[SI-LIST] Re: High Bandwidth SMA Cable
- From: kevin hoffmann <kvhoffmann@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, Tanveer Alam <talam@xxxxxxxxxxxx>
- Date: Wed, 7 Sep 2011 21:59:13 -0700 (PDT)
Hi Tanveer,
This is a very open question, so let me try to give you a few general comments
in response:
Bandwidth. You mention "above 17Gbps" but not the actual rate. Let's assume
18Gbps, so with a 9GHz fundamental from 0101 data, the 3rd harmonic is at
27GHz. At that frequency SMA connectors are becoming modal. You would need
3.5mm (good to ~32GHz) or 2.92mm (aka K) connectors (good to ~38GHz). The
test instruments you are using will likely have something like the 2.92mm.
Many sampling scopes use the 3.5mm connector (or 2.4mm on 50GHz bandwidth
heads, not compatible with the others discussed here). Why introduce a
mismatch by mating with an SMA? Or, if you must, many such as the Agilent
modules, will allow you to use a precision adapter with different connector
types on each side of the "barrel" connector.
Connector type and modality will affect your cable choices. As will phase
stability and the amount of loss you can tolerate. How many times will the
cables be used? Mechanical issues may need to be considered. Similarly, as
with connector mismatches, connector to cable may need to be considered. Do
you need phase matched pairs? Here, a reputable manufacturer can assist you.
Perhaps you can model and de-embed cables. Still, at some point, you need to
make sure they are not impacting the measurement you are trying to make.
One more caution on SMA connectors -- there are RF quality and commodity
connectors. Don't even think of using the commodity type at these rates.
For inexpensive, throw away SMA cables, companies like Mini Circuits and RFMW
have some great offerings. I say "throw away" because these cables are made
with a solder coated braid that will only resist a limited amount of bending.
(They are designed as a low cost replacement for semi-rigid coax.) Cost is
quite low ($10-20 ea) and they have decent RF type SMA connectors. I have
recently used some of these at up to 20Gbps.
For higher end stuff, there are a number of companies who can assist you.
Micro Coax, Rosenberger, Mega Phase, and so on. Many microwave suppliers have
offerings, and some of them are knowledgeable about the high speed digital
area. One place they can aid you is in getting a cable and connector that work
well together.
Hope that provides some guidance and good luck!
Kevin Hoffmann
--- On Wed, 9/7/11, Tanveer Alam <talam@xxxxxxxxxxxx> wrote:
From: Tanveer Alam <talam@xxxxxxxxxxxx>
Subject: [SI-LIST] High Bandwidth SMA Cable
To: si-list@xxxxxxxxxxxxx
Date: Wednesday, September 7, 2011, 11:17 AM
Hi,
I am looking for High Bandwidth SMA Cables (above 17 Gbps) for testing SerDes
on different backplanes
Please if anyone can suggest where to look for this kind SMA cables.
Thanks,
Tanveer Alam
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