[SI-LIST] Re: AC coupling placement
- From: Ben Rothchild <benrothchild@xxxxxxxxx>
- To: M.Randol@xxxxxxxxxxxx
- Date: Fri, 1 Feb 2002 06:33:39 -0800 (PST)
Mark,
Thanks for your input.
I am not clear on your test set up mentioned
in the first paragragh. Where exactly is
the cap connected on the transmission line.
What kind of termination is recommended.
What is the thru. I am digital board designer,
Is this the stuff RF guys use to connect
two coax cables. And why should a spectrum
analyzer/tracking generator be used only with
the thru.
Thanks
Ben
--- Randol Mark-ryvw50 <M.Randol@xxxxxxxxxxxx> wrote:
>
> It doesn't matter if the "DC block" is just that.
> For ~CW signals like most RF signals, that's pretty
> easy. You can measure the cap's mounted series
> resonant frequency by placing a cap to ground on
> either a thru or a terminated line and looking for
> the suck out with a network analyzer or spectrum
> analyzer/tracking generator (thru only). Use the
> value cap that gives you the suckout (low-Z shunt)
> at the desired series pass frequency. Use the cap
> mounted in series for use as a DC block, of course!
>
> If you want to just be "close enough", calculate the
> series resonance with about 0.8nH for 0805 cases,
> 0.7nH for 0603's, and about 0.5nH for 0402's. The
> only frequency that you might have problems with is
> 155MHz, if you have a wide modulation on the signal
> you are wanting to pass, you could roll off the
> outer edges.
>
> If you're running "lots" of power, you might want to
> calculate the power dissapation in the cap from it's
> series resistance. Other than that, a couple tenths
> of resistance aren't going to matter much in a
> system with a characteristic impedance more than
> ~10ohms? That is, the loss should be negligible.
>
> Remember that this isn't a DC block like you'd
> purchase commercially. The bandwidth is rather
> small, maybe a couple hundred MHz in the GHz
> frequencies. It's probably more appropriate to
> refer to it as a "single component band pass filter"
> instead of a "DC block".
>
> If the "DC block" is used as a part of the impedance
> matching network, it's placement can make a great
> deal of difference!
>
> --
> Mark Randol, RF Evaluation Engineer
> Motorola SPS, Inc.
> M/S EL536
> 2100 E. Elliot Road
> Tempe, AZ 85284
> (480)413-8052 Voice
> (480)413-8690 FAX
> m.randol@xxxxxxxxxxxx
>
> -----Original Message-----
> From: Ben Rothchild [mailto:benrothchild@xxxxxxxxx]
> Sent: Thursday, January 31, 2002 5:05 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] AC coupling placement
>
> I have a question on AC coupling.
> While AC coupling a high speed serial
> link to match levels, where should
> the AC coupling capacitor be placed.
> Close to transmitter/close to receiver/
> or doesn't matter, can be placed
> anywhere on the link.
>
> What should be the value of the AC
> coupling capacitor or rather how
> do I calculate it. Is it dependent on
> frequency. I am trying to AC couple
> signals of 155M, 622M, 1.25G and 2.5G.
> Any special care to be taken at these
> different frequencies.
>
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