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  • Author Author: scottiebabe
  • Date Created: 2 Sep 2021 6:31 PM Date Created
  • Views 2770 views
  • Likes 12 likes
  • Comments 13 comments
  • radiocontrolch
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EM Surfer

scottiebabe
scottiebabe
2 Sep 2021

Even faster than the seasons appear to be changing another month's Project14 event is upon us.image

 

Hopefully, the Element14 team will consider fast edges to be an RF topic.

 

I have really enjoyed participating in Project14 events, as its allowed me to explore topics that I don't have any direct experience in with the support and advise of the Element14 community.

 

For this month's project, I will attempt to try and perform fluid level sensing with Time Domain Reflectometry (TDR).

 

I post this project as a basic outline presently for feedback or suggestions.

 

Some of the project graphics put together thus far, that I will cleanup in the coming days:

 

image

 

Measurement Coaxial Line

Technically all that is needed is a waveguide, but this would require trying to design, fabricate and match a launcher to a waveguide.

 

So, to keep things "simple" (the fast edges are making this project complex enough) I will construct porous air-core coaxial line out of a piece of copper plumbing pipe with a wire pulled taught down the center.

 

The length of the measurement coax is completely arbitrary, I just used what I had lying around. Though if you are intending to scope the TDR waveforms with an entry-level DSO like me, I would suggest the longer the better.

image

 

 

In hindsight taking a trip to the hardware store to source some copper pipe caps would be far easier as you could solder them on with a blow torch. Hand soldering FR4 coupons into the pipe ends was quite tedious and slow. I had to let my soldering iron preheat the pipe for at least a minute before I could get the smallest amount of solder to flow near the iron tip (the copper pipe is one big heatsink).

image

imageimage

 

The 24AWG wire is soldered to the BNC connector then the wire is feedthrough the copper tube. The BNC connector is held in place with tension from the center conductor. I'm certain those more mechanically competent than I could find a better mounting solution, perhaps taping a mounting hole or soldering a brass jam nut. The 24 AWG wire is pulled taught and then soldered at the end. The oversized hole was the result of trying to drill press machine the mounting plugs, which did not work as well as I had hoped. However, it is also large enough to allow water to ingress into the coax line.

 

Now that I have the actual dimensions, I can calculate the characteristic impedance and the velocity factor along the coaxial line.

image

I calculated the reflection coefficient on each impedance boundary transitioning from Za to Zb shown in the table below.

image

 

When you look at the TDR response on an oscilloscope there is a lot going on and I was having a hard time keeping track of everything off the top of my head. So I constructed a TDR lattice diagram for the first 4 significant reflections.

image

For measurement purposes we really only care about timing V2 and V6. However, V12 is the pulse I eagerly wish to see on my oscilloscope, as not only is it a TDR pulse through water, it should propagate at almost 1/9 the speed of light.

 

I calculated each reflection with first incident TDR pulse normalized to an amplitude of 1, the results are shown in the table below:

image

 

 

Spice Simulation

 

image

image

 

Replotting all the Spice parameter step results as a gif:

image

 

Scope Measurements

I was able to dig up a TDR pulser I constructed a few years ago on copper clad:

image

 

imageimage

 

I connected the TDR pulser up to my oscilloscope as shown in the Ascii diagram:

                     Scope

TDR Pulser - RG58 -> _|_ -> RG58 -> (Open/Short/Load)

 

image

 

The roundtrip TOF on 3 ft (6 ft effective) of RG58 was approximately 9.5 ns. This results in a measured propagation velocity of 1.58 ns/ft which is only 2 percent off the quoted specification of 1.54 ns/ft. Equivalently, this discrepancy is equivalent to an extra 1.5 in of line length which is comparable to the length added by the BNC-T and BNC-FF coupler. All in all, I think that is pretty awesome, to see signals propagate at 2/3 the speed of light on my oscilloscope. With a small pulse duration it is easier to discern on a scope plot that the second pulse waveform is the actual reflected TEM wave (so awesome!). For the digital readout phase of the project I will launch much longer duration pulse from a GPIO pin and study the reflections from the rising edge.

 

Now its time to make some measurements on the copper pipe measurement coax.

 

First Launch!

After many days of great suspense wondering if this is even going to work at all, we have first launch! I connected the tdr pulser to the measurement coax and launched the first TDR pulse into the DUT.

 

image

Playing at the speed of light, this is too much fun! We expect the roundtrip TOF along a 1.15 m distance (2.3 m roundtrip) travelling at the speed of light (velocity factor of 100%) to 7.7 ns.

 

image

The size of this fluid level measurement coax is in scale with something the size of rain barrel, which I don't keep in close proximity to my oscilloscope. So, this first water launch test was conducted by placing the end of the copper pipe into a 2 L water pitcher and recording the ensuing TDR waveform. The water level on the copper pipe was approximately 22 cm. With water now present in the measurement coax we have reduced the roundtrip air distance from 2.3 m to 1.86 m. Ideally, we should see the water reflection advance 1.4 ns ahead of the shorted end reflection.

image

Considering this is a time difference of 1 ADC sample period on my oscilloscope, I am beyond thrilled with how well I was able to observe a TDR pulse reflection off of water image.

 

The roundtrip TOF through 22 cm of water is 13.1 ns. This would be the second pulse after the water reflection which happens to coincide with 2 roundtrips in the air portion of the coax. We are likely seeing a small amount of a TDR pulse transmit through water, though nothing conclusive just yet. I need to place this in a much larger water column to clearly discern between the 2 reflections. Chasing this reflection is purely for science, the digital readout will only need to time the difference between first and second reflection.

 

Digital Readout

I recently splurged, and redeemed past Project14 shopping carts on some cables, fast comparators, and a time to digital converter IC.

 

I have high hopes these components are up to to the challenge of making TDR measurements with picosecond resolution:

  • TI TDC7200 Time to Digital Converter, 2 V-3.6 V supply, TSSOP-14Time to Digital Converter, 2 V-3.6 V supply, TSSOP-14
  • MAX999 Comparator Analog Comparator, Beyond-the-Rails, High Speed, 1 Comparator, 2.7V to 5.5V, SOT-23, 5 PinsAnalog Comparator, Beyond-the-Rails, High Speed, 1 Comparator, 2.7V to 5.5V, SOT-23, 5 Pins

 

 

More to come...

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Top Comments

  • dougw
    dougw over 4 years ago +4
    Cool idea. We did a project once where we started out trying to use TDR over a pretty long distance but there were too many spurious reflections, so we switched to measuring the resonant frequency of the…
  • javagoza
    javagoza over 4 years ago +4
    I remember using TDR in the 80's. I was working installing ethernet networks with 10Base2 coaxial cable and thick yellow 10BASE5 ethernet cable. There was a crisis in my country and often some workers…
  • colporteur
    colporteur over 4 years ago +4
    It has been twenty five years since I tuned the knobs on a TDR. At one point I knew the vertical propagation delay of LDF450 coax cable from memory. My last exercise was to map 20 different models of VHF…
  • scottiebabe
    scottiebabe over 4 years ago in reply to javagoza

    It truly is! I feel like I am unlocking new measurement superpowers with this project working at the picosecond level lol

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  • scottiebabe
    scottiebabe over 4 years ago in reply to Jan Cumps

    That is a wonderful Roadtest! There appears to be many useful hints related to the TDC7200, thank you.

     

    Interestingly, a Microchip Employee prototyped something similar with the CTMU peripheral available on many PICs: https://www.edn.com/use-time-domain-reflectometry-tdr-for-low-cost-liquid-level-measurement-part-iii/  though I am skeptical of the 3.5 ps resolution claim, I suspect that is the result of significant oversampling and averaging.

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  • javagoza
    javagoza over 4 years ago

    Nice update. What a good feeling when you start seeing results.image

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  • Jan Cumps
    Jan Cumps over 4 years ago

    I worked with the TDC you're mentioning at the end of the post: TI Ultrasonic Sensor - SPI Traffic Snooping

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  • DAB
    DAB over 4 years ago

    Interesting project.

     

    DAB

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