Difference between revisions of "System-Level Tutorial Lesson 1: Investigating RF Transmission of Digital Data"

Revision as of 19:03, 2 October 2015

Tutorial Project: Investigating RF Transmission of Digital Data

Objective: In this project, the basic concepts of RF.Spice A/D are demonstrated, and a simple voltage divider is modeled and examined.

Concepts/Features: CubeCAD Visualization Lumped Sources S-Parameters Far Fields Advanced Meshing in EM.Tempo

Minimum Version Required: All versions

' Download Link: [1]

What You Will Learn

In this tutorial you will explore the transient response of long transmission lines when excited with digital signals. You will define a digital source to perform a mixed-mode digital-RF simulation.

RF Signal Transmission Over Long Distances

The following is a list of parts needed for this part of the tutorial lesson:

In RF Tutorial Lesson 2, you analyzed the transient response of a quarter wave transformer that was designed to match a 100Ω load to a 50Ω transmission line connected to the voltage source with a 50Ω source resistance at an operational frequency of 2GHz. For this tutorial you are going to use the same circuit but with a much longer transmission line.

Place and connect the parts as shown in the figure below:

The Quarter-Wave Impedance Transformer circuit connected to a long transmission line segment.

Define a pulse waveform for the voltage source VS according to the table below:

Run a Transient Test of this circuit with the parameters specified below:

The results are shown in the figure below. Due to the very long length of the 50Ω T-Line XTL1, the source signal initially sees an impedance match at Node 2. The peak amplitude of v(2) is therefore 0.5V. The time it takes the incident propagating TEM wave to reach the load is:

[math] t_L = \frac{L_{tot}}{c} = \frac{1.5 \text{m} + 0.0375 \text{m}} {3\times 10^8 \text{m/s} } = 5.125\text{ns} [/math]

You can see from the figure that v(4) starts at about t = 5.1ns. As we discussed in RF Tutorial Lesson 2, the impedance matching property of the quarter-wave transformer is valid only at 2GHz. The other harmonics of the pulse signal don't see a perfect match condition. As a result, there is a reflected signal, which takes another 5.125ns to reach Node 2. You can see that after t = 10.25ns, the voltage at Node 2 starts to get distorted due to superposition of the reflected signal.

The graphs of input voltage v(2) and output voltage v(4) as a function of time.

Exploring a Lossy Transmission line

So far you have worked mostly with lossless transmission lines with a zero attenuation constant. In this part, you will use a long lossy transmission line segment. Open the property dialog of the T-Line XTL1 and set the value of the Alpha parameter to 1dB/m. For your 1.5m line, this means a total attenuation of 1.5dB.

Adding losses to the transmission line by defining a nonzero attenuation constant.

The results are shown in the figure below. The line attenuation causes additional distortion of the transmitted signal. Also, in comparison to a steady value of 687mV in the previous lossless case, you can see that the peak amplitude of the load voltage has now dropped to a value of 578mV at t = 5.2ns and continues to decline to a value of 528mV at t = 20ns. You can easily verify your results noting that

[math] \frac{v_L^{lossy}}{v_L^{lossless}} = 20\log \left( \frac{578mV}{687mV} \right) = 20\log(0.841) = -1.5dB [/math]

The graphs of input voltage v(2) and output voltage v(4) after adding losses to the long transmission line segment.

Using a Digital Source to Build a Mixed-Mode Circuit

Setting the input bit sequence in the property dialog of the digital source.

In this part of the tutorial lesson, you will use a digital source to replace the analog voltage source of your transmission line circuit. A digital source generates a sequence of binary bits, 0's and 1's, as a function of time. You can access a digital source using the menu item Menu > Parts > Digital I/O > Digital Source or using the keyboard shortcut Alt+D. You have to define the bit sequence in the "Time-Value Array" table in the property dialog of the digital source. Enter the following data for the excitation of your circuit over the time interval [0 - 10ns]:

You also need to add a 1-bit DAC Conversion Bridge between the digital source and the analog/RF part of your circuit. Place a DAC bridge in your circuit and set the values of its out_low and out_high parameters to 0V and 5V, respectively. Also make sure to set the t_rise and t_fall parameters both to 1ps.

Setting the output voltage levels and rise/fall times of the DAC conversion bridge.

You can use the circuit of the previous part and simply replace the old analog pulse source with the new digital source and the DAC bridge as shown below. Also, open the property dialog of the T-Line XTL1 and set its Alpha parameters to zero to have a lossless line.

A digital source connected to a long transmission line segment.

Run a Transient Test of this circuit with the parameters specified below:

The results are shown in the figure below. The yellow binary input pulse train has a total duration of 8ns, and the input voltage v(2) drops to zero after t = 8ns. The transmitted pulse train reaches the load at t = 5.125ns. The reflected signal from the load reaches Node 2 at t = 10.25ns, when you see v(2) rise once again, and it continues until t = 18.25ns. Also note the sizable distortion of the transmitted signal at the load.

The graphs of input voltage v(2), output voltage v(4) and the binary output of the digital source.

Exploring Pulse Width Modulation

The following is a list of parts needed for this part of the tutorial lesson:

A pulse width modulator (PWM) generates the binary data from an analog signal. It has a analog signal input (v_s) and another input for a ramp generator (v_ramp). The PWM basically consists of a voltage comparator that compares the ramp and input signals. The binary output is given by:

[math] v_{out} = \begin{array}{ll} V_{high}, & v_s \gt v_{ramp} \\ V_{low}, & v_s \lt v_{ramp} \end{array} [/math]

Place and connect the parts as shown in the figure below. You can access the Ramp Generator from the menu item Menu > Parts > Waveform Generation Blocks > Basic Waveforms > Ramp Generator. Open its property dialog and set its Period to 1ns. Set the out_high and out_low voltage levels to +2V and 0V, respectively. You can access the PWM modulator from the menu item Menu > Parts > Modulation Blocks > Pulse Width Modulation (PWM) Block. Open its property dialog and set its T_rmp (ramp period) to 1ns. Set the values of rmp_high and rmp_low parameters (input ramp min and max levels) to +2V and 0V, respectively. You have to make sure that these parameters are consistent. Keep all the other default parameter values.

The property dialog of a PWM modulator.

The property dialog of a ramp generator.

Define a sinusoidal waveform for your voltage source according to the table below:

A basic circuit to test a PWM modulator.

Run a Transient Test of this circuit with the parameters specified below:

The results are shown in the figure below. Due to the very long length of the 50Ω T-Line XTL1, the source signal initially sees an impedance match at Node 2. The peak amplitude of v(2) is therefore

The binary output of a PWM modulator.

Transmitting a PW Signal Over a Long Line

The following is a list of parts needed for this part of the tutorial lesson:

A pulse width modulator (PWM) generates the binary data from an analog signal. The

The binary output of a PWM circuit transmitted through a long transmission line segment.

Back to RF.Spice A/D Tutorial Gateway