Hi All,

I am using E8361A to measure S12 of a typical dual-polarized antenna (Isolation of ports). The frequency range is between 3-8 GHz. Let's consider that S12 is almost -30 dB (with a small amplitude deviation) within the frequency band. I use the "Trace Math", described here, to store the S12 to the memory (Data=>Memory). In the "Data Math" section, I choose (Data – Memory), as shown in the figure below.

So, the plotted result shows (S12 – S'12), which should be mathematically zero or -infinity in dB scale, because the setup, situation, etc. is the same as before, so S12 = S'12. It means that I have subtracted a typical data from itself.

However, the actual result is between -55dB and -75 dB, as shown in the figure below (the graph is plotted only for a limited bandwidth). Now, here is my question: Is this graph correct? Why the graph shows an average of -65 dB, while the dynamic range of the receiver is more than 100 dB?

First, I was expecting that the graph should be less than -100 (or close to the noise level of the device). Later, I thought the dynamic accuracy (or ADC resolution of the receiver) might not let the graph reduces further. For example, if S12 = -30 dB and S'12 = -29.85 dB, then S12 – S'12 would be -65 dB (Note that "Data – Memory" is performed on linear data). This example shows there is an error of 0.15 dB between S12 and S'12. How can I reduce this error? How can I estimate this error for other value S12 (like for S12 = -40 dB)?

Also, I have used an average factor at a very low IF bandwidth (e.g. 10 Hz), but the result did not change significantly (reduced by 2-5 dB).

Thanks in advance

Hello Joe89

Did you terminated both ports when you did the measurement?

Also over time there is tendency for the amplitude to rise and the phase in the vector drift.

The value do look pretty low. What was the power set on VNA?

I do not have on hand E8361A, I try that on N5230A, and it show on S21 more then -100 dB, mostly -110 dB (terminated both ports, -8dBm, 10Hz , no averaging). The worst case I am getting is on S11 that is around 80dB. With max power set (10dBm) the VNA show -120 dB up to 10GHz, and better the -110dB up to 20GHz.

Regards,

mirek