2-1. Introduction
If the signal frequency is sped up in the digital circuit, the radiation noise will also be strengthened and noise suppression will be difficult.
Therefore, reduction in the voltage of a signal or another measure is also necessary to reduce the current. For example, in the past, the mainstream of the power supply voltage of digital circuits was 5V, but now various voltages such as 3.3V, 1.8V, and 1.3V are used.
If the voltage is reduced like this, even a slight ringing of the waveform leads to a degradation of the circuit. Therefore, it is necessary to suppress the ringing of the signal waveform.
To completely suppress the ringing of the waveform, the addition of a damping resistor alone is insufficient. It is also necessary to match (1) the output impedance in the transmission side, (2) characteristic impedance of the transmission line, and (3) input impedance of the load side.
The following pages describe the influence of impedance matching on the waveform and radiation noise by adding a resistor of the digital signal line and changing the characteristic impedance of the transmission line.
2-2. Waveform and radiation noise before impedance matching
I prepared an evaluation substrate and observed the influence of impedance matching by measuring the waveform and radiation noise.
Fig. A shows the evaluation substrate. A signal was sent from a general-purpose logic IC, 74ALVC04 (NOT element), and received by 74ALVC04. The signal frequency is 40MHz, the transmission line length is 10cm, the substrate thickness t is 1.6mm, and the power supply voltage is 3.3V with a double-sided substrate whose back surface is entirely grounded. 40MHz is no longer a high-speed signal, but the signal frequency and IC were selected considering the ease of the experiment.
Fig. B shows the measurement results of the waveform and radiation noise.
Large ringing occurred in the waveform. This is because the output impedance of the signal, characteristic impedance of the transmission line, and load impedance of the load are not matched.
The following changes were made as shown in Fig. C for impedance matching on this test substrate.
- (1)Changed the characteristic impedance z: 130 ohms -> 50 ohms
- (2)Added a resistor of 30 ohms to the output of the transmission side and changed the output impedance to 50 ohms
- (3)Terminated with the load impedance of 50 ohms
2-2-1. Change in the characteristic impedance of the signal line
First, I changed the characteristic impedance of the signal line from 130 ohms (pattern width of 0.30mm) to 50 ohms (pattern width of 3.1mm).
The reason why it was changed to 50 ohms is that 50 ohms is generally used for high-speed signal transmission. It seems that 50 ohms is generally used because the loss is small at 50 ohms due to the influence of the materials of insulators such as coaxial cables.
Fig. A shows the measurement results of the waveform and radiation noise. In the ringing of the waveform, the overshoot voltage decreased from 4Vp-p to 3Vp-p.
This is because the characteristic impedance of the transmission line got closer to the output impedance of the transmission side, which reduced the reflection of the signal.
The radiation noise was increased by up to about in the frequency range from 280MHz to 960MHz10dB. It seems that an increase in the electrostatic capacitance between the pattern and ground influenced it because the pattern width was widened from 0.3mm to 3.1mm.
2-2-2. Addition of output and terminating resistors
Fig. A shows the waveforms and radiation noises with output and terminating resistors added.
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(1) Addition of an output resistor
Re-reflection in the transmission side is suppressed because the characteristic impedance of the transmission line got closer to the output resistor due to the addition of an output resistor to the transmission side. Therefore, the ringing was suppressed from 3Vp-p to 0.96Vp-p.
Since the resistors suppressed the current, the radiation noise is also about 5dB smaller.
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(2) Addition of a terminating resistor
Reflection in the load side is suppressed because the characteristic impedance of the transmission line got closer to the impedance of the load. Therefore, the ringing was suppressed from 3Vp-p to 0.56Vp-p.
The radiation noise at 200MHz or less increased by up to about 10dB. It seems that due to the termination with a 50-ohm resistor, the impedance of the load at 200MHz or less was reduced and the current increased.
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(3) Addition of output and terminating resistors
Ringing was suppressed greatly from 3Vp-p to 0.28Vp-p thanks to the matching of the impedance of the output, transmission line, and load. However, the crest value is 1.9V, which is 1.4V lower than the power supply voltage value, 3.3V.
2-3. Influence of substrate thickness
Impedance matching suppressed the ringing, but with a substrate thickness of t=1.6mm, the pattern width is wide, 3.1mm, so it is not suitable for high-density mounting.
Thinning of the substrate is necessary for narrowing the pattern width and keeping the characteristic impedance of 50 ohms. Fig. A shows the calculation result of the relationship between the substrate thickness (layer-to-layer thickness) and pattern width.
I reduced the substrate thickness t from 1.6mm to 0.8mm and measured the waveform and radiation noise. I wanted to thin the substrate more but it ended up to be 0.8mm due to the constraint of availability. Since the characteristic impedance was fixed to 50 ohms, the pattern width was changed from 3.1mm to 1.6mm.
Fig. B shows the signal waveform and radiation noise under this condition.
Since the characteristic impedance was not changed, the reflection amount of the signal and the signal waveform did not change. The radiation noise decreased by 5 to 10dB as a whole. Thinning the substrate more will reduce the radiation noise more. The thinner the substrate, the less resistant against bending it will be, so a multi-layer substrate is used to reduce the thickness. This means that the radiation noise can be reduced by using a multi-layer substrate though its cost is high.
2-4. Noise suppression effect of ferrite beads in matched circuit
Next, let me introduce examples of installation of ferrite beads for noise suppression.
The frequency range where the impedance curve rises varies depending on the material of the ferrite bead. In addition, the ratio of the resistance component R to the reactance component X of the impedance also varies.
We call the chip-type ferrite bead "BLM": general ones as the BLM_A series and those with a sharp-rising impedance curve as the BLM_B series.
Fig. B shows the waveform and radiation noise measurement results with these ferrite beads used.
The type with a sharp-rising impedance is expected to suppress the deformation of the signal waveform as well as the noise in a circuit with impedance matching.
However, as shown in Fig. C, the frequency band where the resistance component becomes the main component tends to be narrow, so without impedance matching, the ringing of the waveform may pose a problem. (If the transmission line is short, the influence of the reflection of a signal is small and the ringing is unlikely to pose a problem. Therefore, it is likely that even a sharp-rising type will not pose any problems with the waveform.)
Summary
- To reduce the voltage for the speed-up of a signal in a digital circuit, it is necessary to suppress the ringing of the waveform in order to prevent malfunctions. To do so, characteristic impedance matching of the signal path is effective.
- Thinning of the substrate is necessary for high-density mounting with the width of the wiring reduced while meeting the signal line characteristic impedance of 50 ohms. This thinning is also effective for reduction in the radiation noise.
- In impedance-matched lines or short lines, a ferrite bead with sharp rise of the impedance curve for the frequency is effective to suppress the deformation of the waveform.
Next: Chapter 3 Noise Suppression in Differential Transmission