In the previous article "What About Vector Calibration (1)-The Past and Present of Unknown Passthrough", we already know that many network sub users have some misunderstandings in calibration. Here is an introduction to network sub calibration. One place where the error occurs more-the unknown through (SOLR) calibration should be used but the SOLT calibration is wrongly used, and what is the difference between the unknown through (SOLR) and SOLT?
We introduce the vector network analyzer VNA485, which is independently developed by iTest, 4 ports, 300kHz～8.5GHz, fast measurement speed, high dynamic range, high precision and stability, comparable to international benchmark products.
1 Setting method of unknown through calibration parts
When the VNA485 software UI menu bar Keyboard is set to ON, open the Keyboard touch panel, as shown in the figure below, you can use the mouse to click and operate flexibly on the software interface, and the touch screen is better.
1. As shown in Figure 1 below, here is an example of 85032F mechanical calibration parts. Click the Cal (Calibration) button, the function key menu will call up the Calibration menu. Set Correction to ON, click the Cal Kit (calibration kit) menu, and select the type of mechanical calibration part used for calibration. Then select the Modify Cal Kit menu button to enter the next menu.
2. As shown in Figure 2 below, click Define STDs and select the Thru menu, where the Thru menu number is 11. The Thru numbers under the Define STDs menu of different mechanical calibration parts are different.
3. Under the Thru menu, if STD Type (standard type) is Delay/Thru (delay/thru), and Offset Delay = 0.000 sec. This setting is usually applicable when the two calibration end faces are opposite to each other, as shown in Figure 3 below.
As shown in Figure 4 and Figure 5 below, if the two end faces to be calibrated are both of the same sex, then a double-female or double-female through calibration component will usually be used. The through calibration part itself has reflection, loss and delay in the signal to be measured, so the unknown through calibration is introduced here.
4. To perform unknown thru calibration, click the STD Type menu in Figure 2 and select Unknown Thru (unknown thru), as shown in Figure 6 below.
As shown in Figure 2, after returning to the previous Thru menu, you need to modify the Offset Delay of the pass-through calibration part. So why should Offset Delay be set? And what value should be set for Offset Delay?
2.1 Why do I need to set Delay?
In the previous article "The Things of Vector Network Calibration (1)-The Past and Present of the Unknown Throughput", we already know that SOLT is derived from a 10-term error model, so it needs to establish 10 equations to solve 10 error terms. 10 equations means that the known parameters of 10 standard parts are required. The unknown through is derived from an 8-term (actually reduced to 7 terms) error model, so the unknown through only needs to establish 7 equations. In addition to the six single-port standard reflection parameters, SOLT also needs to know the complete four S parameters of the pass-through standard to calibrate the vector network analyzer. The unknown thru calibration does not need to know the thru standard part parameters completely, only need to know that the thru part satisfies the relationship of S21 = S12, which is why the unknown thru calibration is called this name.
Based on this, the following formula will be established when the through calibration parts are used:
TmR is the transmission matrix test value of the pass-through calibration part, XA and XB are the known error terms (6) calculated after the calibration of the single-port calibration part, α is the unknown error term to be solved, and TR is the transmission matrix of the pass-through part Actual value.
Since the through piece satisfies the reciprocal relationship of S21 = S12, det(TR) = 1 in the above formula. This relationship can be used to solve for α, but because mathematical calculation involves taking the sign of the square root, it will affect the phase of the test result, so it is necessary to set the Delay through the calibration part to determine the choice of sign.
2.2 How to estimate the Delay value of the straight-through calibration kit or adapter?
It can be obtained by dividing the actual length of the straight-through calibration piece or adapter by the speed of light of the medium of the straight-through calibration piece or adapter. Generally, the light speed of the medium can be calculated according to PTFE (ε=2.1).
2.3 How to make sure that the Delay value is set correctly?
Take a through calibration component as an example, first calculate the Delay value according to the method in section 2.2 and then perform calibration (set VNA485 to work in the full frequency band: 300kHz～8.5GHz).
① Calculate and estimate the theoretical Delay value of the through calibration part as shown in the figure above: 328.5ps.
② Check whether the S21 phase test value of the through calibration kit is continuous, as shown in the figure below, the phase is continuous.
If it is not continuous, then update the Delay value of the calibration component according to the data obtained from the group delay test value of the phase continuous frequency band, recalibrate the Thru component, and then perform OverWrite.
③ As shown in the figure below, modify the data format to: Group Delay and take the average. Here we modify the Delay value to: 167.5ps.
Click Cal→Calibrate→2-Port Cal→Transmission to recalibrate the Thru part. Return to the previous 2-Port Cal menu, and then perform OverWrite.
④ As shown in the figure below, after OverWrite, modify the data format to: Group Delay and take the average. Here we further modify the Delay value to: 169.5ps.
Click Cal→Calibrate→2-Port Cal→Transmission again to recalibrate the Thru part. Return to the previous 2-Port Cal menu, and then perform OverWrite.
⑤ As shown in the figure below, finally modify the data format to: Phase, confirm that the phase of S21 is continuous, and obtain the final Delay value of the through calibration component: 169.5ps.
3 Comparison of unknown pass-through and SOLT test data
When only keeping STD Type = Unknown Thru, and using the pass-through calibration Offset Delay = 169.5ps, and STD Type = Delay/Thru, and Offset Delay = 0.000s as the only variable, use the same DUT (passband: 2300 ～2400MHz, the input and output are both N-type female, so the calibration end faces are N-type male), calibration parts (85032F), RF cable, experimental environment, and the joint parts are all tightened with the same torque wrench and other experimental conditions.