Re: [SI-LIST] : A Question About Power Noise.

Ron Miller (rmiller@Brocade.COM)
Sat, 06 Mar 1999 09:10:46 -0800

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Hi Hans

The use of decoupling at each amplifier stage of RF amplifiers has been common
practice since the 50s. 100 Mhz is not the limit, and digital components into the
multi-gigahertz are no exception.
However, in applying the inductances necessary to keep one stage from talking
to another stage, not only must capacitive bypasses be used but some form of loss.
Decoupling at the higher frequencys may when combined with all the line lengths
become tuned circuits at lower frequencys which can cause oscillations. The answer
to this is to de-Que the bias/power paths by the addition of lossy elements. Resistors
or often used but ferrites are even better because they have an inductive component
as well as a resistive element.
When checking ferrites note that the inductive and resistive component are nearly
constant across a wide frequency range. These are not linear as would be expected.

Ron Miller

HMellberg@aol.com wrote:

> The series addition of an inductor on the VCC line is an EMI type fix for very
> noisy IC's. This type of fix has worked for circuits for up to the 100MHz
> range. Their usefuleness for higher speeds may be doubtfull. Here is the
> theory.
> IC's with intensive di/dt requirements (e.g. clock generator and driver) may
> have the VCC fed with a series inductor to reduce Ldi/dt noise generation on
> the supply plane. Although this may appear counter-intuitive, the use of
> proper decoupling becomes now extremely critical for this to work. The
> decoupling capacitors will now have to provide all the di/dt to the VCC pin of
> the IC that would have come from the planes. That means that the decoupler cap
> must be adjacent to both the VCC and GND pins of the IC. Reducing the round
> trip inductance is a must so, for a j-leaded IC, the decoupler must be on same
> side as the IC with low inductance traces (short and wide). The vias to the
> planes are placed by the j-lead but not so as to affect the L of the trace to
> the cap.
> If proper decoupling had been designed in the first place, the need for an
> inductor would have been eliminated.
> Hans Mellberg
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> Hi Hans

The use of decoupling at each amplifier stage of RF amplifiers has been common
practice since the 50s.   100 Mhz is not the  limit, and digital components into the
multi-gigahertz are no exception.
However, in applying the inductances necessary to keep one stage from talking
to another stage, not only must capacitive bypasses be used but some form of loss.
Decoupling at the higher frequencys may when combined with all the line lengths
become tuned circuits at lower frequencys which can cause oscillations.  The answer
to this is to de-Que the bias/power paths by the addition of lossy elements.  Resistors
or often used but ferrites are even better because they have an inductive component
as well as a resistive element.
When checking ferrites note that the inductive and resistive component are nearly
constant across a wide frequency range.  These are not linear as would be expected.

Ron Miller

HMellberg@aol.com wrote:

The series addition of an inductor on the VCC line is an EMI type fix for very
noisy IC's. This type of fix has worked for circuits for up to the 100MHz
range. Their usefuleness for higher speeds may be doubtfull. Here is the
theory.
IC's with intensive di/dt requirements (e.g. clock generator and driver) may
have the VCC fed with a series inductor to reduce Ldi/dt noise generation on
the supply plane. Although this may appear counter-intuitive, the use of
proper decoupling becomes now extremely critical for this to work. The
decoupling capacitors will now have to provide all the di/dt to the VCC pin of
the IC that would have come from the planes. That means that the decoupler cap
must be adjacent to both the VCC and GND pins of the IC. Reducing the round
trip inductance is a must so, for a j-leaded IC, the decoupler must be on same
side as the IC with low inductance traces (short and wide). The vias to the
planes are placed by the j-lead but not so as to affect the L of the trace to
the cap.
If proper decoupling had been designed in the first place, the need for an
inductor would have been eliminated.
Hans Mellberg

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
  --------------BC6A54CA9EFE3E6E08283675-- **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****