[SI-LIST] Re: Capacitors at high frequency
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Mon, 25 Oct 2010 16:30:59 -0700
One would expect the 1.5625 GHz component to travel through part of the
capacitor that is closest to the PCB. It will therefore not experience
the full capacitance directly. However, the full capacitance is needed
only to support the lower frequency components.
Thanks,
Vinu
Scott McMorrow wrote:
> A question to the list, now that we understand the difference between
> signal capacitors and power capacitors.
>
> Assuming an 0402 X5R capacitor that is 40 mil long and 40 mil tall:
>
> Where exactly does signal field travel through the capacitor at the
> Nyquist frequency of 1.5625 GHz for a 3.125 Gbps data stream?
>
> Does a signal at 1.5625 GHz experience the full capacitance directly?
>
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed® is the registered service mark of
> Teraspeed Consulting Group LLC
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>
> On 10/25/2010 5:40 PM, Alvaro alkschbirs wrote:
>
>> Thank you guys! I understand the diferences between signal coupling and
>> power decoupling.
>> Rgds for all,
>> Álvaro
>>
>>
>> On Sun, Oct 24, 2010 at 12:58 PM, Ken Cantrell<
>> Ken.Cantrell@xxxxxxxxxxxxxxxx> wrote:
>>
>>
>>> Alvaro,
>>> One is a low pass filter (power bypassing), the other is a bandbass filter
>>> (AC coupling). The series cap and the inductance of the receiver form the
>>> bandpass. The BP is tolerant of larger variations w.r.t. to inductance (or
>>> capacitance), and L dI/dt noise is a second or third order effect (don't
>>> care). Apples and oranges.
>>>
>>> Thanks,
>>>
>>> Ken
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx
>>> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Alvaro alkschbirs
>>> Sent: Saturday, October 23, 2010 8:33 AM
>>> To: si-list@xxxxxxxxxxxxx
>>> Subject: [SI-LIST] Capacitors at high frequency
>>>
>>>
>>> Hi all,
>>> I have a question about power (de)coupling and ac coupling capacitors;
>>>
>>> Situation 1:
>>> Thinking about capacitors at high frequency, According to what I've read,
>>> the power supply (de)coupling discrete capacitors works until some hundred
>>> of MHz. Above this, the inductance goes dominant and the power charge
>>> capacity goes down.
>>>
>>> Situation 2:
>>> Now, thinking about this same discrete capacitor working as AC coupling on
>>> 1Gb/s or 3,125Gbp/s signals. I have done this in my projects (using 0402
>>> capacitors) and no error have been reported at this signals.
>>>
>>>
>>> Q1. What do you think about it ?
>>>
>>> Q2. Why the dominant inductance is a problem in situation 1, and not for 2
>>> ?
>>>
>>> Thank you very much,
>>> Álvaro Alkschbirs
>>>
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