[Turn] about clock Line/data line/Address line series resistance and its function

1. Generalization

Such resistors are only considered in high-speed signal lines. In low-frequency cases, it is generally directly connected.

This resistor has two functions, the first is impedance matching. Because the impedance of the signal source is very low, with the impedance mismatch between the signal line (for impedance matching, see detailed), after a resistor, can improve the matching situation to reduce reflection, avoid oscillation and so on.

The second is to reduce the degree of the edge of the signal, so as to reduce high frequency noise and overshoot. Because of the resistor in series, the distribution capacitance of the signal line and the input capacitance of the load form an RC circuit, which will reduce the steep degree of the signal edge. You know, if the edge of a signal is very steep, contains a large number of high-frequency components, will be radiated interference, in addition, it is prone to overshoot.

2, detailed (impedance matching)

Impedance matching refers to a suitable collocation method between the signal source or the transmission line and the load. Impedance matching is divided into two kinds of low frequency and high frequency discussion.

We start by driving a load from a DC voltage source. Due to the actual voltage source, there is always the internal resistance (see output impedance a Q), we can put a real voltage source, equivalent to an ideal voltage source with a resistor R series model. Assuming that the load resistance is R, the Power electromotive force is U, the internal resistance is r, then we can calculate the current through the Resistance R is: i=u/(r+r), it can be seen that the load resistance r smaller, the output current larger. The voltage on load R is: uo=ir=u/[1+ (R/R)], as you can see, the greater the load resistance r, the higher the output voltage Uo. To calculate the power consumption of the resistor R is:

P = i2xr=[u/(r+r)]2XR = u2xr/(R2+2XRXR+R2)

= u2xr/[(r-r) 2+4XRXR]

= u2/{[(r-r) 2/R]+4XR}

For a given signal source, the internal resistance of R is fixed, and the load resistance r is chosen by us. Note in [(R-R) 2/r], when R=r, [(r-r) 2/R] can achieve a minimum value of 0, the load resistance R can be obtained maximum output power pmax=u2/(4XR). That is, when the load resistance is equal to the internal resistance of the signal source, the load can obtain the maximum output power, which is one of the impedance matching we often say. For pure resistor circuits, this conclusion is also suitable for low frequency circuits and high frequency circuits. When the AC circuit contains capacitive or inductive impedance, the conclusion has changed, it is necessary to signal source and load impedance of the real part of the equivalent, the imaginary part of the opposite number, which is called a conjugate match. In the low-frequency circuit, we generally do not consider the transmission line matching problem, only consider the situation between the signal source and the load, because the wavelength of the low-frequency signal is very long relative to the transmission line, transmission line can be regarded as a "short-term", reflection can not be considered (it can be understood that: because the line is short, even From the above analysis we can draw a conclusion: if we need a large output current, then select a small load R; If we need a large output voltage, then select a large load R; If we need the maximum output power, select the resistance r that matches the internal resistance of the signal source. Sometimes the impedance mismatch has another layer of meaning, for example, some instrument output is designed under specific load conditions, if the load conditions change, it may not reach the original performance, then we will also be called impedance mismatch.

In high frequency circuits, we must also consider the problem of reflection. When the frequency of the signal is very high, then the wavelength of the signal is very short, when the wavelength is shorter than the length of the transmission line can be compared, the reflection signal superimposed on the original signal will change the original signal shape. If the characteristic impedance of a transmission line is not equal to the load impedance (that is, it does not match), a reflection is generated at the load end. Why the impedance mismatch will produce reflection and the solution of the characteristic impedance, involving the solution of second-order partial differential equations, here we do not elaborate, interested in the electromagnetic field and microwave in the book of the transmission line theory. The characteristic impedance (also called characteristic impedance) of a transmission line is determined by the structure and material of the transmission line, regardless of the length of the transmission line and the amplitude and frequency of the signal.

For example, the common CCTV coaxial cable characteristic impedance is 75ω, while some RF devices are commonly characterized by a 50ω coaxial cable. There is also a common transmission line is the characteristic impedance of 300ω flat parallel lines, which is used in rural television antenna rack is more common, used to make eight of wood antenna feeder. Because the input impedance of the TV's RF input is 75ω, the 300ω feeder will not match it. How is this problem solved in practice? I do not know if you notice that the TV accessories, there is a 300ω to 75ω impedance converter (a plastic package, one end has a round plug that East, about two thumbs so big). It is actually a transmission line transformer, the impedance of 300 ω, converted to 75ω, so that can match up. One thing to emphasize here is that the characteristic impedance is not a concept with the resistor we normally understand, it has nothing to do with the length of the transmission line, nor can it be measured by using an ohmmeter. In order not to produce reflection, the load impedance should be equal to the characteristic impedance of the transmission line, which is the impedance matching of the transmission line. What if the impedance mismatch would have any adverse consequences? If it does not match, it will form a reflection, energy transmission is not past, reduce efficiency, will be formed on the transmission line standing wave (simple understanding is that some local signal strong, some local signal is weak), resulting in the transmission line effective power capacity reduction; Power can not be emitted, or even damage the transmitting device. If the high-speed signal line on the circuit board does not match the load impedance, it will produce shock, radiation interference and so on.

When impedance does not match, what are the ways to match it? First, consider using a transformer for impedance conversion, as in the case of the TV set mentioned above. Second, the use of series/shunt capacitors or inductors can be considered, which is often used in debugging RF circuits. Thirdly, the method of using series/shunt resistors can be considered. Some drives have low impedance and can be connected in series with a suitable resistor to match the transmission line, such as high-speed signal lines, sometimes in series with a dozens of ohm resistor. Some receivers of the input impedance is relatively high, you can use the parallel resistor method to match the transmission line, for example, 485 bus receiver, often in the data line terminal parallel 120 ohms matching resistor.

To help you understand the problem of reflection when impedance mismatch, let me give you two examples: Suppose you are practicing boxing-punching sandbags. If it's a suitable, hard-to-weigh bag, you'll feel comfortable playing it. However, if one day I have to hand over the sandbags, for example, the inside replaced with iron sand, you still use the force of the previous fight up, your hand may be unbearable-this is the overload of the situation, will have a great rebound power. Conversely, if I replace it with something very light and light, if you punch it, you may be outsmarted, and your hand may be unbearable--that is, the load is too light. Another example, I do not know if you have experienced this: just can't see the stairs up/down stairs, when you think there are stairs, there will be a "load mismatch" such feeling. Of course, this may not be the case, but we can take it to understand the reflection when the load does not match.

Discussion on four-ply plate and 33 Ohm resistor

Selection of four-layer board is not only the problem of power and ground, high-speed digital circuit on the impedance of the line is required, the two-layer board is not good to control the impedance of 33 ohms is generally added to the drive end, is also the impedance matching role; When wiring the data address line, and need to guarantee the high-speed line.

At high frequencies, the traces on the PCB board should be regarded as transmission line transmission line has its characteristic impedance, learned transmission line theory is known, when the transmission line somewhere in the impedance mutation (mismatch), the signal will occur when the reflection, reflection on the original signal caused interference, serious will affect the normal work of the circuit when using four-layer board, Usually the outer signal line, the middle Layer two is the power and ground plane, so that the isolation of two signal layer, more importantly, the outer line and the plane they are near to form a transmission line called "Microstrip", its impedance is fixed, And can be calculated for the two-layer board is more difficult to do so this transmission line impedance is mainly in the width of the line, the distance to the reference plane, the thickness of copper and the characteristics of the dielectric material, there are many ready-made formulas and procedures for calculation.

33 Ohm resistor is usually placed on one end of the drive (not necessarily 33 euro, from a few euro to 50 or 60 kohm, depending on the circuit specific), its role is connected with the output impedance of the transmitter after concatenation with the impedance of the line, so that the reflection back (assuming that the end impedance mismatch) signal will not be reflected back (absorbed), So that the receiving end of the signal will not be affected by the receiver can also be matched, for example, the use of resistors in parallel, but in the digital system is less used, because it is more troublesome, and many times is a multi-harvest, such as address bus, as the source-side matching easy to do.

Here the high frequency, not necessarily the clock frequency is very high circuit, is not the high frequency not only to see the frequency, more important is to see the rise and fall time of the signal can usually be used to estimate the frequency of the circuit rise (or fall), generally take up to half the rise time, for example, if the rise time is 1ns, then its reciprocal is That is to say in the design of the circuit is to consider the 500MHz frequency band to deliberately slow down the edge time, many high-speed IC's drive output slope is adjustable.

[Turn] about clock Line/data line/Address line series resistance and its function