FreeLists / si-list / [SI-LIST] Re: AC coupling placement

[Randol Mark-ryvw50 <M.Randol@xxxxxxxxxxxx>]

Oh no, here come all the Out of Office Auto responders again.  :(  Guess I'll 
see if the email filters work.

Quick and dirty method to measure a capacitor's series resonant frequency.

THRU:
1a) Get a PCB with a transmission line that can have a connector attached to 
each side, without any components needed to complete the connection.  I cut a 
board so that I can use an edge connector on both ends of a resulting trace 
with a known impedance.  Mount the edge connectors.  

One Port:
1b) Alternatively, put a connector on one end and terminate the other end to 
ground at the characteristic impedance of the line.

2) Connect the cap in question between the trace and ground.  Co-planar works 
well for this, but microstrip can work if you have or can create a short path 
to the ground plane.

THRU:
3a) Sweep the structure over frequency with either a network analyzer (NA) or 
spectrum analyzer (SA)/tracking generator (or signal generator with the SA in 
trace max hold mode).  The NA should have ports 1 and 2 hooked to opposite ends 
of the structure.  Alternatively, connect the SA to one end and drive the other 
with the signal source.  Look for the suckout in the through response.

One Port:
3b) Load one end of the trace with the characteristic impedance of the line.  
Look at the reflected wave.  You'll see a large reflection at the resonant 
frequency of the cap due to the short.  The exact impedance you'll see at that 
frequency will change as the length from the connector to the short varies, but 
not the frequency.  Note that an SA won't work since you don't have a place to 
monitor just the reflected wave.  If you have a directional coupler, you could 
do it with an SA, but typically if you don't have a NA, you probably don't have 
the directional coupler around either. <?>

Once you know the frequency response of your cap, you can pick one tailored to 
your requirements.

This method seems to work reasonably well out to a couple GHz.  Purists 
probably won't like it because there are parasitics unaccounted for.  If the 
substrate you test on is similar to the one you are eventually going to use, 
the results should be a quite good representation of what to expect.

-- 
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
<mailto:m.randol@xxxxxxxxxxxx> 


> -----Original Message-----
> From: Ben Rothchild [mailto:benrothchild@xxxxxxxxx]
> Sent: Friday, February 01, 2002 7:34 AM
> 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.
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