[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
Wed Jan 23 13:03:23 EST 2013


On 23 January 2013 15:22, Jim Lux <jimlux at earthlink.net> wrote:
> On 1/23/13 6:48 AM, David Kirkby wrote:>>
>> A student needs to find the open-circuit fringing capacitance of a
>> piece of microstrip line. For this he needs to know the time between
>> the reference plane of the SMA connector and the open circuit
>> microstrip.
>
>
> Can he build multiple microstrips with known distances, and measure them all
> and solve for it that way?

I'm not sure how that would help. The basic problem is that there is
some discontinuity of impedance between the microstrip and SMA
connector. The open reflects 99% of the incident power, and that
discontinuity then causes the power to go back to the open. So it
seems to me one needs some form of time-gating. Also there's the issue
of how reproducible it would be to solder SMA connectors to a board.

>> An obvious way to do this is with a vector network analyzer with a
>> time-domain option. In HP/Agilent VNAs, this is option 010. The
>> student has access to an obsolete and unsuported HP 8753C 6 GHz VNA,
>> but it does not have the time-domain option.
>
>
> Does it have the "measurement plane offset" option?

Yes - it is called "port extensions" in HP language.
> I'm not in front of my
> old analyzer in the lab (8720C.. no disk drive), and it does have the
> transform option, but I'm not sure you need that option to have the "dial
> an offset" in the calibration.  I've used that to move the reference plane
> from the connectors at the edge of a board to the device on the board:
> testing a vector modulator with the display in polar coordinate mode, for
> instance.. you keep turning the knob til the display is a dot, not a circle

That is not really the right way to do it, since there will be some
fringing capacitance. You have not removed that - just appeared to
have done, but have faked that by setting the offset delay. You have
chosen an offset delay of less than the true value.

But to a first degree, delay in the transmission line and delay caused
by the capacitance are similar. See: "



> What measurement uncertainty is called for here?

It's an undergraduate project. I don't suppose he has been given such data.

> Can you make your own
> limited purpose cal kit? The open is the challenge.  Shorts, loads, and
> thrus are fairly straightforward.

The idea of his project is to characterise devices - I'm guessing
surface mount. For this he needs a calibrated microstrip line. So the
whole point is to characterise this.

As you say, the open is the challenge. Knowing what the length to the
open is, you an dial that into a VNA as a port extension, then read
off the capacitance of the open since you have moved the reference
plane to there. However, if you chose a different value of port
extension, you will read a different value of capacitance.

Also the fringing capacitance is frequency dependant - it is not a
constant, though it is quite close to being a constant. I believe he
has arrived at a offset length, but it has been pointed out he had
done this the wrong way. One really needs the TDR option. The
procedure would be:

* Use the TDR, which is basically an inverse Fourier Transform.
* Put a gate around the reflection from the open.
* Transform the data in the gate back the the frequency domain. This
allows one to look at the frequency domain response of the open,
whilst ignoring that due to other discontinuity.

But of course to do this one needs sufficient discrimination in time,
to look at just the open, and not anything else. His VNA does not have
the frequency response to do that, even if he gets the TDR option.

He has arrived at an offset delay, using a method similar to what you
described. But it has pointed out to him that the idea is not to make
the open look a spot, but to determine what the capacitance is. In
that case, the open does not look like a spot.

In fact, on my 3.5 mm kit, the open actually becomes a short at one
frequency, but then the short has become an open. I see about 200
degree of phase shift of both the open and short over the range 50 MHz
to 9 GHz. It does not matter, as long as the phase of the open and
short remain around 180 degrees apart.

Once he arrived at an offset, he moved the reference plane there, then
measured the capacitance as a function of frequency. He then fitted
that to the equation of the form

C = C0 10^-15 + C1 10^-27 f + C2 10^-26 f^2 + C3 10^-45 f^3.

Once he tried to enter C0, C1, C2 and C3 into the VNA, as a
user-defined calibration kit, he was unable to do this, as the
constants are too large.

I think he has got the offset wrong, so he has moved the reference
plane to the wrong place, so the constants are incorrect and unusable.

> Maybe you have to collect the uncalibrated
> data with the cal standards and do the "cal" in post processing in Matlab or
> something.

I think his basic issue is that the time domain discrimination is too
large. I rather suspect an Agilent employee might have given him the
code to enable the TDR, but Joel Dunsmore has determined the TDR would
be no use to him, due to the 6 GHz upper limit on his VNA.

Joel has said he needs a time resolution of about 100 ps, which means
using a 20 GHz VNA. This got me wondering if there was any way a
time-interval counter could be used instead of the VNA to find the
offset to the open.

I think his best option might be to make the PCB longer, so the delay
is sufficient to making time-gating practical. Then I expect some kind
sole from Agilent might give him the code to enable the TDR option.

Another option he has, which I think might be his best one, is to
assume a fringing capacitance of 50 fF. He has been told it will very
close to 50 fF. If the port extension was then close to show a
capacitance of 50 fF, he knows what the offset delay of the open is.
Then using this cal kit, he would set:

C0=50, C1=C2=C3=0.

Since he originally got a C0 of about 100 fF, I think he is well out
on the offset delay.

I think given the limitation of his equipment, that might be the best
he can do. But if a time-interval counter could do better, it would be
worth trying.

I must admit I can't see how a TIC could help, but I thought I'd ask.

Dave


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