TDR

I've done both frequency domain and time domain measurements and you can easily use Fourier transforms to get from one to the other after the fact. Invariably, I found that HP8753D VNA measurements followed by an inverse Fourier transform were superior in terms of S/N.

My TDR was a Tektronix S-6 sampler and an S-52 pulser, which have a slight edge in terms of bandwidth. If I averaged >10k measurments, the S/N approached that of the HP8753D, but a lot of patience was needed.

In the end, sometimes one is better, sometimes the other.

Jeroen Belleman

Sure. Several Tek SD-24s and an 11801C mainframe (our 11802 is still sick).

We also license a low cost, miniature, sub-100ps TDR of our own.

TDR gives you good info on the nearest discontinuity and less good info on succeeding ones. (How much less depends on what the nearby one does to the pulse edge.)

Cheers

Phil Hobbs

A long time ago I was helping with the measurement of concert hall acoustics. I made a very powerful portable spark generator which produced peak sound pressure levels of around 140dB spl at 3m range. The impulse response of the hall was recorded with a high quality cassette recorder. The results were disappointing, even with averaging of hundreds of impulses. The problem was that a huge dynamic range was needed with this method. In contrast, playing a pseudo- random noise signal made much better use of the available dynamic range. I think this maps fairly directly to the VNA vs TDR question. John

To this I would add one comment:

VNAs have fairly narrow Receive and Video Bandwidths, often a few MHz, so the smallest discontinuity must be tens of meters long.

By contrast, a TDR has a bandwidth in the GHz, so far shorter discontinuities may be detected.

Joe

Yeah, I wasn't trying to imply that these two pieces of test equipment could be used the 'other way round' as it were. It would never occur to me to ever try using a VNA for DTF type measurments.

No

Okay - now you've got me intrigued. I have a spark generator here which I keep unassembled unless needed. It provides pretty spectacular and intimidating 60kV sparks with an alarmingly loud crackle. But not remotely as noisy as yours. I have to ask - what the hell kind of voltage were you generating??

This is well worth a read, Phil:

About 3kV at over 1kA. It was a large photoflash capacitor about the size of a shoe box with oil impregnated paper dielectric salvaged from a huge ruby laser power supply. It could be completely discharged in about 10us. I built a 3-electrode triggered spark gap with tungsten rod electrodes which eroded fairly quickly. Two electrodes were connected to the terminals of the capacitor with short brass rods and the third was connected to a motorcycle ignition coil with a thyristor capacitor discharge to ionise the main gap. The sound level was measured with a Bruel & Kjaer peak holding sound level meter using a half inch microphone. Thinking back, it was probably only 120dB to 130dB spl pk at 3m. Still VERY loud. John

No, no, it doesn't work like that. You use the VNA to acquire the S11 frequency response over its full bandwidth. You then apply an inverse Fourier transform to the complex S11 frequency data to obtain the impulse response. Then you apply a time integration to finally get the TDR plot with an equivalent bandwidth of the full sweep of the VNA.

Note that the VNA has a directional coupler, so you'll get only the reflection, not the incident *plus* the reflection like a TDR. The nice thing is that with proper VNA calibration, the reflection will start exactly at time zero and many setup parasitics are suppressed.

I could resolve centimeter-sized discontinuities with ease with S11 data from the HP8753D sweeping up to 6GHz.

Jeroen Belleman

Hmm. The people I was watching were not doing anything like the above. How long does data collection take? TDRs are fast.

Joe

Very interesting; many thanks.

The VNA sweeps the whole range in a second or so. The HP5387D has a wimpy CPU, so it takes a few seconds to do the transform. I also did the transform after transferring the data to my PC, which is more laborious, but the actual transform is then basically instantaneous. (I did not time it.)

My TDR was a Tektronix 7904, with 7T11 and 7S11 plug-ins driven by a VME DAC and read out with a VME ADC. That was pretty slow, especially when I averaged measurements to improve the S/N.

Jeroen Belleman

In the early 60s our school's "Scientific Society" used to visit interesting establishments (such as the NPL). One such place was (I think, but it was well over 60 years ago), to Queen Mary College in East London to its Electrical Engineering department. They tested insulators, lightning conductor design, and related materials. They had a Cockcroft-Walton generator feeding a bank of capacitors. When fully charged, 2MV was available (if memory serves me correctly), at goodness knows what current. It jumped a foot or two without problem when discharged.

Standing barely 50 feet away, in a room with smooth, reflective surfaces, I can still remember my ears ringing for many minutes after the "lightning conductor" test! In those days there was no elf'n'safety ear protection...

I had a school trip to QMC as well. There was also a very large 1MV Marx generator at the Lpndon Science Museum. Visitors of a nervous disposition were advised to keep away when it was fired. Another spectacularly noisy demonstration was a hypersonic wind tunnel, at NPL in Teddington. A colleague once visited an establishment equipped with a flash X-ray generator which produced such short and high energy pulses that it could image the detonation of artillery shells as they impacted a target. John

Hmm. I wonder how fast a modern Rohde-Schwarz VNA can do this. They are quite expensive, and likely among the best one can buy.

Joe

When I was younger (when wasn't I?) I worked for a geophysical outfit. We'd often do inshore marine surveys.

There were two big 19" rack boxes, maybe 500mm high. One contained a very large transformer to convert 240Vac to 5kV IIRC and the other box contained a whole load of brick-sized oil filled paper capacitors and a spark gap, triggered by an insulated tungsten wire.

One of the boxes also had a bridge rectifier and the whole lot was powered from a big petrol generator.

There were two methods. One was to use a 'sparker' trailing behind the boat, and the other was a 'boomer' comprising two separated aluminium disks. Christ it was dangerous. The gear was old, the boats were just trawlers or similar and the sea is wet.

Interesting. I have the 'C' suffix version of your VNA. Can you tell me whereabouts the fourier transforms options are to be found buried in the menu options? I've never come across them. Might not be in exactly the same place, but shouldn't be too far removed....

It's an option. If you have it, there should be a 'TRANSFORM MENU' item on the screen after pushing the "SYSTEM' button.

Jeroen Belleman

If you don't have the option installed, it looks like you can install it for free: