Difference between revisions of "System-Level Tutorial Lesson 1: Investigating RF Transmission of Digital Data"
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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 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]: | 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 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]: | ||
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| + | [[File:RF178.png|thumb|360px|Setting the input bit sequence in the property dialog of the digital source.]] | ||
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During the simulation, [[RF.Spice A/D]] places a digital-to-analog (D/A) converter behind the scenes to convert the binary bit sequence "0" and "1" values to 0 and +5V analog voltages. The resulting waveform then feeds your RF circuit via the source resistor. | During the simulation, [[RF.Spice A/D]] places a digital-to-analog (D/A) converter behind the scenes to convert the binary bit sequence "0" and "1" values to 0 and +5V analog voltages. The resulting waveform then feeds your RF circuit via the source resistor. | ||
Revision as of 15:15, 2 October 2015
Contents
What You Will Learn
In this tutorial you will explore the transient response of long transmission lines with resistive and capacitive loads when they are 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:
| Part Name | Part Type | Part Value |
|---|---|---|
| VS | Voltage Source | Waveform: TBD |
| XTL1 | Generic T-Line | Defaults: Z0 = 50, eeff = 1, len = 1500 |
| XTL2 | Generic T-Line | Defaults: Z0 = 70.71, eeff = 1, len = 37.5 |
| RS | Resistor | 50 |
| RL | Resistor | 100 |
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:
 |
Define a pulse waveform for the voltage source VS according to the table below:
| Initial Voltage | 0 |
|---|---|
| Peak Voltage | 1 |
| Delay Time | 0 |
| Rise Time | 10p |
| Fall Time | 10p |
| Pulse Width | 250p |
| Pulse Period | 500p |
Run a Transient Test of this circuit with the parameters specified below:
| Start Time | 0 |
|---|---|
| Stop Time | 20m |
| Linearize Step | 1u |
| Step Ceiling | 1u |
| Preset Graph Plots | v(VIN), v(VOUT) |
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.
 |
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.
 |
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.
 |
Using a Digital Source to Build a Mixed-Mode Circuit
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 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]:
| Time | State |
|---|---|
| 0 | 0 |
| 250ps | 1 |
| 500ps | 0 |
| 750ps | 1 |
| 1250ps | 0 |
| 1500ps | 1 |
| 1750ps | 0 |
| 2000ps | 1 |
| 2500ps | 0 |
| 2750ps | 1 |
| 3000ps | 0 |
| 3750ps | 1 |
| 4000ps | 0 |
| 5000ps | 0 |
 |
During the simulation, RF.Spice A/D places a digital-to-analog (D/A) converter behind the scenes to convert the binary bit sequence "0" and "1" values to 0 and +5V analog voltages. The resulting waveform then feeds your RF circuit via the source resistor.
Run a Transient Test of your circuit with the start and stop times set to 0 and 5ns, respectively, and a Step Ceiling of 1ps. The following figure shows the results for the source, input and output voltages. Note that the source voltage does not jump between 0 and 5V instantaneously, but it has nonzero rise and fall times. The graph also shows the input binary bit sequence of the digital source on a separate digital axis because you just performed a mixed-mode simulation.
 |
