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big challenge to all the signal engineer

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chuan556

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hi,i have a question to ask,i pump in the square wave to one circuit then i measure the voltage of the driver,but the result that i get is square wave that keep on increasing like in the attachment,why?
i set the termination as 50 ohm,driver and trace 50 ohm,it should no reflection right?but through simulation the result is totally different,any explaination?

the circuit is driver connect to receiver through via and refer to ground,it is complete loop.

thanks.

rgds,
chuan556
 

ahmad_abdulghany

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What's the input impedance you see through the transimission type?? you should calculate it not only the c/cs impedance of the line, as |Γ|=(ZL-Zc)/(ZL+Zc) ....

If the combination of ZL and Zc gave positive Γ, then you may get overdamped wave at the driver side.

I hope that helped somehow :)

BR
Ahmad,
 

throwaway18

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Re: offset or oscillation from driver on 50ohm line

You have not given us enough information to say.

Run the simulation for longer. Does it reach a steady DC offset or is it an oscillation?
 

chuan556

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the simulation result will keep on increasing no matter how long you run it,sorry forgot to mention
 

throwaway18

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If it increases forever then your simulation clearly does not reflect reality. Any practical driver is limited by it's power supply.

We still need lots more information.

What simulator software are using using?
What driver circuit are you simulating?
 

chuan556

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i am using speed2000 and the driver just a voltage source.
is it because of reflection so it keep on increasing?
 

throwaway18

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Your graph of the voltage shows a DC offset. That is not a reflection.
 

chuan556

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may you explain what is dc offset?and how it can prevent?
 

Fom

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Put your schematic if you want get answer.
If you aply to any circuit an ideal source and measure output signal of this source you will get square wave, because driver has zero impedance and will provide square wave independent on load.
 

chuan556

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Fom said:
Put your schematic if you want get answer.
If you aply to any circuit an ideal source and measure output signal of this source you will get square wave, because driver has zero impedance and will provide square wave independent on load.

sorry i forgot to put in the schematic,the problem is the square wave keep on increase ,why?
 

artem

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chuan556, why you put doc file with macros inside ? Is it much easy to put jpg by printscreen utility? What the macros inside your file is used for ?
 

chuan556

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artem said:
chuan556, why you put doc file with macros inside ? Is it much easy to put jpg by printscreen utility? What the macros inside your file is used for ?

i dont know what the marcos is,i try to download and i found that doc is ok.
anyway i convert it to pdf form in order to make it easier for everyone to view
 

chuan556

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i had done the simulation with longer time,and i found out what throwaway18 said,the increment will stable and remain at there.
so i began to wonder why the circuit can be like that since the trace which i am using is short(around 1" and the rise/fall time for my signal is average(1n),so it shouldn't be like that so any reason to explain this?
 

artem

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I dont know the simulation tool you are using and the nature of simulation

Are ground and power planes somewhere DC connected ? If it is not - that is the effect of the grounds-power planes or other capacitance, its charging effect is seen in increasing DC level on signal plot.

Of course, i could be badly wrong )
 

chuan556

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artem said:
I dont know the simulation tool you are using and the nature of simulation

Are ground and power planes somewhere DC connected ? If it is not - that is the effect of the grounds-power planes or other capacitance, its charging effect is seen in increasing DC level on signal plot.

Of course, i could be badly wrong )

may be you are right, i suspect that it is caused by my pwr and gnd but is there any way to reduce it?or make it disappear so that i can get a nice waveform
 

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