[SI-LIST] Re: AC coupling placement

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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