Our club (the awesome AGHR club in San Pedro) has been getting the "RF out of range" stuff a lot lately and while it's a pretty basic problem (the TIU and remote at 900 MHz) not talking well, I've seen a lot of posts on here about people trying to cope with it so I know it's not just me.
The DCS companion book has a few good suggestions (seating of the radio boards, positioning of the TIU,...) but it seems people have tried those and still aren't getting the reliability they want. I know the guys in my club tried those suggestions and we're still not great.
This post is to provide you new options... I haven't tried any of this yet, but I have simulated it in ADS and it looks okay so far. Before we jump into what I'm thinking I want to put a paragraph addressing antennas so people are in the know:
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Antenna stuff:
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I found a few posts about people putting longer antennas on the TIU radio board. I just want to bust a few myths here for people so frustration doesn't lead to someone throwing a train through MTH's window or something (at which point the thrower would then have to send the train to the MTH repair service... right?).
Those RC cars you played with as a kid had a 27MHz radio which worked on a principle called an electrically short antenna.... here's a nice writeup [Short Antenna]. In this condition the longer antenna has benefits. However at 900 MHz the antenna is not electrical short, and is a microwave antenna which really needs to be the length it comes with (which is exactly a quarter wave). If you make the antenna longer, essentially what you are doing is called over-mode-ing the antenna. You can read about it in my favorite book from antenna class when I was in grad school... [Modern Antenna Design Edition 2], it's online and free, how cool is that? The monopole section is on page 242.
When you over-mode the antenna you do 2 things:
1. You change the pattern. When the antenna is 1/4 wavelength (calculated not with the permittivity of air but with the dielectric constant of the insulation on the antenna considered!)... you get an optimally omni-directional pattern (like a sphere in all directions). Once you start making it longer you distort the pattern and then some directions get favored and some directions get nulled out. You may get lucky. You may not. A twist of wire is not well controlled, so it's hard to hit a designed pattern by soldering a random stick of wire there. 7/10 times you'll probably make the situation worse.
2. You change the VSWR (voltage standing wave ratio). As a result of this, more of the transmit power may actually get reflected back into the TIU radio instead of radiated out the antenna, and you may find yourself in an even worse situation with even weaker signals.
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So what else can we do?
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It seems a solution is needed, and that's when engineers start to pretend they have value (at least me anyways!). I took a close look at it, and came up with some simple ideas, as far as I can tell no one has tried this yet. I will probably put a prototype together sometime this quarter (it's a bit of work...).
Note: The full MTH app for WIUs is probably coming which makes this a moot discussion.... unless you still want to run the remotes. (At least for our club I can imagine this being the case so I will still pursue this. )
If you look at the RF board ... the core of the thing is the TI TRF6901 which is a 900 MHz transceiver for the old digital cordless telephones (like not cell phone...I mean cordless):
FYI I was in grade 2 when this advertisement ran in the paper. Okay no one remembers cordless phones but the things were total crap, everytime you go around a corner on the phone the call got cut off, they would drop calls like crazy and such. People are nostalgic but if you think hard enough, you will remember they did suck. That's why if you go into the store today all the phones are using OFDM or at least CCK modulation and the 2.4 GHz phones outnumber the 900 MHz ones like 10:1 on the shelf. (Yes, FSK is a thing of ancient history... it died when WCDMA was invented)...
1 paragraph history lesson... first was the AMPS (American Mobile Phone System) which was ASK/FSK, then came 2G which we often call GSM, which was first PSK, then moved to GMSK modulation. Then came UMTS (3G) which was WCDMA modulation, and then came LTE (4G) which is OFDM based. GSM in Europe invented a thing called EDGE which is just GSM with WCDMA on top. If you follow 5G research people are mostly talking about what we call carrier aggregated OFDM which is an OFDM that senses the spectrum and occupies any empty space.
To the task at hand..... here is the block diagram of the radio chip on the TIU RF board:
Whoever did the design was pretty uninspired (or in a rush) and basically just plopped the example circuit from the TI datasheet directly onto the board. So if you look into it, you have the the usual RFIC junk that's characteristic of the late 1980s early 1990s....there's an external loop filter for the PLLs, some outside filtering for the IF, nothing to write a journal paper over. Anyways if you hop into the TI datasheet for the part, they recommend 3 different ways of hooking up the antenna. (Hooking up the antenna is not super simple because the RF in and RF out are two different ports so you have to impedance match a 3 port network while still signal splitting/combining 1 antenna to two ports on the chip):
Here's that page of the TI Datasheet:
The RF board in the TIU is option B) SPDT switch with the SAW filter. The switch MMIC is the shiny metal part with 6 legs.... (power,ground, RF1_pole,RF_pole, RF_common, control_signal).
Here's the running history lesson again. In the old days the cordless phones had to be super compact, so there was mortal terror of ever designing a radio board with 2 antennas (like a separate TX and RX) because people get fired from Broadcom for suggesting things like that (I've got great stories). Anyways becasue of this, RF circuit designers went to great lengths to try and couple the TX and RX onto the same antenna by putting switches (which pick which signal is connected.. and take turns), putting diplexers (which are super high-Q filters that notch bands... so like TX sees 900-901 and RX sees 902-903 or something) you name it, it's been tried.
The thing is, the TIU is already comically large so I think having two antennas wouldn't be a show stopper for most people reading this if it meant the trains ran better...
This is what I propose. First lift off the switch with a soldering iron... as well as the filter (its a few of those passives around the switch MMIC... I haven't figured out which ones yet, but I'll post when I do). Then two of the pads left over where the switch used to be will go into the TX and RX port. You can do a dirty job and solder two SMA connectors to those... (these things: [Edge Mount SMA]). Actually these guys give away free samples... so score! So now you can connect an SMA cable directly to each of the TX and RX port. Then we can go get grown up RF amplifiers to help our transmit power and receiver sensitivity. We can put an external LNA (low noise amplifier) in front of the receiver (to bring up the sensitivity to weak signals) and we can put an external PA (power amplifier) after the transmitter to boost our outgoing signal.
Schematic of the proposed hack job:
There are many RF amps to choose from.... and generally speaking you can get them inside a box with the connectors on them so you don't even need to do soldering. Now you might be inclined to "go big or go home" and get some massive 50dBm power amp or something where the planes overhead go off course every time you blow the whistle*....
*this is a joke. That won't really happen.
FCC has some basic rules to follow that you need to be a bit mindful of. The 900 MHz band is an ISM band (meaning you don't need a license to transmit), but they would prefer if you kept your EIRP (effective isotropic radiated power) under 10 watts. Also the regulation is based on average CW power (transmitting a tone) but the DCS signal is very infrequent and short in duration meaning the average power is very very low even if the peak power is high.
The TRF6901 on the TIU board as it is transmits a measly 10dBm (which is 10mW) so if you go out for even a 1W amplifier you're talking about a 100X improvement in reception right there.
The parts I am thinking about for my test are the
ZFL1000 for the LNA on the RX (ZFL1000)
and
ZHL1000 for the PA (ZHL-1000-3W+.pdf).
That will give you about a factor of 300 in signal power from the TIU to the remote, and the TIU receiver will have 20dB gain in front so a factor of 100 improvement in sensitivity ....that is I checked the RSSI (received signal strength indicator)... pin41 on the TRF6901 chip in our TIU and we were well below the max. Remember for a receiver, if you put in too much signal the receiver becomes non-linear and starts to distort. In RF design we call this compression... in the old days people would quantify this with parameters like p1dB compression (the power at which the gain drops 1 dB), and IP3 or (intercept point 3) which is where the 3rd order harmonic intercepts the power of the linear term. In the modern RF chip design world everyone thinks these are stupid parameters and just use EVM (Error vector magnitude now) which is a mathematical measure of how distorted your signal actually is. (Basically you divide the real received signal by the ideal signal for each data point in the constellation and estimate the difference).... if you're interested here you go [EVM].
Basically the plan is to have a TIU with 2 SMA cables coming out then two of these little amplifier modules next to it, then you can just buy a set of antennas to stick on the input/ouput of the two amps. Most antennas take a coax cable with an N-connector... so you can just buy an adapter ... amazon.. $30... whatever..[RF Adapter]... As for the antenna itself... who knows... google 900 MHz antenna, find something that looks nice. Here's a nice one [Patch Antenna].. $50 , not ugly, whatever. good to go.
One detail. The MTH remote is "linearly polarized" so don't buy "circularly polarized" or you'll have an issue. This is just describing the orientation of the electric and magnetic fields (E & H) as they propagate out of the antenna and into space. They can go up and down, left and right (linear), or they can rotate as they travel (circular)
Your signal power is high now so it really won't matter what the antenna gain is unless your layout is literally a mile long...
Also this trick doesn't work with all radios because if you are transmitting and receiving at the same frequency the high power PA can jam the super sensitive receiver which is right there. This is not a big issue for a TIU becasue the transmitter and receiver don't work at the same time. The handheld sends a signal to the TIU, then the TIU replies. Also that dinosaur transceiver chip has 2 separate PLLs (one of TX and one for RX) so they are at different frequencies anyways.
As I said, I will build this up when I find a moment, but I have lots of other projects too, so it may be a month or two before I have results.
MORE REFERENCES:
If you're interested in learning all this hardcore RF ... you can read the two books I use most often on my shelf:
2. My Stanford Colleague Tom Lee's book
Or for a more modern twist (this is like the modern RFIC chips Qualcomm/Broadcom design):
3. Digital Assisted Analog (I wrote Ch-6 in here!... please excuse the shameless self promotion)