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.

schem

The following shows using the LineCalc tool to get the values for the second schematic.

linecalc

The simulation is shown below.

sim.PNG

A new substrate is created with a loss tangent of .0002 for the second schematic. The simulation results in:

schem2.PNG

An ideal transmission line circuit is created and compared with both the lossy and lossless lines.

IL.PNG

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.

PV

 

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.

schem

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.

results

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.

schematic.PNG

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.

res

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.

bounce.png

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.

schem

Running the simulation with Data Display equations yields….

maxpower

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).

sc

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.

power

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.

sc.PNG

 

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.

schem

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.

VSWR.PNG

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.

update.PNG

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.

table.PNG

 

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.

ocschem

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.

oc

Microstrip Stub Low-Pass Filter (10 GHz)

This is a 10 GHz Stub Low-pass filter, made using ADS.

 

First, build the component using the ADS DesignGuide/Smart Component Passive Circuit tool.

stublp1

 

This is the original, equation-based simulation.stublp2

 

This is the substrate used for the Low-pass filter.

stublp3

 

This is the Momentum simulation of the layout component.stublp4

 

This is the layout component for the 10 GHz Stub Low-pass filter component.

stublp5

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:

langec0

 

These are the results for the equation-based simulation. These results admittedly look considerably better.

langecoupler2

 

This is the substrate used:

langecoupler1

 

These are the results for the momentum simulation. Admittedly, some tuning would improve this considerable.

langec3

And here is the layout component:

langec4