Re: [PSES] SV: Ferrite bead vs. resistor

2016-02-05 Thread alfred1520list 
I don't know your circumstances but I can understand why in some situations 
adding a resistor help. In many circuits a driver side series termination is 
part of the circuit design to match the driver impedance to the trace impedance 
for source termination. The effect of this is that any reflections are absorbed 
by the driver when it is reflected back and therefore preserve the signal 
integrity at the receiving end. A side effect could be that there is less 
ringing which results in less EMI. You need to look closely at your design to 
under the mechanics of your problem and device a solution.

Alfred

On February 5, 2016 1:00:25 AM PST, Amund Westin  
wrote:
>A resistor will cause a voltage drop, and that could make problems for
>the functionality. Ferrite bead will not make a DC voltage drop.
>
>For higher frequencies (above 100MHz) the bead is more precise
>described (Z, X and R), while the impedance of a resistor is maybe not
>defined.
>
> 
>
>#Amund 
>
> 
>
> 
>
>Fra: Amund Westin [mailto:am...@westin-emission.no] 
>Sendt: 5. februar 2016 09:38
>Til: 'EMC-PSTC@LISTSERV.IEEE.ORG' 
>Emne: Ferrite bead vs. resistor
>
> 
>
>Had some EMI issues, caused by a LVDS line.
>
>By inserting CM chokes, the radiation seemed to lower significant.
>
>Inserting resistors as well (aprox 22 ohm) made it even better. I can’t
>check the eye curve, but 22ohm makes no problem with the EUT operation.
>
> 
>
> 
>
>But, should the series resistor be replaced by ferrite beads instead?
>Will it make any really differences?
>
>The LVDS clock is approx 40MHz and data rate 160Mbps.
>
> 
>
>#Amund
>
> 
>
> 
>
>
>-
>
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-- 
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Re: [PSES] SV: Ferrite bead vs. resistor

2016-02-05 Thread ce-test, qualified testing bv - Gert Gremmen 
In general, if the application can support serial resistance,

a resistor will give you better –reproducible- results.  The inductive part

of a ferrite bead may give resonances, and resistors  are available in much 
wider

ranges. Resistors especially SMD, have good RF properties if used on the proper 
PCB footprint.

Values in general between 22 and 100 ohm.

 

This is my experience with fast Ethernet and TTL level serial connections.

 

I can imagine that laser trimmed resistors have worse characteristics, but

as far as I understand the trimming process that is just in terms of increased 
inductivity

a property that creates less of a problem for this application.

 

The bead’s properties are not always specified completely, most are limited

to a graph of (real and/or imaginary) impedance versus frequency.

 

 

In some cases the inductive impedance is simply to high ( >500) to reduce 
emissions.

 

The source impedance of CM currents is not that of a simple voltage or current 
source,

but a unknown mixture of the 2, and blocking one CM current on one cable will 
increase others.

 

The interference current are principally caused by charging and discharging 
(clock) of semiconductor

internal and external capacitances (0-3 or 0-5V), and each charge discharge 
cycle releases

an amount of energy (0.5 CV2 ) that HAS to be dissipated locally in real 
impedances. Too inductive

beads will not dissipate , and simply reflect the energy. Blocking the currents 
(beads) will rise the

voltage opening other ways out, shorting the voltage (caps) will rise the 
current flowing,

both creating more emissions.

Regards,

Ing. Gert Gremmen



 

From: Amund Westin [mailto:am...@westin-emission.no] 
Sent: Friday 5 February 2016 10:00
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] SV: Ferrite bead vs. resistor

 

A resistor will cause a voltage drop, and that could make problems for the 
functionality. Ferrite bead will not make a DC voltage drop.

For higher frequencies (above 100MHz) the bead is more precise described (Z, X 
and R), while the impedance of a resistor is maybe not defined.

 

#Amund 

 

 

Fra: Amund Westin [mailto:am...@westin-emission.no] 
Sendt: 5. februar 2016 09:38
Til: 'EMC-PSTC@LISTSERV.IEEE.ORG' <EMC-PSTC@LISTSERV.IEEE.ORG>
Emne: Ferrite bead vs. resistor

 

Had some EMI issues, caused by a LVDS line.

By inserting CM chokes, the radiation seemed to lower significant.

Inserting resistors as well (aprox 22 ohm) made it even better. I can’t check 
the eye curve, but 22ohm makes no problem with the EUT operation.

 

 

But, should the series resistor be replaced by ferrite beads instead? Will it 
make any really differences?

The LVDS clock is approx 40MHz and data rate 160Mbps.

 

#Amund

 

 

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[PSES] SV: Ferrite bead vs. resistor

2016-02-05 Thread Amund Westin 
A resistor will cause a voltage drop, and that could make problems for the 
functionality. Ferrite bead will not make a DC voltage drop.

For higher frequencies (above 100MHz) the bead is more precise described (Z, X 
and R), while the impedance of a resistor is maybe not defined.

 

#Amund 

 

 

Fra: Amund Westin [mailto:am...@westin-emission.no] 
Sendt: 5. februar 2016 09:38
Til: 'EMC-PSTC@LISTSERV.IEEE.ORG' 
Emne: Ferrite bead vs. resistor

 

Had some EMI issues, caused by a LVDS line.

By inserting CM chokes, the radiation seemed to lower significant.

Inserting resistors as well (aprox 22 ohm) made it even better. I can’t check 
the eye curve, but 22ohm makes no problem with the EUT operation.

 

 

But, should the series resistor be replaced by ferrite beads instead? Will it 
make any really differences?

The LVDS clock is approx 40MHz and data rate 160Mbps.

 

#Amund

 

 


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