DDR4 reference signal power supply layer, impedance will there be affected?

        DDR4 reference signal power supply layer, impedance will there be affected?

    Since the laboratory instruments, redpower on the principle of impedance and the impedance test suddenly came in, think I have the chance to personally test the line impedance and their processing of impedance is the same, if not the same will probably have much deviation. For starters, redpower this continuous exploration and self-doubt spirit is worth our study, the so-called truth from practice, accurate simulation can ensure the performance of the test, at the same time, accurate test is the necessary condition to improve the simulation precision, the simulation and test verification tend to complement each other, on the successful implementation of high-speed signal be short of one cannot, in combination with certain theory and engineering practice, Simulation and test of our a good closed loop to solve the problem of S1.

   The PCB, and we all know that the line is mainly microstrip line and ribbon line, two line impedance formula respectively as shown in the figure below.

  Can be seen from the graph, the line impedance is and related to the following factors:

   1, the dielectric constant of the Er: a kind of material properties, simple means under the unit of voltage, material can be stored within the unit capacity of electrostatic energy; Of the Numbers represent the meaning of the capacitance ratio (relative to the material in the vacuum of capacitance), also known as leakage (through) rate of electrical permittivity, usually air dielectric constant is 1, we often say FR4 is about 4.3 + / - 0.4, when it's calculated impedance adjustment according to the selected material. In the case of not remember formula, we only need to know is inversely proportional to the dielectric constant and impedance, dielectric constant increases, the impedance decreases.

   2, line width W: line width is easy to understand, is the width of the signal itself, line width and impedance is inversely proportional to, also line width increasing impedance decreases.T:

3, copper thick copper thick and line width, is the thickness of the signal itself, copper thickness and line width is inversely proportional to, copper thickness increase the impedance decreases.

   4, line H: the distance to the reference plane microstrip line is only a reference, at this time and stripline has two reference, this also is the essential difference between the two sound, usually the H and the distance to the reference impedance is proportional to the distance impedance also increase (remember, this is also the impedance formula in wei - and is directly proportional to the impedance factor)

    See here, redpower have a doubt in his heart, for signal, or for impedance calculation is generally thought that as long as the signal is above or below the surface layer(not GuanPing surface is ground or power supply), the surface layer is as we calculated impedance of reference; But from the view point of signal flow, signal is ultimately return plane and form a loop, if the signal is above or below the surface layer is not signal directly back, for example, we see more similar as shown in figure 6 layer board or eight layer plate laminated, in the middle of the signal layer S1 / S2 and S3 if reference a layer of power, the power supply layer, it is not the signal Reflow layer directly, longer return path at this time, if the power supply layer and can be used as the reference, the impedance of actual test signal impedance will have effect?

   White layer for Art05 DDR4 signals, green is Pwr04 layer on the Vcc power supply, yellow in the inside of the chip pin and hole, you can see in the fourth floor to the hole and the power is disconnected, this time we went to test this part of the line impedance, the probe end on the signal pin to pin pin must be in place, and the fourth floor Pwr04 for signal backflow is quite so a floating empty layer, at this time we produced at the time of actual test impedance or impedance Coupon article, will be due to the inconsistency of the reference to impedance difference?

    Time may can't distinguish between two cases of specific differences, we still use this figure to explain, the only difference is that in the above reference signal level, on the left is the actual situation, ground hole through the power supply with power supply layers have actual contact, equivalent to a power supply is a floating empty layer, and the right side is the impedance calculation ideally, signal fluctuation are involved Take an examination of the land, and the hole is also and all the reference layer. The only difference is if there is any hole and the signal to plane contact layer (impedance test need some holes in the welding plate at one end).

   In both cases, according to the circuit impedance formula of signal to the plane distance are the same, up and down from the cross section point of view in the past, to say the reference is the same, so according to the formula of impedance or slice computed impedance should be the same; But if to test the impedance, because of the difference of test point on the hole and circumfluence, the actual test of impedance and impedance calculation or slice calculated the impedance of the same? If not, are on the left side of the actual test of impedance is high or low? Do you know why?