TDR function of ENA network analyzer

The most important indicator for measuring The characteristic impedance of PCB transmission lines is not DC resistance, but a concept in long-line transmission. In the high-frequency range, during signal transmission, where the signal edge arrives, a transient current will be generated between the signal line and the reference plane (power supply or ground plane) due to the establishment of an electric field. If the transmission line is isotropic, then as long as the signal is transmitted, there will always be a transient current I. If the transient voltage of the signal is V, during the signal transmission process, the transmission line will be equivalent to a resistor, and this equivalent resistor is called the characteristic impedance of the transmission line. During the transmission of the signal, if the characteristic impedance on the transmission path changes, the signal will be reflected at the node where the impedance is discontinuous. Therefore, the characteristic impedance must be measured.

The network analyzer can measure both the frequency domain characteristics and the time domain characteristics of the device. During the measurement, a sine wave excitation signal is input to the device under test, and then the measurement result is obtained by calculating the vector amplitude ratio between the input signal and the transmitted signal (S21) or the reflected signal (S11). The frequency response characteristics of the device under test can be obtained by scanning the input signal within the measured frequency range.Filters can remove noise and unwanted signals from the measurement results to improve measurement accuracy. 

But VNA can also be used to test TDR  parameters. A continuously swept sine wave is generated by the signal source to stimulate the DUT. VNA measures the reflected signal and the transmitted signal of the DUT. The reflected signal can be displayed in a variety of formats, including SWR and reflection coefficient. The time domain characteristics can be obtained by performing an inverse Fourier transform on the reflection and transmission frequency response characteristics, and the impulse response characteristics in the time domain can be obtained. Then, by integrating the impulse response characteristics, the step response characteristics can be obtained. Of course, VNA is not omnipotent. Its power is based on complex calibration. It cannot be measured directly under DC conditions. When the device under test is very long (cable), it takes a long time to complete low-frequency measurements. Since TDR  starts measuring from DC, it has good low-frequency signals. TDR has a considerable storage depth and can effectively measure the S parameters of long cables. In addition, the price of VNA is much higher than that of  TDR  . 

The relationship between the step response characteristics and the impulse response characteristics derived from the inverse Fourier transform.

Before using the ENA-TDR function of the network analyzer, calibration must also be performed. The accuracy and difficulty are shown in the figure below. Among them, the most accurate and convenient is to use electronic calibration parts for full-port calibration. The calibration speed is also very fast. The 4-port calibration can be completed in a few seconds. Therefore, when choosing and using ENA-TDR, it is recommended to equip and use electronic calibration parts. 

TDR operation steps based on E5071C. 

The ENA-TDR user interface is easy to understand and provides intuitive operation steps for users who are not familiar with vector network analyzers and S-parameter measurements. The setup wizard guides the user through all the necessary steps, allowing intuitive and error-free setup, calibration and measurement.

First, set [TDR] to [ON] under the [Analysis] menu.

After clicking [Yes], the software will restart automatically.

Enter TDR mode

1/2/4-port TDR/TDT and S-parameter modes

Click [Eye/Mask] and the eye diagram mode will appear.

You can switch between modes by pressing the button on the left.

Select the DUT type by pressing the [Setup Wizard] button. Select the type of device under test (DUT). Supports single-ended 1-port, 2-port and 4-port, and differential 1-port and 2-port types.

Alternatively, you can select the DUT connection method through [DUT Topology].

Perform error correction. Follow the prompts to perform error correction. The prompts vary, depending on the error correction method you selected in the first step.

The most comprehensive and convenient operation is to use the electronic calibration module ECal to perform full-port calibration.

Automatically measure the length of the DUT

Setting the Rise Time

To exit the TDR application, press [TDR] OFF.

Some people will directly click the cross below to close the TDR function when using it for the first time. The application interface is closed, but the actual function is not completely closed. After clicking [Analysis]-->[TDR]OFF, the software will automatically restart.

All parameter settings in TDR mode are listed in the table below, and no physical buttons on the keypad are used for operation, such as the setting of S parameters.

The impedance of the PCB can be measured by matching the probe. (The following figure is only an example)