[time-nuts] Is there any way to use a TIC to measure time of > reflection on a PCB?

David Kirkby david.kirkby at onetel.net
Thu Jan 24 09:05:47 EST 2013


On 24 January 2013 11:32, Grant Hodgson <grant at ghengineering.co.uk> wrote:
> Dave

Hi Grant

> Couple of thoughts here :-
>
> 1)  A 'real' TDR measurement would require a pulse generator with a fast
> rise/fall time.  The faster the rise/fall, the better.

Yes, I suspect it needs a pretty decent TDR for this.

> A directional
> coupler could be used to separate the forward and reflected signals, or you
> may be able to get away with a 3dB splitter and use the forward pulse to
> start the TIC and the reflected pulse to stop it.  However - this will only
> give you a measurement of the propagation time of the pulse - using the VNA
> with a short at the end of the microstrip (as has already been described)
> would give a much better result.

I think the problem is you can't get a short at the open end of a
microstrip. One could try one's best, but there will always be some
connection length to the short which will be different to the open.

> So yes, you could use a TIC, but you would need a pulse generator and you
> wouldn't have a better measurement than that already.  Actually, a fast
> 'scope would be better than a TIC as you would be able to see the shape of
> the reflected pulse, which is somewhat more useful than a simple time
> measurement.

Yes.

> 2)  Given that the student doesn't have the TDR option on his VNA, it would
> be possible to simulate this using Matlab, Mathcad or one of the other
> analysis packages which are now freely available.  The formatted data (in
> the frequency domain) from the VNA can be read via GPIB, inverse FFT'ed,
> time-gated and FFT'ed back into the frequency domain.  Excellent learning
> task for a keen student.

I think his basic problem, even if he got the TDR option added, is the
resolution in the time domain would not be good enough.

Joel Dunsmore, who works for Agilent on VNA design, has written a book
about them, and worked on worked on the TDR for his Ph.D., has said
thje student needs a 20 GHz VNA. I'm sure Joel could give him the TDR
option on the 8753C, and might be persuaded to do it if he felt it
would be any good to the student, but Joel has stated the HP 8753C VNA
will give insufficient resolution in the time domain. A 20 GHz or
higher VNA is needed. (See the link to  the thread on the Agilent
forum I first gave if you want to see the discussoins on this).

I originally offered to make the measurements on my VNA, as it does
have the TDR option. But when Joel said he needs a 20 GHz VNA, that
rules me out, as I don't have a 20 GHz cal kit, even though the VNA
will go to 20 GHz. My 3.5 mm cal kit is limited s to 9 GHz.

I should soon have an 18 GHz cal kit in APC-7, but that does not
really help. The Agilent 85052B 26.5 GHz 3.5 mm cal kit is $12287 and
the "economy"  version, the 85052D, is $6432. Even on eBay, these kits
go for a lot of money. And don't think about N connector kits, as they
are even more expensive.

I paid $16,000 for a used GHz VNA, but the 26.5 GHz 3.5 mm cal kit is
nearly as much, and the 18 GHz N cal kit is even more expensive than
what I paid for a used VNA.

I was hoping there might have been some way with an HP time interval
counter, but it appears there is not.

Dave


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