The Modern High Speed Mode Converter (ADC) has implemented direct sampling of RF (RF) signals, and thus no mix is required in many cases, and the flexibility and functionality of the system is also improved.
Traditionally, both ADC signals and clock inputs are represented by a set of integral components. However, for the RF sampling converter, its operating frequency has increased to the extent to which the distributed representation is required, then the original method is not applicable.
This series will start from three parts, indicating how to apply scatter parameters (also known as S-parameters) to direct radio frequency sampling structures.
S-parameters for decisive role
The S parameter is the network parameters based on incident microwave and reflective microwave relationship. It is very useful for circuit design because it can be used to calculate indicators such as input impedance, frequency response, and isolation using the ratio of incident waves and reflected waves. Moreover, since the S parameters can be measured directly with a vector network analyzer (VNA), there is no need to know the specific details of the network.
Figure 1 shows an example of a dual port network, and its incident wave is AX, and the reflected wave amount is bx, where x is the port. In this discussion, we assume that the device is a linear network, so it is suitable for superimposing.
Figure 1: Dual port network wave quantity
Typically, when the reflection waves on all ports are measured, the VNA only stimulates a port (by pushing the incident wave to the port). Moreover, the amount of these waves measured is very complicated because each wave amount has a corresponding amplitude and phase. Therefore, this process needs to be repeated for each port at each test frequency.
For dual-port devices, we can form four meaningful ratios from the measurement data. These ratios are typically represented by Sij, where i represents the reflective port, and j represents the incident port. As mentioned above, it is assumed that only one port is stimulated once, then the incident wave of other ports is zero (the end of the system's characteristic impedance Z0 is terminated).
Equation 1 to 4 applies to four dual port S parameters. S11 and S22 represents the complement of port 1 and port 2, respectively. S21 represents the transmission characteristic, the port 1 is input, the port 2 is output (S12 is the same, but port 2 is input, port 1 is output).
For one-way devices, such as an amplifier (port 1 is input, the port 2 is output), the input impedance can be indicated by S11, and the frequency response is represented by S21, and the reverse isolation is indicated by S22, and the output impedance is indicated by S22. The data converter is also a one-way device, but its port 2 is usually digital output, which will have a certain impact on measurement and interpretation.
Extend the S parameter to multi-port devices and differential devices
You can extend the S parameter frame to any number of ports, meaningful parameters number 2n, where n represents the number of ports. Many integrated circuits have differential inputs and output due to oscillation and common mode suppression capability. RF Sampling ADCs (such as TI ADC12DJ5200RF) typically have differential RF inputs and differential clock inputs. We can further extend the S parameter framework to support differential ports.
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As shown in Figure 2, we must distinguish between common mode waves and difference modes for differential ports. The two modes have the same incident amplitude, but the difference mode incident wave has a phase shift of 180 degrees, and the common mode incident wave has the same phase.
Figure 2: Differential wave and common mode wave
For a linear device without feedback between ports, a superposition method can be used, according to the single-ended S parameter measurement (in any given time, only one port has an incident wave in an active state) to calculate the difference co-mixed mode S parameters. . Modern high-performance VNA also supports two ports simultaneously with differential mode or common mode waves.
Measuring the challenges of the data converter S parameters
The semi-equivalent characteristics of the data converter have challenged the measurement S parameters. VNA cannot be directly connected to the digital bus of the data converter, so a special method is required for measurement.
The second part of this series will introduce a method of measuring the S-parameter of the Texas Instrument RF Sampling Data Converter. The third part will discuss how to use the S parameters in the design of the RF sampling data converter system.
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