Understand AC coupling electric capacity correctly

 

In design of high frequency circuit, classics regular meeting uses AC coupling electric capacity, or adds two to be connected continuously between chip, or adds two between chip and connector. Look be like simple, but everything differs because of the high speed of signal. The high speed of signal is transmitted make this capacitance becomes not “ ideal ” , this capacitance is good without the design, with respect to the failure that may cause whole project. Accordingly, to high speed circuit character, capacitance of this AC coupling did not optimize will be “ deadly ” good.

 

Below the project experience before author basis, check analyses a few problems that I encounter on the use of this capacitance.

 

Most the action that begins capacitance of coupling of Yao Xianming white AC. Will tell commonly, we offer dc bias voltage with AC coupling capacitance, it is the dc weight that filter gives signal, make signal about 0 axial symmetry. Since be this action, so can be this capacitance put in any positions of the passageway? This is the author when making high frequency circuit at first, use in this capacitance on the capacitance of coupling of ——AC of the first problem that encounter should be put which after all.

 

The typical route that middling of a project takes to encounter here will analyse.

 

Graph 1: Access of model of AC coupling capacitance

 

In design of low speed circuit, this capacitance is OK and equivalent into ideal capacitance. And in high frequency circuit, the impedance that causes as a result of the existence of parasitism inductance and plank is discontinuous quality, actually this capacitance cannot be regarded is ideal capacitance. 2.5G of here signal frequency, passageway length 4000mil, the position of AC coupling capacitance sends the position of end and sink 200mil in the distance respectively. We look to emulate the change that the look that give pursues.

 

Graph 2: AC coupling capacitance stands by the eye plan that sends end

 

Graph 3: AC coupling capacitance approachs the eye map of sink

 

Apparent, when capacitance of this AC coupling stands by sink, the integrality of signal is close friends at be being put in send end. My understanding is such, impedance of blame ideal capacitor is discontinuous, energy of the catoptric after signal passes passageway attenuation can be less than direct catoptric energy, great majority is so serial link asks capacitance of this AC coupling is put in sink. But also have exception, do before the author board right board this problem has been encountered when join, check PCIE standard to if be,discover two board place normally in send end to go up, the another action —— that still used AC coupling electric capacity right now presses protection too. E.g. SATA, normally so the requirement stands by connector to place.

 

Solved placement problem, another perplexed everybody’s is look value to choose. Such saying, the resistor R that equivalent of our whole and serial link gives is fixed, so of C of AC coupling capacitance choose will matter to time constant (RC) , RC is bigger, the dc weight that pass is greater, dc pressing falls lower. Since such, can AC coupling capacitance increase indefinitely? Be no good apparently.

 

Graph 4: The eye that after AC coupling capacitance increases, measures pursues

 

Same position, after with the graph 3 photographs increase coupling electric capacity than can seeing, the eye becomes high low. The reason is ” of “ high speed those who make capacitance changes is not ideal. Inductive inductance can produce series connection resonance, look value is greater, syntonic frequency is lower, AC coupling capacitance shows sensibility below low frequency circumstance, accordingly attenuation of high frequency heft increases, eye tall decrescent, ascendant edge changes delay, corresponding JITTER also can increase.

 

Normally capacitance of proposal AC coupling is between 0.01uf~0.2uf, the 0.1uf in the project is commonner. Recommend use of 0402 enclose.

 

Finally, solved above two problems, design those who analyse this capacitance to optimize a design from PCB again. It is actual in the project, these visible element photographs compare the position with AC coupling capacitance, size that hold a cost, more elusory is plank itself (include solder dish the uniformity of precision, Copper Foil) and solder dish the parasitism electric capacity of place is right the influence of signal integrality. We know, high frequency signal must travel along the way that has even feature impedance, when if encounter impedance to break,compounding discontinuous perhaps situation, partial signal can be answered to launch end by reflection, cause the attenuation of signal, affect the integrality of signal. In the project, this kind of case can appear in solder normally dish or board carry connector to be in. When the high speed circuit that the author involves at first is designed, often encounter this problem.

 

Solve this problem to want from two respects proceed with. Be in above all of plank choose on, we choose high-powered ROGERS board normally in application, roger this plank is very accurate on the control of Copper Foil ply, even Copper Foil enclothes the discontinuous sex that reduced impedance greatly; Be in next eliminate solder dish on the parasitism capacitance of place, the common method inside course of study is in solder dish place do secondary floor to handle (hollow is located in solder dish the referenced and planar area of lower part, cupreous fill is founded in lining) , through increasing solder dish as referenced as its plane (perhaps return way) the distance between, reduce the discontinuous sex of capacitance. Use medium more in the author’s project the ROGERS board with accurate control of width of even, Copper Foil increased solder effectively also dish treatment precision.

 

Through emulating contrast ROGERS plank does accurate secondary floor to handle the signal integrality of around.

 

Graph 5: Do the TDR before secondary floor is handled

 

Graph 6: Do the TDR after secondary floor is handled

 

Graph 5 graphs 6 contrast, the leap of impedance is apparent before discovery was not handled, the impedance improvement after secondary floor is handled is very much, have hardly any rank jump with discontinuous.

 

Graph 7: Make the echo loss before secondary floor is handled

 

 

Graph 8: Make the echo loss after secondary floor is handled

 

Graph 7 graphs 8 contrast, after doing secondary floor to handle with ROGERS plank, drop than doing secondary floor to treat echo loss – in 30dB, reduced echo loss greatly, those who made sure signal is transmitted is complete.

 

On put together, want to make the AC coupling capacitor of deadly ” of this “ in deciding high frequency circuit, want to do sufficient circuit to design the course on not only, in the meantime, the stuff of plank of high frequency PCB with choice better performance can allow your get twice the result with half the effort.

 

Match the inductance of circuit to choose

 

To high frequency circuit character, it is very important that the inductance between circuit matchs. Inductance matchs is to point to the transmission line in signal to go up, the input impedance that lets send the output impedance that carries circuit and sink circuit is consistent, after matching, can utmost ground conveys the electric power that delivers end to sink.

 

Match circuit to use capacitor and inductor, but actual capacitor and inductor and ideal element are different, have loss. Show this loss have Q value. Q value is greater, the loss that shows capacitor and inductor is smaller.

 

The Q of inductance is worth the loss with high frequency circuit

 

Match the size that the Q of the inductor that uses in circuit is worth, the loss to high frequency circuit also can produce an effect.

 

To affirm this matter, the SAW filter that we used village field (transmission bands 800MHz frequency paragraph) with RF inductance, in matching circuit Q of transhipment of through cargo is worth different RF inductance, the insertion that measure and compared SAW filter loss.

 

Graph 9 state circuit pursues. This circuit, though be a distribution road, but have inductor of a RF only.

 

Graph 9: SAW filter and match circuit

 

Graph the frequency that the Q of the 10 RF inductance that expressed to have transhipment of through cargo this is worth is characteristic, watch 1 state structure, dimension, Q is worth (the Typ when 800MHz. Value)

 

Graph 10: The Q value of RF inductance is compared (all be 7.5nH)

 

Express the comparison of 1 RF inductance

 

※ pursues the chart of 10 is the design that uses village cropland to offer auxiliary tool SimSurfing is denotive.

 

The integral character of the SAW filter when transhipment of through cargo matchs the RF inductance of circuit sees a picture 11, transmission bands character sees a picture 12.

 

Graph 11: The whole of SAW filter is characteristic

 

Graph 12: The transmission bands of SAW filter is characteristic

 

From the graph the transmission bands character of 12 looks, the insertion that can affirm SAW filter loss because of uses RF inductance different. The loss of this kind of level of high frequency circuit is more and more significant.

 

From now on second experimental result is knowable, the Q value of RF inductance is greater (loss is jumped over small) , the insertion of SAW filter loss is smaller. That is to say, the size of inductor loss includes the size of the SAW filter loss that matchs circuit inside namely.

 

Ask an attention, use high frequency cell (it is SAW filter this) , match circuit, frequency paragraph etc different, loss also each different.

 

The deviation of inductance and the influence to matching circuit

 

Additional, the impedance of actual inductor is worth the discontinuous value that is and so on of 1.0nH, 1.1nH, 1.2nH. When undertaking matching, must use meticulous invariable measure to undertake fine tuning sometimes. In the meantime, the deviation that impedance is worth (standard deviation) can turn the level that match into deviation, to satisfy necessary character, need the inductor with small error sometimes. Among the inductor of village cropland, film LQP series most accord with the demand with meticulous invariable measure and little error.

 

According to above circumstance, be necessary to be opposite the respect such as value of the Q character of the inductance of RF of integrated return circuit of SAW filter, deviation, dimension, cost, make comparative the choice is made after discussing. Sticking outfit space to the rest circumstance falls, q is worth a line inductance LQW15/LQW04 of on the high side to be optimal choice. In addition, below the circumstance that sticks outfit space to be restricted somewhat, small size 0603, the LQP03HQ/LQP03TN_02 that has taller Q value is optimal choice.