Skip to main content

So in our club we noticed that some days a given engine runs fine, and other days it has TMCC signal strength problems, although when we measure the 455KHz signal voltage at the track (relative to building ground) it's always the same to like 2 decimal places, and the other layout conditions have not changed. When we put in the booster from @gunrunnerjohn most of these issues went away, but I still wanted to understand what was causing this quantitatively. Our club is by the ocean and we get large swings in humidity so I've continuously suspected maybe the weather had something to do with it (specifically the humidity) since TMCC/Legacy is capacitively coupled so I designed the experiment described below to answer that question definitively.



The experiment setup has the TMCC train in a sealed container with a water vapor inlet. The track is hooked up to 18V power and the legacy base. We put a strip of foil tape on the side wall of the container near the train connected to an oscilloscope to estimate how well it's coupling/radiating (quantified by pk-pk voltage of the TMCC carrier). With a humidity sensor in the box we slowly vary the humidity with pulses of injected water vapor and letting it settle for a minute or two at each step.

setup

From the results we see the capacitive coupling basically doubles between the train and the foil strip when the humidity is high. From the curve taken based on the humidity sensor, we also see that this only becomes a problem when the environment is very dry (<20% humidity) and that from 50% to 100% humidity there is very little change, only 2 or 3mV in 50mV of coupling.

results

These results are sort of common sense. If you think about a parallel plate capacitor then C = EoEr A/D and the relative permittivity of air Er = 1. If you start humidifying that air, Er goes up, so C goes up, so the amount of coupling from train to building goes up.  Anyways a fun little experiment to prove out something we probably all already know and thought about.

Attachments

Images (2)
  • setup
  • results
Videos (1)
vapor_video
Original Post

Replies sorted oldest to newest

@Adrian! posted:

So in our club we noticed that some days a given engine runs fine, and other days it has TMCC signal strength problems, although when we measure the 455KHz signal voltage at the track (relative to building ground) it's always the same to like 2 decimal places, and the other layout conditions have not changed. When we put in the booster from @gunrunnerjohn most of these issues went away, but I still wanted to understand what was causing this quantitatively. Our club is by the ocean and we get large swings in humidity so I've continuously suspected maybe the weather had something to do with it (specifically the humidity) since TMCC/Legacy is capacitively coupled so I designed the experiment described below to answer that question definitively.

Interesting observations.  As you say, something that certainly makes sense, but to actually demonstrate it is very illuminating.

Yes, I think it is the lack of humidity that causes an issue. I can tell when the seasons change because this time of year the humidity in my basement drops into the 30's and I have to take that static shock every time I touch the un-powered rails. When we get into winter it is not uncommon to see humidity drop into the 20's.

I'll be adding Legacy over the next year and it is good to start picking up these notes. I'll have to start another page in my notebook along with my DCS notes.

I did see the chart of the voltage measure on the foil. It increases as the humidity increases. Perhaps I'm reading something into this but it seems to me this is indicating an increase in transmitted power. I don't think it is any secret that RF signals propagate farther when it's cloudy. In my basement the build up of static is an indication of low relative humidity, usually below 40%. Realistically speaking I doubt I will have this problem with a layout of 22 x 10.

@MartyE posted:

This doesn't surprise me.  We have had folks report in the past that after extended dry periods that had issues with the TMCC signal.  Everything works great and then they try to operate trains in the long hot dry summer and some issues show up.

We had a very long dry summer here but the humidity doesn't drop all that much.

Well.... it's not RF propagation, that's for sure. At 455 KHz the wavelength is 2160 ft so unless the locomotive antenna is that long, it's going to be dominated by E-field capacitive coupling modes. "Signal" is the AC current flowing through through the locomotive, and through the capacitance formed between the engine and the building.

It's like when you put your hand over a TMCC locomotive that is struggling with signal and suddenly it starts to work. Your hand is like the second plate (building side) so you're lowering the D (distance between plates) parameter in the capacitance C=EoEr A/D. The moisture is taking the place of your hand, increasing the capacitance through the Er parameter instead of the D parameter.

Last edited by Adrian!
@Adrian! posted:

Well.... it's not RF propagation, that's for sure. At 455 KHz the wavelength is 2160 ft so unless the locomotive antenna is that long, it's going to be dominated by E-field capacitive coupling modes. "Signal" is the AC current flowing through through the locomotive, and through the capacitance formed between the engine and the building.

It's like when you put your hand over a TMCC locomotive that is struggling with signal and suddenly it starts to work. Your hand is like the second plate (building side) so you're lowering the D (distance between plates) parameter in the capacitance C=EoEr A/D. The moisture is taking the place of your hand, increasing the capacitance through the Er parameter instead of the D parameter.

Thank you for that explanation Adrain! I understand your example of how the capacitance is changing because the dielectric medium is changing. I have a lot more studying to do on how TMCC/Legacy works.

"Signal" is the AC current flowing through through the locomotive, and through the capacitance formed between the engine and the building."

This just brings up more basic questions in my mind so I'll go research this for myself.

@Adrian! posted:

At AGHR we did a board around @gunrunnerjohn's booster that takes this effect into account. We operate in closed loop and have a microcontroller tweak a digital trimpot feeding the booster. The microcontroller senses voltage on a strip of metal nearby to the layout via some mini-circuits amp and an envelope detector.booster

Adrian - Can you talk more about the benefits of the added elements you add to the GRJ booster.  What improvements did it add to running TMCC engines?

thanks,

Bob D

@rad400 posted:

Adrian - Can you talk more about the benefits of the added elements you add to the GRJ booster.  What improvements did it add to running TMCC engines?

thanks,

Bob D

So I'm legit in a zoom meeting about landing a radar on the mars and it's soooo boring. We're arguing over how many hinges we can afford mass on for the radar antenna to fold up and if the gain and side-lobe levels on the beam pattern will change 0.1 dB or 0.15 dB. Im trying so hard to stop from going zzzzzzzzzz

Anyways, let me instead write this out for you....

Basically our train club runs everything, DCS, Lionel TMCC/Legacy, Lion Chief and a couple of other formats we developed in house (RS232 over track). As you probably saw in my other posts, generally speaking when you turn the TMCC carrier up too much, it starts to interfere with the DCS packets. Ideally this wouldn't happen as one is differential mode (DCS) and one is common mode (TMCC), but in practice there is imbalance so you get some mode conversion. The most notable imbalance being the ground is 2 rails and the center rail is 1 rail so even at best the intrinsic capacitance is 2:1 between the two nodes. I posted a lot about these issues on here in the past.

When we first put in GRJ's booster, it was too much (it's a meaty amplifier he's got in there) so our DCS response was basically toast. We first  put a simple trimpot between the booster input and the base (basically a level knob) that we could turn up and down to control the TMCC signaling voltage. The idea is we would turn it just enough so that our TMCC trains would run, but not any further, so DCS was minimally impacted. This sort of worked except we found we needed very different settings on the trimpot on different days so it needed continuous adjustment.

This makes complete sense.... lionel signalling isn't just about the train or the track or the base. The entire loop of circuit is the base to the track, the track to the train, the train through the air capacitively to the building gnd and the building gnd back to the base. So even if the TMCC voltage is exactly the same on the track every day, that doesn't mean the signal strength is the same every day because the capacitive-air coupled leg of the loop is changing with weather, humidity and so on.

So what we do now is we have a sense or pickup near the layout, and we digitally adjust the GRJ's booster input voltage in closed loop, to make sure that the amount of signal we are picking up capacitively stays the same. So when the humidity goes up the booster is turned down and when the humidity goes down, the booster is turned up. That way we don't need to manually adjust it, but still keep the TMCC signal as low as it can acceptably go to minimize the impact on DCS operations.

Last edited by Adrian!

Cool, I was wondering what the feedback loop was supposed to do, it's all really clear now.  Very clever, I wonder if the guys at NJ-HR can use something like this.  I see Bob is very attentively tuned in, perhaps you could share the exact part numbers and the code you used?

We went through a few iterations so the various sections are broken out over a few different boards, one for the trimpot, and one for the detection circuitry.

The gain stage we use off the pickup itself is this one: ZFL500LN which just runs off 15VDC. The digital trimpot was an AD5290YRMZ10. The control opamp isn't physical that was done on the microcontroller in software. We just used an arduino nano to read the level on the ADC and tune the trimpot until the ADC input matches a constant in the code (setpoint =120; )

Here's a screenshot of the trimpot Board in diptrace (schematic attached). This one is just the arduino with some connectors for the ADC input, some filtering some scaling and the trimpot itself.

trim_board

Detector Board in diptrace (schematic attached). This is a differential envelope detector with a gain stage that goes into the arduino ADC port. Not much here except you need the nifty power supply module to get the +15/-15V for the opamp from a single supply input. The diodes are just 1n4148 signal diodes. I like the LM358N opamp since the input referred noise is low but you can go with whatever.

dboard

Honestly there's a billion ways you could implement this, and the above setup is probably not the best, it's just what we arrived after a few iterations.

Attachments

Last edited by Adrian!

Thanks for explanation Adrain.  When this discussion first came up of the TMCC buffer signal stepping on the DCS signal, I took a scope measurement at several points on the layout and didn't see a significant change in the DCS signal when the buffered TMCC was turned on.  When you did the testing and saw the DCS being stepped on by the TMCC buffer, was it a lab simulation or actual layout test.  I plan to redo this test in the next several weeks when things calm down at the club, after our open house Sunday are done.

Bob D

@rad400 posted:

Thanks for explanation Adrain.  When this discussion first came up of the TMCC buffer signal stepping on the DCS signal, I took a scope measurement at several points on the layout and didn't see a significant change in the DCS signal when the buffered TMCC was turned on.  When you did the testing and saw the DCS being stepped on by the TMCC buffer, was it a lab simulation or actual layout test.  I plan to redo this test in the next several weeks when things calm down at the club, after our open house Sunday are done.

Bob D

I did measurements. If you want I can post some examples of what the issues are on the scope if you want. If you're interested that might be worth a new thread.

Thanks so much to all of you.  Some of the commentary is over my head, but I am glad I am not alone in encountering these problems.  For many years, one of the favorite crowd pleasers for visitors to my layout were the tiny TV cameras available  for our trains  a few years ago.  I had one camera in a Santa Fe F unit and a second in a rear end observation car . Receivers  connected to two flatscreen TV's delighted both young and old who would wait at tunnels to see themselves as the train emerged.  Signal was pretty good even over long distances.  These systems got worse and worse and they no longer are sold at shows.  Too much other stuff going on in my train room.  

I mentioned my use of Touch Toggle switch controllers made by Barret Hill, company for creating control panels where your fingertip activates a turnout.  They work 95% of the time for me but occasionally one fails to activate the relay that actually throws the switch.  This product has had a following among model railroaders using the smaller scales.  For our train setups they are still a work in progress as we try to isolate these devices from all the extraneous stuff going on in our train rooms.

I will mention this discussion to my friend at Barrett Hill as he may learn something from your posts.  

Phil

Add Reply

Post
OGR Publishing, Inc., 1310 Eastside Centre Ct, Suite 6, Mountain Home, AR 72653
330-757-3020

www.ogaugerr.com
×
×
×
×
Link copied to your clipboard.
×
×