Correlator Noise floor inflation by Legacy Signal Interfering with DCS

gunrunnerjohn posted:

Adrian, I do foresee a much larger issue with switching the Legacy signal.  Since any large layout will have multiple TIU's and multiple channels, you need to have multiple switches to manage the Legacy signal interruption.  However, you can only have one Legacy signal source in the entire layout, so there's the issue of a major disconnect between this unit and the TMCC Buffer.  The only way I see this working is somehow having all these interrupter switches feeding a common switch that turns off the Legacy signal.  Obviously, you are then dealing with the propagation delay as usually the TIU's are scattered all over the layout with a really large layout.

Until we can somehow address that issue, I don't see how this can work.  Maybe balancing the Legacy signal between the center rails and the outside rails is a better bet after all.

I'm very aware of this and have been thinking about it already actually.

Right now I'm building one of these for the SD3R folks who have 1 TIU right next to the legacy base. The board I have connects to the 4 TIU outputs and "ORs" them together to run the switch.

I'm thinking even if you chop off the better part of the first DCS packet in a sequence (like 1-10us) the DCS command will only have to repeat 1 extra time before everything is communicating within the hole and the decoder is clean.  At our AGHR club we probably have a good 200-300 ft between TIUs so that's almost 1us of delay right there. Once I have the nice boards back I was going to try to wire them across the club with a remotely located switch and see how it performs. Maybe the initial latency is not as critical as we think. Testing should answer this...

Balancing the center and outer rail isn't so easy either.  After thinking a bit.... while the outer rail is often a single node in a layout, the center rail is usually broken up into blocks, so you'd need a lot of copies of whatever circuitry is doing the balancing. You need to distribute the legacy signal to all of them, and they all need a connection to earth ground as a reference. If you have earth ground connections all over the layout to support this, the cap may go up a lot and impact the legacy/tmcc performance. Sound hard...

Both options sound hard Adrian.   I was also mulling over balancing the center rail and I came to the same conclusion you did.  The only thing that occurs to me is some sort of notch filter to pass the 455khz but block the other frequencies.  As you say, you need one for each TIU channel block, so that's sort of ugly as well.  As far as affecting Legacy performance, that's what the TMCC Buffer is for.

gunrunnerjohn posted:

..snip..

The only thing that occurs to me is some sort of notch filter to pass the 455khz but block the other frequencies.  As you say, you need one for each TIU channel block, so that's sort of ugly as well.

Yeah this is bad. Expect a new thread shortly with some data.

The single-TIU solution is fully built and bench tested. I just need to drop it in a project box, get a wall adapter to power it, and it's good to go.

SD3R_DCS_LEG_PCB

SD3R_DCS_LEG_PCB2

The plan is to stick it into our club layout on a TIU somewhere and see if any legacy users notice something happened (it's a double blind test). If they don't notice anything we have an answer for at least the single TIU case. I'll post layout results as I get them.

-->The idea is after I prove it's okay in our club, I'll box it up and send it down to the SD3R club. They have a single TIU layout and had the legacy-DCS issue in the first place.

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WOW, looks good!  If you used this for multiple TIU's, could you have a single switch output that gets connected to a common input on a single TMCC switch?  What does it look like the costs of this module might be?

gunrunnerjohn posted:

WOW, looks good!  If you used this for multiple TIU's, could you have a single switch output that gets connected to a common input on a single TMCC switch?  What does it look like the costs of this module might be?

What you describe is sort of what I was thinking. I found a few us delay really doesn't do much in terms of impacting the DCS performance. The DCS will just keep repeating the command until the hole shows up.

So you'd have a board like the left side of this board that has the 4 filters for the TIU channels, and one shots and "AND" gates to combine them together to send out a TTL signal (0=clear 1=DCS packet). Each TIU would have just that.

Then you'd have a different board at the legacy base that is like the right side of this board. It would accept the TTL signals from all the TIUs, AND them together again, go through the circuit that translates the TTL from layout ground to Earth ground, then has the analog switch.

The one above was $80 for 3 but I wanted to debug it since it's a prototype so it's hole-thru not SMD and I didn't push on the placement much. I bet we could get it down to $30 with a production layout.

gunrunnerjohn posted:

Where did the money go?  What's the source of the BCM9000?  I looked for that one, and I don't find it.  The other components all look pretty cheap.

Most of the money went to shipping and expedition fees because I am not a patient person.

Part is: BRCM_isolator

 

 

Sorry folks, it’s not good yet. The smaller/shorter dcs commands (whistle and stuff go unnoticed) but the moment you ask to “add engine” all the tmcc/legacy trains stop on their tracks.

im going to modify the board so it switches the legacy carrier to a reduced amplitude instead of a hard off....

 

 

gunrunnerjohn posted:

Bummer Adrian.  I figured this might be a tricky build with the timing.

I have reservations about lowering the Legacy amplitude, that's the whole reason I'm building the TMCC Buffer!

I just mean lowering it by maybe 25-50% (probably trimpot tunable) during the DCS packet, not all the time.  Like put a trim resistor in parallel with the switch so it's leaky. Ill try this out in the next few days and see if it works at all.

I'm very interested in the results.  I suspect that may have to be tuned.

One issue I didn't see you address, the short DCS commands.  Reducing the Legacy carrier isn't likely to fix that I would imagine.

bigdodgetrain posted:

question.

is there a way to make the single-TIU solution Legacy switch module to not work when doing the add engine command?

I can definitely tell the TMCC/Legacy signal is interfering with the MTH signal as the issues are not in the same place on the layout each time.

Hey there!

I’m actually working the module out for your club! We did the first test two weeks ago and posted above but it needs some tweaking before I mail it over to you guys (I need to adjust the on/off ratio). The problem is I’m away for the next 1-2 months in New Mexico to launch a NASA mission that I’m leading one of the instruments for. Finishing that board is at the top of my list for when I get back!

Adrian! posted:

The problem is I’m away for the next 1-2 months in New Mexico to launch a NASA mission that I’m leading one of the instruments for. Finishing that board is at the top of my list for when I get back!

Just stay away from the rocket when it goes up!

Isn't that always how those projects go?  When I was doing avionics, I remember more than once being out on the flight line the day of the FCC visit to witness a test flight still tweaking our systems!

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