[SI-LIST] Re: VNA Probing Solution

This is how we do most of our VNA measurements, including 4-port
mixed-mode.
 
Wafer probes (we now usually call them microprobes), are now available
in many different styles that are well-suited for PCB applications.
These include different access angles and lengths (to allow probing on
populated boards), as well as Ground-Signal separation (pitch).  The
largest standard pitch that you will be able to purchase is in the
neighborhood of 1250u (about 50 mils).  Although you can order probes in
a wider pitch, your frequency range will limit the probe pitch that you
can use.  For calibrating with a 2-port VNA, using the Short, Open, Load
and Thru (SOLT) technique to 15-20 GHz, we normally use probes that have
a maximum pitch of around 650u to 1000u.  A wide range, yes, but the
ability to calibrate accurately is dependent on many factors including
operator skill, condition of the probes and cal substrate as well as the
instrument accuracy (including cables and adapters).

We often must measure boards that have not been designed for these kinds
of probe-based measurements, and frequently have to "modify" the boards
to accommodate the probes.  This sometimes involves soldering copper
tape to boards, "painting" with silver-filled epoxy or drilling down to
a ground plane.  Imagination is required.  Of course, this kind of "test
tooling" will modify the interconnect measurement, and must be taken
into account when evaluating the data.  De-embedding the added launch
parasitics at 20 GHz will almost certainly be necessary.  However, we
long ago concluded that the accuracy and flexibility of the probe-based
measurement system is worth the additional work for these high frequency
PCB measurements.

Our PCB Probe Station was created to meet this kind of VNA measurement
requirement.  You can learn more about it on our website, at
www.gigatest.com

Good Luck.

-- Gary Otonari
   GigaTest Labs

------------------------------------------------------------------------
------------
Attend the GTL Signal Integrity University in Sunnyvale, CA  -  November
8-16 
 
GTL 122 - Fundamental Principles of Signal Integrity
GTL 250 - High Speed Board Design
 
GigaTest Labs    
134 South Wolfe Road   *   Sunnyvale,  CA    *     94086
408.524.2700
www.gigatest.com
 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of pritchard, jason
Sent: Tuesday, October 19, 2004 8:37 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] VNA Probing Solution

Hi All,
I am looking for a probing solution to take s-parameter measurements out
to
10-15Ghz. These measurements are on boards that are currently developed
and
don't have anyway of connecting an SMA to the board. I've looked at
wafer
probe manufacturers offerings and they seem to be accurate to at least
20Ghz
depending on grounding configuration and probe style. The pitfalls are
that
the probes are pitch specific and too fragile to probe by hand. They
also
are fairly expensive. 

My connections vary from board to board. The ground connections points
can
very signifigantly in distance. Signal pitch can be 1mm under an ASIC to
2mm
apart at a connector or board via. Currently I use an sma connected to a
semi-rigid coax with a wire from the ground shield to my ground point on
the
board. This method seems to be accurate to 3 or 4 Ghz if I get a close
ground. It's my understanding that I can only get accurate measurements
if I
get the ground as close as 1/50th of my maximum frequency. Is this too
conservative? 

Does anyone have any better Ideas on how I can probe boards and get good
results out to 10Ghz? Is this even possible? 

Thanks,
Jason Pritchard


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