Reaffirm Signal Integrity Using S-Parameter Simulation

Key Takeaways

• S-parameter simulation is a small signal AC analysis that gives the linear characteristics of the RF circuit at given bias, temperature, and frequency.

• For an n port network, the total number of S-parameters equals n2. The reflection coefficients are useful in impedance matching; and the transmission coefficients indicate the coupling and crosstalks in the circuit. 

• The matching network inserted in the load side for impedance matching in RF circuits can be designed and optimized using S-parameter simulation. As the impedance gets matched, the signal integrity is attained in the circuit. 

Debugging the internal circuits to find the root cause of operational failures in devices requires skill and experience.

Nowadays, it's common practice to replace faulty equipment, instead of putting energy and money into lengthy repairs. However, troubleshooting operational failures is a difficult job. From an engineer’s point of view, the level of engineering involved in molding a user-friendly solution is enormous—it's even tougher to identify what went wrong in internal circuitry design.

The signal integrity of complex internal circuits plays a pivotal role in ensuring the reliable operation of devices. Proper communication between internal circuits keeps the equipment running as given in the catalog. Whenever there comes a propagation delay, the interaction between the internal circuits loses compatibility, and the device may misoperate.  The chances of signal quality distortion are comparatively more at the high-frequency operation of the circuits.

Scattering parameter ❲S- parameter❳ simulation underlines the signal integrity of the circuits at high frequency. At high frequencies, the voltage and current analysis are not that effective compared to S-parameter analysis. The S-parameter simulation considers the input AC signal as electromagnetic ❲EM❳ or radio frequency ❲RF❳ waves, and defines how the signal is reflected in the input side and transmitted to other parts of the circuit. For characterization of RF circuits in terms of reflection and transmission, S-parameters are more effective than the H, Z, and Y parameters. 

S-Parameter Simulation

The multitude of interconnections between circuits increases the possibility of communication errors and delays among devices. The delays, distortions, and quality degradation in the signals influence the signal integrity of the circuit. As the frequency of operation goes from kHz to GHz, the behavior of the circuit quantities is more like power waves. It is not a feasible approach to model the circuits in terms of voltages and currents at high frequency. The S-parameter simulation is the most well-suited tool to characterize the complex circuits at high frequency to ensure its signal integrity. 

The S-parameter simulation is one type of AC simulation that presents the small-signal behavior of the device at the given temperature, bias conditions, and input signals. The attenuation and amplification of the signals can be interpreted with the help of S-parameters. The non-linear components are linearized in this simulation for providing the linear characteristics of the circuit at different frequencies. 

The cascading of the S-parameter can be viewed as one of the advantages of S-parameter simulation, especially when it comes to complex circuits. For instance, you can simulate the S-parameters of each amplifier stage separately and the performance of the multi-stage amplifier can be analyzed using the cascaded S-parameters. 

In the S-parameter simulation, the circuit under consideration is transformed into a multi-port network. You don't need to short and open the ports for S-parameter calculation, which is the procedure followed in determining H, Z, and Y parameters. The easy conversion of the S-parameters into H, Z, and Y parameters makes it useful in finding the effect of parasitic reactances on signal integrity. 

Reflection and Transmission Coefficients

In S-parameter simulation, the n-port network equivalent is excited one port at a time with EM waves, and the amount of EM energy is measured at all the ports. Usually, the S-parameters are presented in a matrix form, called S-matrix, with the coefficients of the linear equations governing the circuit as the matrix elements. For an n-port network, S-parameters form an n x n matrix.  shown below:

S-matrix

The diagonal elements of the matrix ❲i=j❳ denote the reflection coefficients, and the other elements ❲ij❳ give the transmission parameters. The reflected and transmitted EM energy is indicated by the reflection coefficients and transmission coefficients, respectively. Any element S_ij in the S-matrix quantifies the EM energy transmitted from the input port ‘j’ to output port ‘i’. 

The S-parameters are complex quantities with amplitude and phase values, and are frequency-dependent. The variation of S-parameter wrt to frequency can be plotted in two ways :

1. Amplitude versus frequency plot and phase versus frequency-The variation of amplitude and phase are plotted in two different graphs. Even though the S-parameters are dimensionless quantities, the amplitudes are generally expressed in dB.

2. Smith charts- The smith chart is the popular plot used for indicating S-parameter. The radius of the circle is the amplitude of the S-parameter, and the phase angle varies in the counter-clockwise direction. 

The figure below shows the reflection coefficients marked on a smith chart. The reflection co-efficient 1⦟0° indicates a perfect open circuit where the signal gets reflected back to the input port with no phase change. 

Smith chart showing the reflection coefficients

Signal Integrity in Relation to S-Parameters

While building an RF circuit, our intention is to transfer the energy efficiently from source to load. For a circuit to be lossless, impedance matching is essential. The impedance mismatch reflects back some amount of energy to the input port, and this energy loss is frequency-dependent. The reflected energy is always mentioned wrt to forward power from the source to load for a better insight into the energy loss. 

The terms which are commonly  used in  RF circuits to express the energy loss are:

1. Return loss is the difference between the forward power and reflected power. As the reflected power decreases, the percentage of power loss also decreases. For a lossless system, no power is reflected back, which we have mentioned earlier as 0% reflection in the smith chart. In terms of S-parameter, the return loss can be found by using equation ❲1❳

where S_ij represents the reflection coefficients.

2. Voltage Standing Wave Ratio ❲VSWR❳ is the ratio of the maximum to the minimum value of ❲forward wave + reflected wave❳.  VSWR can be expressed as equation  ❲2❳:

 The reflection coefficient present in equations ❲1❳ and ❲2❳ can be expressed in terms of source impedance ❲Z₀❳ and the load impedance ❲Z_L❳ as follows፦

When Z₀ perfectly matchesZ_L, the system turns lossless and VSWR takes unity value. The VSWR value goes to infinity in a circuit where power reflected is 100%.

The relationship between the return loss and VSWR can be expressed by equation  ❲4❳

Now let's connect the dots between the signal integrity, impedance matching, and S-parameters simulation.

If you are looking for signal integrity in your RF circuit, you need to ensure that the signals are propagated without any scattering or delay. This circuit condition corresponds to impedance matching. The signal integrity can be achieved by matching the load impedance with the source impedance ❲Z₀=Z_L❳. The input and output impedance matching can be readily identified using the reflection coefficients whereas the presence of coupling or cross-talks from the transmission S-parameters. 

As we have seen how the S-parameters are connected to the reflected energy in an RF circuit, inserting a matching impedance network on the load side can create magic. When the equivalent impedance of the matching network and the load impedance is equal to the conjugate of the source impedance, the reflection of signals back to input ports becomes  0％ and the signal integrity of the circuit is at its maximum. 

If you are designing a matching impedance network for your RF circuit, you need to get hands-on with an RF analysis and simulation tool.  Utilizing the strong Allegro PCB Designer from Cadence will ensure you’re always in the right layout management tool. You don't need to burn the midnight oil for impedance matching and optimization if you have access to the best S-parameter simulation tool. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.