偏离线性相位程序错误地表征了DUT

我使用GPIB控制8753和8720系列网络分析仪来测试微波滤波器。
我不同意Agilent / HP（以及其他公司）发布的测量相位失真为“偏离线性相位”的程序背后的逻辑。
我不使用分析仪来确定与线性相位的偏差，也不使用分析仪来模拟电气长度的变化。
我使用分析仪捕获包裹的相位数据，然后执行所有处理以确定计算机上软件中线性相位的偏差。
我对“偏离线性相位”方法的理解是，通过调整电延迟来增加/减去电气长度，以获得最小的峰 - 峰值。
这应该消除通过DUT的线性相移，只留下线性相位的偏差。
我的理解是，如果我看一下“理想”相位（重建“理想线性相位”），理想相位线的斜率是由于电长度的相位旋转引起的;
它实质上模拟了将无损传输线与长度可变的DUT串联。
我的理解是，改变电长度的效果是旋转实际相位数据集（改变线穿过数据点的斜率并平移其周围的所有点）而不改变线性相位的偏差（因为线性相位）
线与实际测量的相位数据一起旋转）。
这就好像你只是绘制图形，然后将其设置在一个唱机上。
数据点（包括展开阶段和线性阶段）之间的关系保持不变;
它们只是作为一组*旋转*。
据推测，旋转的原因是抵消DUT的电气长度，这应该消除数据的线性相移 - 旋转到不同的角度表示电气长度的简单变化。
所以我不明白为什么，如果你使用最小二乘法或回归法通过在观察到的未包裹的相位数据上拟合一条线来找到线性相位，你不会只是旋转数据集（改变电气长度）这样的
线性相位的斜率为零，然后平移数据集，使得线性相位线在X轴上（其中Y = 0）。
这应该导致偏离线性相位的完美图表，而不需要从未展开的相位中减去线性相位，因为它是0并因此已经减去。
此外，峰 - 峰测量现在很容易，因为与X轴平行的偏差数据允许直接在Y轴上测量峰 - 峰值。
_这就是摩擦：这样做并不会导致偏离线性相位的最小峰峰值 - 经验表明，通常需要增加或减去额外的电气长度，以最大限度地减少峰峰值偏差！
_那困扰我。
原因如下：似乎+当理想线性相位线的斜率为0时，您正在看*正确*当DUT的电气长度被消除时，与DUT的线性相位的偏差。
因此，电长度的任何额外的相加或减少表示增加或减小电长度以确定使相位失真最小化所需的*最佳*电长度。
但是......如果你正在做的是试图表征DUT的相位失真，那就不一样了;
事实上，您正在设置电气长度，以补偿DUT的电气长度应该是什么，以获得最小化失真，而不是测量DUT实际电气长度的相位失真！
因此，不是表征DUT，而是表征DUT，如果它的电子长度是理想的...换句话说，要测量由于DUT引起的相位失真，你不应该旋转数据集（加/减电长度）直到到达
一个最小值，但你应该只旋转，直到理想（线性）相位线的斜率为0.然后（然后只有）你看DUT引起的失真。
任何额外的电气长度变化都会在测量中引入误差！
我在这里错过了什么吗？
无论我到哪里，它都会说延迟加/减去+最小化+线性相位的峰 - 峰偏差;
这似乎错了！
顺便说一句，我从8720D / E / ES / ET用户指南的第2章获得了大量这些信息，其中讨论了相位失真和线性相位测量的偏差。



     以下为原文

  I use GPIB to control 8753 and 8720 family network analyzers to test microwave filters. I don't agree with the logic behind the procedures published by Agilent/HP (and other companies!) to measure phase distortion as 'deviation from linear phase'.   

I don't use the analyzer to determine the deviation from linear phase, nor do I use the analyzer to simulate electrical length changes. I use the analyzer to capture the wrapped phase data, and then perform all processing to determine deviation from linear phase in software on a computer.

My understanding of the 'deviation from linear phase' methodology is that you add/subtract electrical length by adjusting the electrical delay to obtain a minimum peak-to-peak value. This is supposed to remove the linear phase shift through the DUT leaving only the deviation from linear phase.  

My understanding also is that if I look at 'ideal' phase (meaining the 'ideal linear phase'), the slope of the ideal phase line is due to phase rotation from electrical length; it in essence simulates placing a lossless transmission line in series with the DUT whose length is changeable.  It's also my understanding that  the effect of changing the electrical length is to rotate the actual phase dataset (change the slope of the line fit throught the data points and translates all points around it) without changing the deviation from linear phase  (since the linear phase line rotates along with the actual measured phase data). It's as if you simply drew the graph, then set it on a record player. The relationship between the data points, both the unwrapped phase and the linear phase, is unchanged; they are simply rotated *as a group*.  

Supposedly the reason for the rotation is to cancel out the electrical length of your DUT  which should remove the linear phase shift from the data - rotation to a different angle represents a simple change of electrical length. So what I don't understand is why, if you use a least-squares or regression to find the linear phase by fitting a line to your observed unwrapped phase data, you wouldn't just rotate the dataset (change the electrical length) such that the slope of the linear phase was zero and then translate the dataset such that the linear phase line was on the X-axis (where Y=0). This should result in a perfect graph of the deviation from linear phase with no need to subtract the linear phase from the unwrapped phase, since it's 0 and therefore already subtracted. In addition, the peak-to-peak measurement is now easy since the deviation data, being parallel to the X-axis, allows measurement of peak-to-peak values directly on the Y axis.   

_Here's the rub: doing this doesn't result in the minimum peak-to-peak value of the deviation from linear phase - experience shows that usually adding or subtracting additional electrical length is required to minimize the peak-to-peak deviation!!!_

And that bothers me. Here's why: it seems that +when the ideal linear phase line has a slope of 0 you are looking at *exactly* the deviation from linear phase for the DUT when its electrical length has been cancelled out+. So any additional addition or subtraction of electrical length represents increasing or decreasing the electrical length to determine the *optimal* electrical length required to minimize phase distortion. But... if what you are doing is trying to characterize the phase distortion of the DUT, this won't be the same thing; in fact you are setting the electrical length to compensate for what the DUT's electrical length SHOULD be to get that mimimized distortion, not measuring the phase distortion at the actual electrical length of the DUT! So instead of characterizing the DUT, you are characterizing the DUT if it's electical length were ideal...

In other words, to measure the phase distortion due to the DUT, you should NOT rotate the dataset (add/subtract electrical length) until reaching a minimum value, but rather you should rotate ONLY until the slope of the ideal (linear) phase line is 0. Then (and ONLY then) are you looking at the distortion caused by the DUT. Any additional electrical length changes introduce error in the measurement!!!

Am I missing something here? Everywhere I look it says to add/subtract delay to +minimize+ peak-to-peak deviation from linear phase; that seems WRONG!

By the way, I got a lot of this information from Chapter 2 of the 8720D/E/ES/ET user's guide where it talks about phase distortion and deviation from linear phase measurements).