# Keysight ADS – Microstrip Line Design

The goal of the project is to design a 50 ohm microstrip line at an operating frequency of 10 GHz and phase delay of 145 degrees.

The following ADS simulation will be composed of four major parts:

a) Designing the microstrip lines using two models (I.J. Bahl and D.K. Trivedi model and E. Hammerstad and Jensen model). The insertion loss (S(2,1)) will be plotted over the range of 10 MHz to 30 GHz.

b) Assuming reasonable dielectric losses, results should be compared to part a

c) Creation of ideal transmission lines with same parameters compared to part a and b

d) Showing dispersion on the lossless microstrip line. This is compared to the ideal line.

The LineCalc tool (which uses the Hammerstad and Jensen model) within ADS is used to design the second line with the correct specifications. The first circuit is designed using hand calculated values. The following shows using the LineCalc tool to get the values for the second schematic. The simulation is shown below. A new substrate is created with a loss tangent of .0002 for the second schematic. The simulation results in: An ideal transmission line circuit is created and compared with both the lossy and lossless lines. In order to demonstrate dispersion, the phase velocity must be calculated. As shown by the values compared from 0 GHz to 30 GHz, the phase velocity does not change for the ideal line, but does for the microstrip line. # Keysight ADS – Frequency Dependence of Microstrip Lines

The following ADS simulation will demonstrate how the characteristic impedance and effective dielectric constant change with frequency.  In the simulation, a quarter wave section of multi-layer microstrip line is used to demonstrate frequency effects. The result are expected to show that the dielectric constant and the characteristic impedance are inversely related. When the frequency of the electric field increases, the permittivity decreases because the electric dipoles cannot react as quickly. The multi-layer component is used in place of an ideal component because frequency dependence must be demonstrated. An “MLSUBSTRATE2” component is used with the updated dielectric constant and Dielectric loss tangent. For S parameter analysis, two terminated grounds are required. The effective dielectric constant must be solved for by unwrapping the phase of S(2,1). The results show the characteristic impedance (both real and imaginary parts) increasing with frequency and the dielectric constant decreasing. # Keysight ADS – Transient Propagation

The following ADS simulation will demonstrate the effects of transients on a transmission line. A rectangular pulse of duration .5 microseconds will be generated and a net voltage vs time will be plotted for a period of .7 microseconds. The circuit has a mismatched load, producing reflections. A time domain reflectometry analysis will prove that the propagating signal voltage steadily increases after the initial time and as time increases, the reflections will eventually die out and leave a steady state response. This is shown with transient analysis. The schematic above contains two circuits for the two parts of the rectangular pulse (one with and one without a time delay). The simulated results are shown below. A bounce diagram can also be used to convey Time domain reflectometry analysis, as shown below. This diagram is a plot of the voltage/current at the source or load side after each reflection. This is a general diagram and does not apply to the problem. # Keysight ADS – Conjugate Matching

This project will use conjugate matching to match a capacitive load of 50-j40 to a generator of impedance 25+j30. Since the generator impedance is complex, conjugate matching is required to match the network, as opposed to in situations of low frequency where the reactive components are negligible. In the example, an L network is used to match the generator to the load. Theoretically, differentiating the power and setting this equal to zero proves that maximum power is transferred when the resistance of the source and load are equal and the reactive portions are equal and opposite phase shift/sign.

The first step is to use the impedances given to calculate the actual lumped inductor and capacitor values to use for the network to work at 2 GHz. 25+j30 corresponds to a 25 ohm resistor in series with a 2.387732 nH inductor and 50-j40 corresponds to a 50 ohm resistor in series with a 1.98944 pF capacitor.

The following shows the schematic with the source, matching network and load. Running the simulation with Data Display equations yields…. This shows maximum power transfer at the correct frequency of 2 GHz. The next step is to use the Smith Chart tool. A shunt inductor and series capacitor is used to form the L Network. Exact values can be typed in for these to get the impedance value Z = 0.5 +j0.6 which is the normalized equivalent source impedance (divided by 50). With the capacitor and inductor values recorded, these values can be loaded into a separate schematic and compared to the original schematic results.

Conjugate matching is not achieved with this Smith Chart configuration so there is no peak at 2 GHz. Alternatively, the Smith Chart tool can be used from the palette. From this point with the chart icon selected, the network can be created by selecting “Update Smart Component” from the Smith Chart tool window. These results show that it is important to select the proper design network for the specifications for optimal results. # Keysight ADS – Quarter Wave Transformer Matching

In ADS, a batch simulation can be implemented to run different load impedance simulations. This function will be used to simulate a quarter wave transformer matching system for various loads (25, 50, 75, 100, 125 and 150 ohms),  The system is used to match a 50 ohm line with an electrical length of 60 degrees at 1 GHz.

The simulation will demonstrate that an unmatched load will generate a constant VSWR at all frequencies. With the implementation of the matching network, the VSWR varies because it is only designed to match the network at a specific frequency. A previous post derived the relationship to find the impedance of a quarter wave matching transformer. The VSWR can be plotted by adding equations into the data display window and manually adding equations into the plot window to plot VSWR against frequency for both the matched circuit and the unmatched circuit. The mismatched circuits appears constant over frequency with a very high SWR, as it does not have the matching transformer. The quarter wave transformer is shown to provide excellent matching at specific frequencies. For batch simulations, a slider tool can be implemented to show only specific impedances. Clicking on the axes and changing the names to include the index will update the plot with the specific impedances one at a time. The plot is updated to match the slider value for the load impedance. With the axes correctly updated, sliding the slider tool will change the plot automatically. Also in the data display window, tables can be added to view specific values at different frequencies. # Keysight ADS – Short Circuit Terminated Ideal Transmission Line

Using ADS, a parameter sweep can be used to confirm the results of a short circuited transmission line. The input impedance of a transmission line is given as When the line is terminated by a short circuit, ZL = 0 and the equation reduces to only the imaginary part of the numerator. For integer multiples of the wavelength, the input impedance is equal to zero. At odd multiples of a quarter wavelength, the input impedance becomes infinite and looks like an open circuit.

The following circuit is constructed to test the results. A parameter sweep with the variable theta (the electrical length) is used. The results are shown below. As expected, the reactance alternates between inductive and capacitive for different electrical length values. The reactance is infinite at quarter wavelength multiples and zero at integer multiples of the wavelength. The current is shown to be lagging the voltage by 90 degrees. The major conclusion to be made is that a transmission line does not behave like a lumped element circuit because voltages and currents are different at different lengths along the line. # Keysight ADS – Open Circuit Analysis

Expanding upon the previous project, open circuit analysis can be used to find equivalent per unit length capacitance and conductance values for the dielectric part of the transmission line. The same process is used for the open circuit analysis with new equations for capacitance and conductance. The calculated values from the simulation window are compared to the simulated values from the AC analysis. # Keysight ADS – Extraction of Lumped Element Model from Coaxial Line using Short Circuit Analysis

The following ADS project is meant to analyze an RG58 coaxial cable and extract a “lumped element model” containing discrete components intended to represent distributed values. The lumped element values for R, L, G, and C for ideal coaxial lines can be obtained from the following equations. “a” is the radius of the inner conductor and “b” is the radius of the outer conductor. It is important to note that since these are ideal values, the actual simulation will differ from calculated values. R and G are nonreactive and therefore will be quite similar, however C and L which are frequency dependent will vary.

Like with other ADS projects, creating variables is an easy way to change component values, especially when these need to be duplicated. In the ideal transmission line palette, the COAX_MDS component can be found.

The Dielectric loss model can be changed to Frequency independent, as shown. This will prevent the frequency dependent parameters from changing from the calculated values. First, a short circuit analysis can be performed in order to determine resistance and inductance values (shorting out the dielectric parameters C and G). Using the “name” option at the top of the screen, the input wire can be named “Z_SC”.  An AC simulation can be performed with the schematic shown. The results for per unit length resistance are shown from the simulation. Decibel scale is used for the x axis and the y limits are changed to get a better looking plot. The following image demonstrates placing equations in the data display window and using a calculated value to compare with a simulated value. As expected, the results of the calculated and simulated values agree (they are both close to zero). Although, the resistance differs a bit (not sure why). The inductance normally would vary, but because the frequency independent model was selected for the coax cable, they are exactly the same at 100MHz. # 028/100 VSWR Circles on the Smith Chart

Example 3.5-2B: Show the direction of movement on the Smith Chart when adding a series or shunt element to an impedance.

Shunt Inductor: Series Inductor: Shunt Capacitor: Series Capacitor: Series Resistance: Shunt Resistance: Shunt Transmission Line: Series Transmission Line: # Microstrip Stub Low-Pass Filter (10 GHz)

First, build the component using the ADS DesignGuide/Smart Component Passive Circuit tool. This is the original, equation-based simulation. This is the substrate used for the Low-pass filter. This is the Momentum simulation of the layout component. This is the layout component for the 10 GHz Stub Low-pass filter component. # Messing with Substrates (10 GHz bandpass filter) [silly]

In this post we will take a reasonable 10 GHz bandpass filter (at least for a new grad student) and see how a new substrate will change how this filter works.

This is the original bandpass filter with the standard substrate:  If you can read the numbers on the above pic, you could also build this filter. Let’s get a better picture of what the substrate is for the original circuit: Let’s try a silicon dielectric, shall we? One thing is for sure – it’s not a bandpass filter anymore. Let’s add a 20 mil Indium conductive layer below the dielectric: Voilà! It’s centered at 8 GHz! Brilliant! Here’s a thought – how well does your bandpass filter work underwater? It looks like your bandpass filter might give you a little gain there at some frequencies in water! # Microstrip Lange Coupler (5 GHz)

The Lange Microstrip (quadrature) coupler is known for it’s low loss, wide bandwidth and compact layout. Similar to other couplers, it consists of an isolated port, through port and coupled port.

You can build a microstrip Lange coupler using the DesignGuide tool in ADS: These are the results for the equation-based simulation. These results admittedly look considerably better. This is the substrate used: These are the results for the momentum simulation. Admittedly, some tuning would improve this considerable. And here is the layout component: # 027/100 Shunt Reactance on Smith Chart

Example 3.5-2A: Measure the effect of susceptance on Smith Chart impedance matching.

First, build the circuit and run the Smith Chart Matching tool. A shunt capacitor moves in a clockwise direction across the smith chart tool: Also note that a shunt inductor moves counter-clockwise across the smith chart tool: The following matches a 50 Ohm line to a 100 Ohm load at 10 GHz using a double-stub component. This was designed using the ADS passive circuit DesignGuide tool. This method is a great alternative to using the Smith Chart matching tool for lumped elements if you need a microstrip line for matching.

Circuit: Substrate: Momentum simulation result (can be tuned to center better at 10 GHz): Layout component: # 026/100 Lumped Element Smith Chart Movements: Series Inductor

Example 3.5-1: Measure the amount of movement caused by the reactance added to the circuit below. Measure the change from the starting point to the end point on the Smith Chart. The circuit simulated gives the following result: Recall that the circuit without a series inductor had the following result: Through this simulation, it is shown that adding a series inductor causes the smith chart diagram plot to move in a clockwise direction.

Note the change using the Smith Chart matching tool:  