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[size=+2]Signal Integrity in Digital Circuits
1. Summary
This lesson describes how the transmission line behaviour can be verified with some simple laboratory experiments.
The lesson answers to the following questions:
- How can we get experimental evidence of transmission line behaviour?
- Which electronic instruments should be used to analyse signal integrity?
The experience described in the following is part of the course "Electronics of Telecommunications" held at the Politecnico di Torino.
[size=+2]2. Aim of the experience
The aim of this laboratory experience is to check the behaviour of a transmission line in various operating conditions, and to become familiar with transmission line behaviour. The comparison of the dynamic behaviour of different logic families is also described.
We will observe on the oscilloscope the waveforms at both ends of a transmission line driven by a square wave generator (the square wave corresponds to a sequence of voltage steps of alternate polarity). As transmission line we shall use a rather long coaxial cable (10 to 20 meters) to achieve propagation times long enough to allow the use of standard low cost instruments to verify the propagation effects (signal generator and scope).
The experience is repeated in different driving and load conditions:
- line matched at both ends;
- line with open far end;
- line with unmatched driver.
The sequence of experiments is:
A) Measurement of pulse generator parameters;
B) Measure of cable parameters;
C) Effects of mismatch at driver and termination ends;
D) Effects of capacitive load;
E) Time-domain reflectometry;
F) Driving a line from logic circuits.
Laboratory equipment required for the experience
- signal generator, 50 W output impedance, transition times < 20 ns;
- oscilloscope, 2-channels, 100 MHz minimum bandwidth;
- 10 to 20 m of RG58 coaxial cable;
- 50 W terminations;
- adapters to mount components at cable ends;
- resistors and capacitors: 22 W, 100 W, 220 W, 1 nF,
- Logic devices of various families (LS, HC, AC, BCT, ...);
For all measurements set the signal generator for square wave, at about 2 V peak, 200 kHz rate.
Connect the signal generator to one end of the cable, and the scope to both ends of the cable using high impedance probes. Use adapters at both ends to mount the required R and C components.
The lattice diagram analysis technique is described in lesson 3 of "Interconnections for high-speed digital circuits".
The experimental setup for all proposed measurements is shown in figure 5.1. A 50 W coaxial cable (type RG 58) is used as the transmission line, and is driven by a pulse generator. The waveforms at source and termination ends are monitored on a scope. Incident and reflected waves can thus be verified in several different operating conditions.
Figure 5.1 Measurement setup
The cable length determines the transmission time tP, and therefore the time scale of all measurements, and in turn the requirements of instrumentation (especially the scope). Using 10 m of cable, all measurements can be performed with a 100 MHz scope, available in most general purpose electronic laboratories. Components at both ends of the cable can be connected using clips mounted on BNC connectors, as shown in figure 5.2.
Figure 5.2 The cable and the test fixtures used for the measurements [size=+2]3. Generator Parameters
1) Verify the no load (open circuit) output amplitude VB from the signal generator (Figure 5.3 a).
2) Connect the signal generator to a known load RL (e.g. 100 W), and measure the new VB; compute the output impedance RO of the signal generator (Figure 5.3 b). The expected result is about 50 W.
Figure 5.3 Measurement of the signal generator output impedance.
[size=+2]4. Cable Parameters
- Connect the signal generator to an open line (the coaxial cable); check the waveform at near and far ends (Figure 5.4), and compare with results from lattice diagrams.
| Figure 5.4 | The top trace shows signal at source end: the two steps correspond to the incident and to the reflected wave (after 2 tP). Since the source is matched to line impedance, there is no further reflection. |
From the waveform at the near end and from the cable length compute the wave propagation speed U. Expected result for RG58 cable: about 0.7 c. 3) connect a termination; verify the absence of reflected waves.
Figure 5.5 Measurement of propagation speed in the cable.
[size=+2]5. Mismatch at driver and at termination
- Connect a series termination resistance RS (220 W) between the generator and the line.
- Leave the line open at the far end (GT = 1).
- From the waveform at near and far ends, compute the reflection coefficient GG (near end).
- Compare with results from lattice diagrams.
- Verify the waveforms at near and far ends. Since both reflection coefficients are > 0, all steps have the same direction and decreasing amplitude, making an exponential envelope.
- Compare with results from lattice diagrams.
Figure 5.6 Measurement setup for R[size=-2]O > Z[size=-2]0, line open at far end
Figure 5.7 Waveforms for R[size=-2]O > Z[size=-2]0, line open at far end. Repeat for a generator resistance lower than characteristic impedance (put a 22 W resistance in parallel to the generator output). Since now the reflection coefficient at source is negative, steps have alternated direction, causing oscillations at the far end.
Figure 5.8 Measurement setup for R[size=-2]O < Z[size=-2]0, line open at far end.
Figure 5.9 Waveforms for R[size=-2]O < Z[size=-2]0, line open at far end.
[size=+2]6. Capacitive Load
1) Connect a 1 nF capacitor (C[size=-2]T) at the far end of the line.
2) Verify the waveforms at near and far ends.
For a first approximation analysis, the far end capacitor can be considered a short circuit when the step arrives at the termination (G[size=-2]T = -1), and an open circuit (G[size=-2]T = 1) after the transient. Therefore at t = t[size=-2]P (for the far end) and t = 2 t[size=-2]P (for the near end) the waveform corresponds to a short circuit at the far end. For t >> time constant RC the waveform corresponds to an open line.
Figure 5.10 Measurement setup for transmission line with capacitive load
Figure 5.11 Waveforms in a transmission line with capacitive load. [size=+2]7. Time-domain reflectometer
The experimental setup used before consists of a step (or pulse) generator and an oscilloscope, and is actually a Time Domain Reflectometer or TDR. The TDR is also built as a complete free-standing instrument and can be used to analyse the state of a transmission line from one side only.
The waveforms at the near and far end of an open transmission line with matched impedance at the driver are in figure 5.12. The cable length can be measured from the width of the intermediate step at the near end, which corresponds to 2t[size=-2]P (100 ns in this experiment). The far-end waveform shows a single step, because the incident and the reflected wave appear at this point at the same time.
Figure 5.12 Near end (top trace) and far end waveforms for an open line with matched driver.
If we add another segment of coaxial cable to the far end, the total propagation time is increased (wider intermediate step in the top waveform), and the point C becomes an intermediate point in a longer line. The bottom waveform shows separate incident and reflected waves.
Figure 5.13 Another segment of transmission line is attached at the far end.
The total cable length is still measured by the width of the intermediate step at the near end (about 150 ns in this experiment). The length of the additional cable can be measured from the width of the intermediate step at the point C (previous far end, 50 ns in this experiment). The waveform in C (bottom trace) shows an intermediate step, because the incident and the reflected wave now appear at this point at different times.
Figure 5.14 Near end (top trace) and intermediate point waveforms for an open line with matched driver. [size=+2]8. Driving the line with logic devices
Using LS-family circuits, the equivalent output impedance is slightly higher than the cable impedance, and the rising edge exhibits multiple steps (as in the previous experiment C). The different height of the first step shows that output impedance is different for the H-L (red) and L-H (blue) transitions (lower for L-H). This asymmetric behaviour is typical of bipolar logic families (TTL and similar ones), which have I[size=-2]OL > I[size=-2]OH.
Figure 5.15 Open line driven by a 74LS device.
With HC-family drivers the output impedance is close to 50 W and there is almost no reflection at the driver (Figure 5.16, as in previous experiment B). Rising and falling transitions are symmetrical.
Figure 5.16 Open line driven by a 74HC device
For AC-family devices the output impedance is lower than characteristic impedance, and this causes negative reflections with oscillations (Figure 5.17, second part of experiment C).
Figure 5.17 Open line driven by a 74AC device
Clamp diodes at the termination can limit the oscillations. Since logic devices have clamp diodes at the inputs, connecting a receiver at the end of the line changes the waveforms as in figure 5.18. This is a correct Incident Wave Switching (IWS) working condition.
Figure 5.18 Line with clamp diodes at the far end driven by a 74AC device. [size=+2]9. References
For this lesson, the related sections in the reference book are: 3.2, 3.2, 3.3, 3.4, 3.5, and 3.6
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发表于 2009-4-8 09:40:30
