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RF Circuit Technology

RF Circuit Technology - Understand the dielectric constant Dk of the circuit board

RF Circuit Technology

RF Circuit Technology - Understand the dielectric constant Dk of the circuit board

Understand the dielectric constant Dk of the circuit board

Different circuit board manufacturers will have slightly different PCB parameters. Through communication with the technical support of the circuit board factory, we can obtain some parameter data of the factory.

Such as Dk dielectric constant: The dielectric constant of prepreg is related to thickness. The following table shows the thickness and dielectric constant Dk parameters of different types of prepreg. The dielectric constant of the board is related to the resin material used. The dielectric constant of FR4 double-sided and multi-layer circuit boards is 4.2-4.7, and will decrease with the increase of frequency.

Dielectric loss factor df: The energy consumed by dielectric materials due to heat generation under the action of an alternating electric field is called dielectric loss, which is usually represented by the dielectric loss factor tan δ. Typical value for S1141A is 0.015.

What are the important parameters of high-frequency PCB?
The dielectric constant (Dk) of the high-frequency circuit board substrate must be small and stable. Generally speaking, the smaller the better. The signal transmission rate is inversely proportional to the square root of the data dielectric constant. A high dielectric constant can easily cause signal transmission delays. .

The dielectric loss (Df) of high-frequency circuit board substrate data must be small, which mainly affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss.
The impedance of high-frequency circuit boards actually refers to the parameters of resistance and reactance. Because PCB circuits need to consider the installation of electronic components, conductive performance and signal transmission performance after plugging, so the lower the impedance, the better. .
The base material of high-frequency circuit boards should have low water absorption. High water absorption will cause dielectric constant and dielectric loss when it gets wet.

High frequency circuit board

High frequency circuit board

In order to meet the signal integrity requirements of different applications, PCB not only needs to test S parameters and TDR impedance, but also needs to analyze the physical characteristics of the data itself, dielectric constant and dielectric loss. Accurate dielectric constant can not only achieve effective design, but also make simulation and product real test results more consistent, improve the efficiency of design and development, and is of great significance to PCB data suppliers, including PCB production developers.

1. Our commonly used circuit board PCB medium is made of FR-4 material, and its dielectric constant relative to air is 4.2-4.7. This dielectric constant changes with temperature. Within the temperature range of 0-70 degrees, its maximum change range can reach 20%. Changes in dielectric constant will cause a 10% change in line delay. The higher the temperature, the greater the delay. The dielectric constant also changes with the signal frequency. The higher the frequency, the smaller the dielectric constant is. Below 100M, you can use 4.5 to calculate the inter-board capacitance and delay.

2. The transmission rate of the inner layer signal in the general FR4 material PCB board is 180ps/inch (1inch=1000mil=2.54cm). The surface layer generally depends on the situation, usually between 140 and 170.

3. The actual capacitor can be simply equivalent to L, R, C connected in series. The capacitor has a resonance point, which will appear inductive at high frequencies (above this resonance point). The resonance point will be different depending on the capacitance value and process of the capacitor. And there will be great differences in the products produced by different manufacturers.

This resonance point mainly depends on the equivalent series inductance. Now, for example, the equivalent series inductance of a 100nF chip capacitor is about 0.5nH, and the ESR (equivalent series resistance) value is 0.1 ohms. Then the filtering effect is best when it is about 24M, and the AC impedance is 0.1 ohms. The equivalent inductance of a 1nF chip capacitor is also 0.5nH (the difference between different capacitance values is not too big), the ESR is 0.01 ohms, and it will have the best filtering effect around 200M. In order to achieve better filtering effect, we use a combination of capacitors with different capacitance values.

However, due to the effect of equivalent series inductance and capacitance, there will be a resonance point between 24M and 200M. There is a maximum impedance at this resonance point, which is greater than the impedance of a single capacitor.

This is an undesirable result. (In the period from 24M to 200M, small capacitors are capacitive, and large capacitors are already inductive.

Two capacitors connected in parallel are equivalent to LC connected in parallel. The sum of the ESR values of the two capacitors is the series resistance of the LC loop. If LC is connected in parallel, if the series resistance is 0, there will be an infinite impedance at the resonance point, and the worst filtering effect will be achieved at this point.

This series resistance will suppress this parallel resonance phenomenon, thereby reducing the impedance of the LC resonator at the resonance point). To mitigate this effect, capacitors with larger ESR can be used as appropriate. ESR is equivalent to the series resistance in the resonant network, which can reduce the Q value, thereby making the frequency characteristics flatter.

Increasing ESR will make the overall impedance more consistent. In the frequency bands below 24M and above 200M, the impedance will increase, while in the 24M and 200M frequency bands, the impedance will decrease. Therefore, the frequency band of the board switching noise must also be comprehensively considered. Some foreign boards have a resistor of several ohms in series with a small capacitor (680pF) when large and small capacitors are connected in parallel. This is probably due to this consideration. (From the above parameters, the Q value of a 1nF capacitor is 10 times the Q value of a 100nF capacitor. Since there are no specific equivalent series inductance and ESR values from the manufacturer at hand, the parameters in the above example are inferred based on the data seen in the past. of.

But the deviation shouldn't be too big. The information I have seen in many places in the past is that the resonant frequencies of 1nF and 100nF ceramic capacitors are 100M and 10M respectively. Considering that the L of the chip capacitor is much smaller, and no reliable value has been found, for the sake of convenience, just press 0.5nH calculation. If you have specific and reliable values, I hope you can post them.) The dielectric constant (Dk, ε, Er) determines the speed of electrical signals propagating in the medium. The speed at which an electrical signal travels is inversely proportional to the square root of the dielectric constant.

The lower the dielectric constant, the faster the signal transmission speed. Let's make a vivid analogy, just like you are running on the beach, the water depth is submerged to your ankles, the viscosity of the water is the dielectric constant, the more viscous the water, the higher the dielectric constant, and the slower you run.

The dielectric constant is not very easy to measure or define. It is not only related to the characteristics of the medium itself, but also related to the test method, test frequency, and data status before and during the test. The dielectric constant also changes with temperature. Some special materials take the temperature factor into consideration when developing. Humidity is also an important factor affecting the dielectric constant, because the dielectric constant of water is 70, and there is very little moisture. , will cause significant changes.

The following are the dielectric constants of some typical materials (at 1Mhz): Vacuum 1.0 pure PTFE 2.1 GY PTFE 2.2-2.3 GX-PTFE 2.55 cyanate ester/glass 3.2 cyanate ester/quartz 2.8-3.4 polyimide-quartz 3.5 -3.8 Polyimide-glass 4.0-4.6 Epoxy resin-glass (FR4) 4.4-5.2 Non-woven aromatic amine (aramid) 3.8-4.1 Aromatic amine (woven fabric) 3.8-

4. Ceramic filled PTFE 6.0-10.2 Foamclad (Arlon patent) 1.15-1.3 Water 70.0. It can be seen that for high-speed and high-frequency applications, the most ideal material is an air medium wrapped by copper foil, with a thickness tolerance of +/-0.00001". As for material development, everyone is working hard in this direction, such as Arlon The patented Foamclad is ideally suited for base station antenna applications.

But not all designs have a smaller dielectric constant, the better. It is often determined based on some actual designs. Some circuits that require a small volume often require high dielectric constant materials, such as Arlon's AR1000, which is used in miniaturization. Circuit design. Some designs, such as power amplifiers, commonly use a dielectric constant of 2.55 (such as Arlon Diclad527, AD255, etc.), or a dielectric constant of 3.5 (such as AD350, 25N/FR, etc.). There are also those that use a dielectric constant of 4.5 (such as AD450). Changed from FR-4 design to high frequency applications, but want to keep the previous design.

In addition to directly affecting the signal transmission rate, the dielectric constant DK also determines the characteristic impedance to a large extent. In different parts, the characteristic impedance matching is particularly important in microwave communications. If there is an impedance mismatch, the impedance mismatch will also It is called VSWR (Standing Wave Ratio). MAX2242: The printed circuit board material should be FR4 or G-10.

This type of material is a good choice for most low-cost wireless applications operating below 3GHz. The MAX2242 evaluation board uses 4-layer FR4 with a dielectric constant of 4.5, an insulation layer thickness of 6 mil, and 1oz copper. When designing a low-impedance circuit like the MAX2242 with an output impedance of only about (8 + j5) at 2.45GHz, a 0.5nH inductor can produce an inductive reactance of 8. An inductive reactance of 8 is equivalent to a dielectric constant of 4.5 and a thickness of 6 mil. Impedance created by a 60mil x 10mil microstrip line on an FR4 PCB.

Resonance frequency calculation:f=1/(2*3.14159*SQRT(L*C)

The dielectric constant (Dk) determines how fast electrical signals propagate in that medium. The speed at which an electrical signal travels is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal transmission speed. To give a vivid metaphor, it's like you are running on the beach. The water is deep enough to cover your ankles. The viscosity of the water is the dielectric constant. The more viscous the water, the higher the dielectric constant and the slower you run.