[SI-LIST] Re: Bandwidth & AC coupling capacitor
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, "Richard Jungert" <r_jungert@xxxxxxxxxxx>
- Date: Mon, 1 Jun 2009 15:14:47 -0600
Richard,
As I'm sure you are aware, the fundamental concept is one of complex
conjugate matching of source and load for maximum power transfer to the
load. The lumped element approximation holds because the
capacitor/inductor/resistor geometries are much smaller than lambda as Steve
mentioned. Variational values of capacitance (or inductance) do indeed
effect the Q (bandwidth), resonant frequency, and power delivered to the
load. Maximum power transfer occurs when Xj = Xc. For a given resonant
frequency, increasing C or L decreases the resonant frequency, and
decreasing one or both increases the resonant frequency. A 10% change in C
or L at an f_res of 1GHz using a basic L-match produces an approximate 50MHz
up/down shift. The impedance, Vout/Iout, however remains constant over a
narrow band of frequencies, which in this case (10% component variation)is
approximately +/- 50MHz at a center frequency of 1GHz for a 5-to-50 ohm
up-transform. You will lose some gain, but reflections due to impedance
differences are not an issue. RF and SI are very different in this respect.
In some senses, RF is less complex and more forgiving than the issues that
we normally deal with where the lumped element approximation does not hold.
At least that's my understanding. As you stated, it's an analog technique
that's been around forever.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Sunday, May 31, 2009 3:44 PM
To: Richard Jungert
Cc: si list freelist; icermail@xxxxxxxxx
Subject: [SI-LIST] Re: Bandwidth & AC coupling capacitor
Richard, if one goes about selecting the capacitor correctly the
inductance from part to part between manufacturers and as manufacturers
change their own process will vary by only a couple percent. Choose the
capacitor properly, and design the etch properly and the job is done.
The design and component sourcing will not have to be revisited next
month or next year.
Best Regards,
Steve.
Richard Jungert wrote:
> Steve
>
> The inductance your talking about it way above the SRF. Is this
> inductance value going to work each time for any capacitor ( with the
> same value ) that is stuffed in there? What if we change capacitor
> manufacturers and the process for making them is slightly different
> and so the inductance your talking about is different? This kind of
> problem one also needs to also consider if your going to manufacture
> the product. Maybe in this case one would need to pick one
> manufacturer/process for this specific part function.
>
> Richard Jungert
>
>
>
> > Date: Sun, 31 May 2009 13:23:24 -0700
> > From: weirsi@xxxxxxxxxx
> > To: r_jungert@xxxxxxxxxxx
> > Subject: Re: [SI-LIST] Re: Bandwidth & AC coupling capacitor
> >
> > Richard from a circuit standpoint it is somewhat simpler than a typical
> > line equalization problem such as long video transmission lines. So
> > long as the capacitor is selected such that the mounted SRF is well
> > below any signal frequencies, the dominant reactance is entirely
> > inductive. This is readily compensated with a pure capacitance which is
> > readily manipulated through the pad and ground clear out patterns. The
> > compensation works provided that the structure is small compared to the
> > shortest wavelength of interest.
> >
> > Best Regards,
> >
> >
> > Steve.
> > Richard Jungert wrote:
> > > I would suggest finding out where the losses are or where the first
> > > pole occurs and then building a LC compensation network to address
> the
> > > problem with rolloff. Peak it where the rolloff occurs. Use
> > > compensation and equalization circuits to fix it.
> > >
> > > This is how it has been done in other technologies such as analog
> > > video for years.
> > > Its an age old problem.
> > >
> > > Richard Jungert
> > >
> > > > Date: Sun, 31 May 2009 03:34:29 -0700
> > > > From: weirsi@xxxxxxxxxx
> > > > To: icermail@xxxxxxxxx
> > > > CC: si-list@xxxxxxxxxxxxx
> > > > Subject: [SI-LIST] Re: Bandwidth & AC coupling capacitor
> > > >
> > > > It is all about the series impedance and impedance continuity.
> Your PCB
> > > > etch is nominally a 50 Ohm signal transmission line. From about an
> > > > octave above the mounted SRF, the capacitor looks like an
> inductor with
> > > > some loss. If the interconnect in the region of the capacitor is
> > > > arranged such that the parasitic shunt capacitance Cshunt =
> > > Lcap_mounted
> > > > / 2500, then for wavelengths that are long compared to the
> length of
> > > the
> > > > region surrounding the capacitor, the L/C approximates the
> impedance of
> > > > the rest of the transmission line and there is little impedance
> > > > discontinuity.
> > > >
> > > > Steve.
> > > >
> > > >
> > > > icer world wrote:
> > > > > When use AC coupling for interconnection,the capacitor is about
> > > 0.1-0.01uF.However,the resonance frequency of this kind capacitor is
> > > below 100MHz,Would signals above 1GHz such as PCIe signals can
> pass it
> > > without no bandwidth reduced?Can anyone give me a explanation? Thanks!
> > > > >
> > > > >
> > > > >
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> > --
> > Steve Weir
> > Teraspeed Consulting Group LLC
> > 121 North River Drive
> > Narragansett, RI 02882
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> > (707) 780-1951 Fax
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Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
California office
(866) 675-4630 Business
(707) 780-1951 Fax
Main office
(401) 284-1827 Business
(401) 284-1840 Fax
Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax
http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of
Teraspeed Consulting Group LLC
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