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FAQ

Common Questions of Impedance Board

Control impedance line has been more and more popular over the past few years,changing from expert applications to more common applications.The following are some control impedance line common questions and answers.

 

 1.Why more and more manufacturers custom control impedance?

 

The running speed and exchange velocity of telecommunication and computer are on the increase.We don’t have to consider some physical rules in low frequency environment,but now we must take it seriously.When high speed PCB trace is used as transmission line,its electric energy is inter-reflection just like ripples in the lake meet obstacles.Control impedance trace design can reduce electrical energy reflection and ensure that transformation between PCB trace and internal connection is right.

 

 

2.I have some pcb boards,which differential impedance measurement results are 8-10 ohm higher than predicted.


There is a zone of  resin on the differential traces closely arranged on the laminated glass reinforced fiber laminate such as FR4. In areas with more resins, the value of Er is lower than the value of the buck material Er.When using glass reinforced fiber laminate ,you should reduce the ER by 0.4 to 0.8 depending on the trace distribution and lamination to compensate for the lower Er.

FR4 glass Er value is about 6,while the resin is 3.

Non-plaited and aramid reinforced fiber laminate can reduce or eliminate the difference.Please see AP139 for more information.

 

3.Si6000b has a program error in strip line structural modeling--when I inverted structure,the impedance have been changed.

 

Maybe you forget to calculate the trace thickness.For example,if H is 15,H1 is 10 and T is 1,when you invert,the correct answer is that 15 minus 10 minus 1 equal 4.But the common mistake is that H1 means 15 minus 10 equals 5.When calculating,please take the trace thickness into consideration.In this way,no matter what kind of structural modeling you using,Si6000b also can calculates the right value.

 

4.How to calculate the size of control impedance trace ?

 

There are some fundamental equations and calculator provided on the Internet by IPC D 317.Polar Si6000b control impedance design system has considered to expand size usable range,adapting to design concept of modern precision circuit.Si6000b adopts advanced field solver to achieve this target.

 

5.When customer said that we need to test pcb board under 900MHz,may we can use TDR test system or not?

 

Of course,frequency testing range of TDR impedance test system is very wide.Determine impedance parameter(laminated board Er value)don’t have obviously change from 3 to 5GHz.Thus,there is no need to spend so much money and time to test frequency by using network analyzer.You can draw a Er and Zo picture to make out impedance change by using

Er and Frequency comparison table of laminated board manufacturer

 

6.Should I need more probe to match each impedance which is to be tested,because some impedance test system use different impedance probe.

 

You need to test in the most flat part of test trace,when using TDR.You can use 50ohm probe,and the trace maybe has enough flat length,if not,you will come across test limits.If you don’t use shorter test sample,which means that trace have not enough flat length,you can try to use probe that is close to test trace impedance value.

 

7.What is the importance of testing sample?Is it better to test in actual PCB trace?

 

Testing sample is very ideal for production test.The sample ground point is located near test trace,which is convenient for testing.What’s more, for strip line structure,the inner layers of the sample are interconnected(The layers on the actual PCB board are isolated,which will influence on TDR numerical reading).After the power decoupling capacitors on the PCB board are fully assembled,you can get the real numerical reading.

 

 

8.Why is it important to test test the connection between the probe and the specimen ?Is it easier to test lead grounding ?

 

Only when the test system and the control impedance system become one, that is, the probe is completely connected to the control impedance system,control impedance measurement is accurate.A longer earth lamp or coil will cause poor interconnection  and reflection , leading to a decrease or incorrect reading of the test trace .The probe point has lots of , if it is used occasionally, it is better to use adjustable pitch IP 50-v probe.

 

9.Although the number of PCB boards with differential structures is not very large, there is a growing trend .What are the differences between a differential structure and a microwave band ?

 

A differential structure is sometimes used for telecommunications cable interfaces, such as twisted-pair.By using differential cables and structures, excellent signal quality can be obtained in a high electronic noise environment without the need for excessive amplification.In short, the start signal of the cable is transmitted to one side of the cable while it is turned upside down and transmitted to the other side.The electrical noise on both sides of the cable is the same (i.e., not inverted) as the signal is transmitted over the wire (or traces).The receiver simply subtracts the PCB or the incoming signal of the cable ,and noise can be eliminated.The remaining signal is a good copy of the original signal

 

10.The customer requests the PCB board produce 100 ohm traces, and the tolerance ± 0.5 ohms. is this tolerance realistic?

 

 No,it is not realistic.High frequency measurement is difficult to realize,comparing to DMM measurements in CD environment. RF engineers are quite satisfied if they can get several ohm tolerances.Usually, the tolerance of the 50 ohm traces is about 5%, 75 ohms, about 15%, and 100 ohms will be as high as 20% 

 

11.How do you track impedance test system ?

 

The reference impedance standard is higher.Polar uses Maury high precision microwave air line,to obtain high precision, stable mechanical performance of coaxial "pipe" .The high precision air line standard is regularly proofread by NIST or NPL.The laboratory uses gas measuring technique to measure inner size,and calculate the true impedance.

 

12.What is the pulse frequency on the CITS?

 

CITS500S adopts  time-domain reflectometer(TDR)technique to measure electronic reflection of high speed step pulse.Actually,a series of high speed pulses and sampling pulses are used to construct wave-forms.

Fourier Analysis can show the high-speed stepped pulse with a variety of frequency components.Thus,when you do TDR measurement,CITS500s or other TDR instruments do not use a particular frequency, but rather a variety of frequencies.The main frequency that affects the ZO on the PCB board is the Er of the laminates.Zo equals extract a root of 1/Er.You can use Si6000b to draw a picture of Zo and Er to see the change of frequency.

 

13.Why sometimes there are a lot of ripples on the test traces of CITS, and again, the ripples will disappear again?

 

Check if there is a cell phone or other wireless device near CITS or RITS.

Microwave transmission with proof test and mobile phone antenna have a similar dimension, which will receive the mobile phone signal.Even if you don't make a phone call, your phone regularly sends signals to the nearest base station,which leads to erroneous readings of the CITS (or other RF measurement system)

 

14.I want to learn more about RAMBUS®,and customer has the production requirements of the PCB board with an impedance of 28 ohms .

 

RAMBUS is a new high-speed memory architecture for PC at a speed of 800MHz.Therefore, PC boards and memory bank (RIMM) are required to have matching impedances.The impedance of the RAMBUS system is 28 ohms required by the customer, and the tolerance is 10%, which is a very strict tolerance requirement in practical applications .Only when main board meets the requirements,can RAMBUS run.Maybe you can visit the site  www.rambus.com to get more details about RAMBUS.28 ohm IP probe produced by Polar can be used to CITS500S control impedance test system,and can test RAMBUS PCB board.

Please contact Richard Chen of Polar to get detailed information about control impedance test using Polar CITS or RITS.

 

15.Why the strip line traces on the sample are always higher ?

 

Check whether your manufacturer make short circuit of Vcc and Gnd on the sample boards(not the PCB board).If it is differential traces,please see the FAQ2.The board on the sample is short circuited which can imitate actual RF condition of the assembled PCB board.

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