If you are an experienced VNA user and are wondering what about time domain, you will
be pleased to hear that you can order Option 010, time domain, with your FieldFox microwave
analyzer, and this includes time domain gating. If you are not familiar with what time domain
and time domain gating are, there are plenty of good application notes on Agilent’s website.
The test setup I am using here is a little 20 GHz SAW filter connected to the FieldFox
via two of these purple microwave cables, so the signal from the FieldFox will come
through the first cable, through the connector join here, through the second cable, and to
the SAW filter. And then I have the other port of the SAW filter terminated with a 50-ohm
load. You will see I have set the frequency range on the FieldFox to cover its entire
range, that is 26.5 GHz in the case of this model here. And in fact, I have just done
an instrument preset. I am not even going to perform a calibration because every FieldFox
is supplied with a CalReady calibration where, at the factory, a full S-parameter calibration
has been performed at the reference plane of port 1 here. You can see here on the display
we are measuring S11. That is the return loss of the entire measurement system. So this
response here is, in fact, the return loss of both of my purple cables, the connectors,
the launch onto the filter, and the filter itself. And the first thing you will notice
is that there is quite a bit of ripple on the return loss trace. This is typically characteristic
of a poor match somewhere in the measurement system. And this is where time domain can
be incredibly useful to try and identify any point in the measurement system that is not
50 ohms and is providing a reflection to the incoming signal. So I will press Meas Setup,
Transform, and Transform On. And the FieldFox is now performing an inverse Fourier transform
of the return loss trace. We are still measuring return loss on the Y axis in dB, but on the
X axis, instead of sweeping frequency, we are now displaying time starting at 0 second
on the left hand side of the display and 3 nanoseconds on the right hand side. Now, 0
second is our time reference, and that corresponds exactly with the calibration plane when we
calibrated. Now, in this instance we are using the CalReady that came with the FieldFox,
so we know that 0 second is this point here. And as we move along the trace, we are moving
along through the measurement system until eventually we reach the filter, which is this
large response here in the middle of the display. Now, if you are wondering what all these other
bumps are on the trace, I can quickly show you. This one here I know will be from the
connector join here at port 1 on the analyzer. And this one here, which I have marked with
marker 1, is going to be this, join here, which I can quickly show you by separating
the cables. That has created an open circuit on the end of this cable, and you can see
marker 1 there is on that peak. So we know that that distance from the reference plane,
port 1, is the length of this first purple cable. I will join them together again. And
we can do the same test with where the second purple cable joins the filter, so I will just
undo that and create an open circuit again. And you can see I have marked that with marker
2. Again, the large reflection indicates the end of the cable, the open circuit. I will
put the filter back on, and finally I will do exactly the same but for the output of
the filter. Now, this is going to be harder to see, but you will see I have put a marker
here, marker 3, on the bump indicating the open circuit at the end of the filter. Now,
you can see, in fact, this filter physically is quite short, but electrically it is quite
long, presumably because it is a SAW filter of some type. We will put the termination
back on the filter. So there are a few things we can learn from looking at the time domain
trace. First of all, the largest response is indeed the filter, but maybe when we were
looking at S11 return loss, that trace was being affected by these other discontinuities
on the trace. Perhaps they were causing the ripple. And now that we know the cause of
these discontinuities, we might go away and perhaps improve the cables or the connectors
or improve the design. But the FieldFox has an especially useful function called time
domain gating. What time domain gating does – if I just turn it on -- where these blue
markers are, it keeps the trace, but to the left and to the right of the markers, it artificially
sets the trace data to 0. In other words, it has deleted from the time domain response
the non-50 ohm discontinuity effects that we do not want to measure when we are measuring
S11, in other words, the cable, the connectors, adapters, etc. In fact, I have even trimmed
the markers in close enough to negate the effect of where the APC 3.5 millimeter connector
joins the SMA female on the board. So we have removed the effects of the launch onto the
substrate as well. So if I now turn off the inverse Fourier transform, but I will leave
the gating on, you will see we are back in frequency domain again, but much of the ripple
on the trace has now disappeared. In fact, the Gating On/Off button is still available
even in frequency domain. So I will show you what is the response with the gating turned
off, and you can see the ripple here caused by multiple reflections from the non-50 ohm
discontinuities in the measurement path. If I put the gating back on, it smoothes all
of that out and now we are truly measuring the return loss response of just the filter
and not the rest of the measurement system. Time domain and time domain gating provide
a tremendous insight into what is going on in your RF system. In this case we are able
to remove the effect of a few cables and joiners, which to be honest, we could have removed
anyway by calibrating at this point here. But when out onsite, maybe this was a poor
quality bulkhead connector, we would easily be able to identify that perhaps we could
not see the antenna or the filter at the far end because of the poor response from this
bulkhead connector. If we were only looking in the frequency domain, we would never be
able to tell, but by going into the time domain, we can see these discontinuities. There is
another valuable lesson we can benefit from by having time domain on our FieldFox analyzer,
and that is the ability to measure good quality or poor quality adapters and connectors. Where
these two purple cables join, I am going to insert some $30 adapters that I bought. They
are a pair of type N-to-SMA connected back-to-back. So with the adapters now inserted, you can
see this huge reflection from the poor match of these adapters. And in fact, if I go back
to the frequency domain by turning the time domain transform off, you could hardly even
see the response of the filter. In fact, you probably would not know the filter was there.
Even if I turn time domain gating on, it still makes very little difference. We still cannot
see the filter because there is almost no signal reaching it. It is all being reflected
back by these adapters. So whenever you are doing network analysis or any microwave measurements,
just spend a little more and get a decent quality set of adapters. And of course as
with every measurement on the Agilent FieldFox, we can save the measurement results as a PNG
or trace file to the internal memory, an SD card, or a USB memory stick along with the
complete instrument state. For further information on the FieldFox microwave analyzers, application
notes on time domain measurements, or further information on vector network analysis, please
contact your local representative or visit the website shown below.