for my 4291B (P/N 04191-85302), which does not control the exact position of the collet, so are using the better quality open from an 85050B cal kit (85050-80010), where the capacitance is calculated using the following polynomial:

C(f) = 90.4799 10 ^-15^ + 763.303 10 ^-27^ f  -63.8176 10 ^-36^ f ^2^ + 6.4337 10 ^-45^ f ^3^

The capacitance is 90.48 fF at 1 MHz and 91.6846 fF at 1.8 GHz. Therefore there's a 1.2 fF variation between 1 MHz and 1.8 GHz. With no way to enter the capacitance as a polynomial, I picked a frequency 75% of the upper limit, on the assumption the capacitance was more critical at high frequencies. I rather arbitrarily picked 91.41 fF, which is the value at 1350 MHz.  

As I missing something? Is this far less critical on an impedance analyzer than a VNA?  

Dave

Edited by: drkirkby on Jan 6, 2016 2:03 PM