Spektrum / JR Tx Module Conversion Project
Posted 04 September 2006 - 12:42 PM
Late last year the Spektrum division of Horizon Hobby (JR's USA distributor) introduced a 6-ch 2.4GHz DSSS (direct sequence spread spectrum) R/C system. Unlike their earlier 2.4Ghz R/C car systems Spektrum had been selling, this one is designed for "park flyer" aircraft.
The DX6 transmitter is a based on a computer radio that was a retired JR design. Spektrum merely had JR reproduce the transmitter, less the 72Mhz RF deck. They then installed the 2.4Ghz DSSS RF board in it and packaged it as the DX6 system. They sell it in the USA for $199 with four mini servos.
The Spektrum RF board is based on the Unigen Juno WirelessUSB module. It is a low cost part (<$10 in qty) that was intended for wireless consumer applications. The module is an FCC certified component. Manufacturers that use it are FCC grandfathered. That is to say, the FCC Part 15 Registration ID transfers with the module as long as the data sheet specified regulation requirements are met.
The Spektrum transmitter has a small daughter board in it that contains the Unigen RF module, a voltage regulator, and a host microntroller. The microcontroller is used to measure the PPM signal from the transmitter's encoder and convert it to a three-wire (I2C) communication signal so that it can talk to the Unigen module. This board is at the back of the transmitter case. The "Bind" button that the user can access is soldered to this board.
Then the model community hacks came. The first one involved putting the Spektrum board in a JR6102 transmitter. Details here: http://www.rcgroups....ad.php?t=458314
Other folks stuck it on the back of other transmitters and had good success too. But, one big issue that came up was that the Spektrum board would not work if the PPM signal had too many channels, which caused it to have a short reset period. I thought I could help, so I created a universal PPM translator circuit to solve that problem: http://www.rcgroups....ad.php?t=540007
At that point I didn't have a DX6 system to play with. But, I was curious if it would be possible to build something using DX6 parts that would look factory made. However, I was a bit reluctant to buy a DX6 system, tear it apart, and create a custom plug in module. Mainly because I figured the moment I spent my entire hobby budget doing that, I would see an Ad from JR for just such a thing at some ridiculously low price. I eventually decided to tackle the project.
Now on to the photo show ...
Posted 04 September 2006 - 12:44 PM
Here is a front view of my JR XP8103 Tx with the 2.4Ghz module installed. I have also used it in my XP783. I expect it would work with any JR Tx that uses an RF module (including the popular XP9303). Keep in mind that there are absolutely no mods to the transmitter! Plug and go, as they say.
All I have to do is unplug the 72Mhz RF module and antenna, then plug in the custom 2.4Ghz module. I can make the switch in about 20 seconds. It all fits just like the JR gods intended.
Posted 04 September 2006 - 12:46 PM
Swing the Tx around and this is what you see. The DX's "Bind" button has been replaced with a nice little tactile push switch. The Bind status LED is directly above it. The stock antenna is attached to the module. When the module is unplugged everything travels together, antenna and all.
Posted 04 September 2006 - 12:48 PM
I needed a plastic RF module case for the 2.4Ghz RF board. JR sells it for less than $10 and it can be found at many hobby shops that sell JR R/C systems. Part number JRPA615.
Posted 04 September 2006 - 12:52 PM
The RF module plugs into the back of the high-end JR Tx's, where it gets power and the PPM signal. The signals are as shown.
Posted 04 September 2006 - 12:55 PM
The PPM signal that is available at the back of the RF module based JR Tx's has inverted logic from what the DX6's RF board needs. The reset pulse is borderline too, which can cause grief in some situations.
Posted 04 September 2006 - 12:57 PM
The logic levels and reset gap time is solved with the PPM Translator circuitry (link to details provided in post 1). Here is the above signal after the PPM PIC has processed it.
Posted 04 September 2006 - 01:03 PM
The first thing to do was to remove the Unigen Juno module from the DX6's RF board. The PCB through-holes (all twenty of them) are barely big enough for the pin diameters used on the module, so it is a near press fit. This made the solder sucking activity painfully tedious. But, in the end, I got it off without any damage to the module or PCB.
Yes, I know what you are thinking. I can kiss the JR warranty goodbye.
At this point I started my reverse-engineering of the DX6 layout and came up with a schematic to their circuitry.
Posted 04 September 2006 - 01:06 PM
Here is the bottom of the Unigen Juno 2.4Ghz DSSS module. You can see the FCC Reg ID here too.
Posted 04 September 2006 - 01:07 PM
Then I used the hot air wand and removed their microcontroller and 32Khz xtal. None of the other parts will be used. At this point the original board is undamaged, but a bit naked.
I only needed to harvest the Unigen module, the microcontroller, and xtal, for re-use in a custom PCB that will fit in the stock JR Tx module case. The other parts are ignored since it is easier to install new ones rather than mine the old ones.
Posted 04 September 2006 - 01:16 PM
The stock DX6 antenna comes off by removing a screw. It plugs into the Unigen module via a micro U.FL button connector. The antenna will be reused, so it gets set aside for now.
Posted 04 September 2006 - 01:18 PM
A custom PC board set was created. The board on the left goes inside the Tx module case. It holds the Unigen module, 3.3V VReg, DX6 microcontroller, and PPM Translator microcontroller.
The PC board on the right will mount on the outside of the Tx module case and will be used as an antenna adapter mount. In a perfect world this would be a sexy molded plastic part that also doubled as the cover to the module case. But, I'll skip the $15K plastic mold tooling fees and substitute it with this mortal solution.
The PC boards are double sided FR4, soldermasked, with silkscreen on both sides. That, along with the oddly shaped perimeter and slot cutouts, made it an expensive proto set. I ordered spares in case I botched one up. It's only a credit card.
Posted 04 September 2006 - 01:23 PM
I dry fit it the PC boards and drill holes in the JR module case for the LED and Bind switch. I also mount two 2-56 machine screws to hold the antenna adapter board. Everything fits great. What a relief!
Posted 04 September 2006 - 01:27 PM
The internal PC board is fitted with the 5-pin connector (lower right) that interfaces to the Tx's rear panel signals. I found that connector alignment was perfect and that the module case plugged into my JR transmitters without a hitch.
Posted 04 September 2006 - 01:32 PM
The Unigen RF module and 3.3V DC-DC switcher voltage regulator are mounted. It all fits, but with no room to spare. I had to trim a small bit of plastic from the VReg's plastic frame to clear a rib in the plastic case.
The Unigen module looks odd here because the tin RF shield is temporarily off. Normally it would look like a sardine can.
By the way, a linear Vreg cannot be used since it will run too hot in the unventilated case. The DC-DC Vreg cures that problem. However, the Unigen Juno module is designed for linear Vreg supplies because they offer lower ripple. I used a low noise TI switcher VReg and heavily decoupled it. Power supply noise is under 40mV, which should not be a threat.
Posted 04 September 2006 - 01:38 PM
Here's a close up of the plastic I had to trim. A few seconds work with an X-Acto knife.
Posted 04 September 2006 - 01:42 PM
All the components are mounted. You can see two microcontrollers, the bind switch, LED, and misc components. Fortunately, all the SMT parts are low enough that the case's lid still fits fine.
The DX6 microcontroller has 25-mil legs. Like the other parts, it was hand soldered under a low power magnifier. The naked eye is no match for this stuff.
Posted 04 September 2006 - 01:44 PM
The LED and switch poke through holes that were drilled in the lid. Some of the cutouts that you see were already there (normally covered by the JR FCC label).
Posted 04 September 2006 - 01:50 PM
Now it's time to install the stock antenna on the antenna adapter board. The antenna slips on the top and is held by a single screw. Some cutouts in the PC board are available for tiewraps to hold the micro feedline. All the little details have been covered.
I have read where several folks have broken the knuckle on the stock antenna. So, the antenna adapter board includes an option where an SMA (or reverse-SMA) connector can be installed for mounting a generic 2dBi whip antenna. I'll show photos of that option in a couple days.
Posted 04 September 2006 - 01:52 PM
Now it's all together and bench tested. Success!
Funny thing, when I ground range tested my stock DX6, I got 45 paces. Under the same circumstances, with the custom RF module in my XP8103, I experienced 50 paces! I'm going to ignore the difference and call them the same. The bottom line is that the position of the module and antenna in the new Tx does not reduce range.
And here is the best news. With the stock DX6 Tx, the servo signal resolution is only 5uS (about 7.5 bits) step size with ~500nS of jitter. With the new module installed in a high end JR Tx, the resolution is supercharged to 1uS (10-bits) with only ~100nS of jitter. The icing on the cake is that I get to use all the features on my favorite JR Tx. Not a bad outcome for a million hours of work.