Delivering clean power to the chips in the boards is a real challenge with the evolution of technology. Chips designed these days are power hungry and draw huge current typically several amps. And more interesting thing in this is the operating voltages of the digital cores have reduced by many fold as the devices shrunk. Typical digital core of this era has more transistors than the previous decade chips and are smaller and faster than the latter. Because of the shrink in device size and operating voltages the target impedance of the power distribution network falls greatly. This needs high impedance in DC and zero impedance in the other frequencies. But constructing zero impedance PDN is impossible and the impedance of the PDN must be limited to target impedance. The typical power distribution network to deliver clean power to devices connected to it, must have the impedance equal to the target impedance. The power pins of the chips must see target impedance at the local power bus.
ZTarget = ∆V/I
Where ∆V is the maximum allowed ripple voltage in the PDN and I is the current drawn by the devices. Target impedance must be calculated for all the frequencies within the maximum frequency limit. The PDN of the system is designed for this target impedance. Normally ∆V is normally expressed as R x V, where R is the maximum allowed ripple %. If in the datasheet of a device displays a tolerance of +/- 10% of 3.3V operating voltage and draws 1 A in 1ns the target impedance is calculated as,
Ztarget = 10% x 3.3 V / 1A = 0.33V / 1 A = 0.33Ω
Here the device draws a current of 1A to in 1ns that means the bandwidth is calculated by,
F = 0.35 / Tr = 0.35 / 1ns = 350 MHz
The impedance of the PDN must equal 0.33Ω in this case for all frequencies less than 350MHz to deliver a sufficiently clean power to the devices connected to it.
Tuesday, June 10, 2008
Wednesday, June 4, 2008
Reflections in Transmission Lines
It is a real head ache for the one who designs transmission lines that carry high speed signals. The mismatch or change in the impedance of the transmission lines will cause reflections that the signal at the receiver gets corrupted. In this post you will find a simple SPICE simulation to understand the reflections.
Creating SPICE netlist
You must have been familiar with program called SPICE simulate the electronic circuits. SPICE is a powerful program for circuit simulation. You can find many flavors of SPICE free to download from the Internet. The one used here is LTSPICE the one available from Linear Technology. To run the spice netlist give in this post get one and install it in your computer.
Open notepad and paste the code given below:
T1 N001 0 N002 0 Td=1n Z0=50
V1 N001 0 PULSE(0 5 10ns 1ns 1ns 10ns 20ns 5) Rser=0
R1 N002 0 100
.tran 100ns
.end
Save it as filename.cir
The schematic of the netlist is given below
In the simulation we are going to simulate three cases
(1) R1 = 50 ohms
(2) R1 = 25 ohms
(3) R1 = 75 ohms
You will find that only in case (1) signal is reproduced at the receiver as launched into the transmission line.
To run the simulation, open the file.cir in the LTSPICE SwitcherCAD window and click run. After the simulation is over, waveform window will open. Now right click on it and select add traces. Now you will be able to select the signal at the input and the output of the transmission line. V(n001) and V(n002) are the voltages at the input and output of the transmission line respectively.
Case (1)
Now the impedance at the load is matched with the impedance of the transmission line and thus there is no change in the signal received at the receiver. This is the ideal condition for the signal to be transmitted over a transmission line.
Case (2)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is lesser than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflected and the launched signals are shown in the following figure.
Case (3)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is more than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflections add to the original signal that the overshoots appear. The reflected and the launched signals are shown in the following figure.
To avoid reflections ideally you have to avoid impedance variations in the transmission line that carries your signal.
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Creating SPICE netlist
You must have been familiar with program called SPICE simulate the electronic circuits. SPICE is a powerful program for circuit simulation. You can find many flavors of SPICE free to download from the Internet. The one used here is LTSPICE the one available from Linear Technology. To run the spice netlist give in this post get one and install it in your computer.
Open notepad and paste the code given below:
T1 N001 0 N002 0 Td=1n Z0=50
V1 N001 0 PULSE(0 5 10ns 1ns 1ns 10ns 20ns 5) Rser=0
R1 N002 0 100
.tran 100ns
.end
Save it as filename.cir
The schematic of the netlist is given below
In the simulation we are going to simulate three cases(1) R1 = 50 ohms
(2) R1 = 25 ohms
(3) R1 = 75 ohms
You will find that only in case (1) signal is reproduced at the receiver as launched into the transmission line.
To run the simulation, open the file.cir in the LTSPICE SwitcherCAD window and click run. After the simulation is over, waveform window will open. Now right click on it and select add traces. Now you will be able to select the signal at the input and the output of the transmission line. V(n001) and V(n002) are the voltages at the input and output of the transmission line respectively.
Case (1)
Now the impedance at the load is matched with the impedance of the transmission line and thus there is no change in the signal received at the receiver. This is the ideal condition for the signal to be transmitted over a transmission line.
Case (2)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is lesser than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflected and the launched signals are shown in the following figure.
Case (3)
This is the case where the receiver impedance is not matched to the transmission line impedance. The impedance at the receiver is more than the transmission line impedance. This is one case in which reflection occurs. As the signal propagates the signal sees the change in the impedance. Whenever it sees a change in the impedance it gets reflected. The reflections add to the original signal that the overshoots appear. The reflected and the launched signals are shown in the following figure.
To avoid reflections ideally you have to avoid impedance variations in the transmission line that carries your signal.
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Tuesday, June 3, 2008
Digital is Analog
The basic idea of digital communication is to eliminate the unnecessary noises that are the inherent components of so called analog communications. The old AM receivers sound little awkward when an electric switch near to it is turned OFF/ON. The basic concept of digital communication is to represent anything in this world with two digits ‘1’ and ‘0’. But if we look closely at how these ‘1’s and ‘0’s are transmitted over the media, it will be very interesting. Before understanding how to maintain the integrity of signals, we got to know about what a signal is. After reading this article you will be able to understand that the digital signals are nothing but ensemble of analog signals. When you can deal any digital signal with analog approach, maintaining integrity of signals is made easy..!!!
Inside signals
Fourier was quite successful in defining the anatomy of any signal that had finite energy. He with his formula was able to identify the signals that collectively represented a totally new signal. This demands us to view the composed signal in a completely new domain called Frequency domain, for which mathematical transforms like Fourier Transform, Laplace Transforms are greatly useful. You can find the fourier transform of standard signals here. The signal that had different amplitudes, phases and more importantly frequencies when added up forms a new signal. Let us limit our scope of discussion to only digital signals. The following figure shows an example of ideal digital signal representing 1’s and 0’s.
Note that we are considering a typical digital pulse that has its levels continuously changing between 1 and 0. The frequency content of the above signal is shown in the following figure. 
Note that the digital signal that is shown looks like a SINC signal in the frequency domain. That means the digital signals are composed of infinitely large set of sine waves of different amplitudes and phases with frequencies progressively aligned. When these progressively aligned signals when viewed in time domain, represent a digital pulse.
Ideally the signal received in the other end of the transmission line must be very similar to the originally transmitted signal. The transmission line, when viewed as a system, must show a unit impulse response of H(ω) = 1, such that the signal is received at the receiver with no modification. But in reality all systems are band limited and so the transmission lines conduct only a certain band of signals that the signal falling out of its conduction band are lost in the line or reflected back to the receiver. Thus the signal shape of the received digital signal starts approaching that of a trapezoidal signal. A trapezoidal signal is seen at the receiver only if the conduction band has a flat response. If the conduction band is not flat enough the signals the received end loses its integrity and looks similar to the one shown below.
To maintain the signal shape at the receiver do,
• Have a second look at the transmission line
• not believe signals are digital
• Match the spectrum of signals to that of the transmission line
The next issue shall discuss the transmission line characteristics and reflection phenomenon using simple simulations.
Inside signals
Fourier was quite successful in defining the anatomy of any signal that had finite energy. He with his formula was able to identify the signals that collectively represented a totally new signal. This demands us to view the composed signal in a completely new domain called Frequency domain, for which mathematical transforms like Fourier Transform, Laplace Transforms are greatly useful. You can find the fourier transform of standard signals here. The signal that had different amplitudes, phases and more importantly frequencies when added up forms a new signal. Let us limit our scope of discussion to only digital signals. The following figure shows an example of ideal digital signal representing 1’s and 0’s.
Note that we are considering a typical digital pulse that has its levels continuously changing between 1 and 0. The frequency content of the above signal is shown in the following figure. Note that the digital signal that is shown looks like a SINC signal in the frequency domain. That means the digital signals are composed of infinitely large set of sine waves of different amplitudes and phases with frequencies progressively aligned. When these progressively aligned signals when viewed in time domain, represent a digital pulse.Ideally the signal received in the other end of the transmission line must be very similar to the originally transmitted signal. The transmission line, when viewed as a system, must show a unit impulse response of H(ω) = 1, such that the signal is received at the receiver with no modification. But in reality all systems are band limited and so the transmission lines conduct only a certain band of signals that the signal falling out of its conduction band are lost in the line or reflected back to the receiver. Thus the signal shape of the received digital signal starts approaching that of a trapezoidal signal. A trapezoidal signal is seen at the receiver only if the conduction band has a flat response. If the conduction band is not flat enough the signals the received end loses its integrity and looks similar to the one shown below.
To maintain the signal shape at the receiver do,• Have a second look at the transmission line
• not believe signals are digital
• Match the spectrum of signals to that of the transmission line
The next issue shall discuss the transmission line characteristics and reflection phenomenon using simple simulations.
Monday, June 2, 2008
Signal integrity Fundamentals
This series, "signal integrity fundamentals" will cover a wide area of topics in signal and power integrity. Hope this will provide a view with much on this area and discuss the topics using fundamental concepts and simple simulations.
Happy Reading..!!!
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