Obtaining the gear. Usually you have to search out Ebay, or a reseller, matching serial numbers so you get the right band, then hunting down someone to do the conversion, or if you are lucky enough and have the test equipment, doing it yourself. I have neither readily available to me, so I decided on another route. Even my local Motorola shop has closed.......Im in the middle of a rural area, Im out of options going this route. I do have some test equipment I can borrow, but this is even limited.
The solution:
I did some research, and while its not totally perfect, it will work, and indeed it does! It will take a little extra filtering, and a look at some preselectors later, but its been fun learning and designing the replacement. Meet the DRA818V/U
The modules though are available commercially to folks that want to work with them. What others have found out is that while the T/R switch in them isnt the worlds fastest, and there are no output filters at all. They can be used if you are willing to take the extra time and work with them. I decided that one could be used strictly as a receiver, and the other as a transmitter for the repeater. They were going to need massive amounts of filtering, and even more amplification, but at least they were cheap and easy to obtain, fully programmable with Linux, and if they blow up, who cares, for 15 bucks, get another one.
I decided to use these at full rated power. That is simply 1W RF......driven at full output has a cleaner RF output, and pulls a whopping 350ma of current. Ok I sent in an order. By the way there are vendors on amazon, and while I have no stake in Amazon, you can get them fast and if there is a problem they have a return policy....perfect.
I did more research, and a fellow ham W0ANM has already made some leaps and bound using these modules, as well as KP4TR and WA3DSP. Many have designed nano nodes for use with Allstar, and these are great starting point. Look these guys up, they have plenty to read and have done some of the work already.
I set out to design my own radio module. With a basic working knowledge of the module I was able to come up with a small easy to construct design I liked. Initial test were good, and I am still playing with the setup.
The top is the first generation board. I have since revised this to allow for even better filtering. The module is easy to use and the schematic is really straightforward. There is really not a whole lot to discuss here other than the COR( squelch) circuit. Q1 and T1 make up the squelch circuit. When a signal is received, you will get two outputs, why? easy of use. I made it so that you can have either a HI( 5V0) output for COR, or a LO ( 0V0) output depending on your needs. This eliminates the need for further conversion if you needed one or the other type of signal. All of the signals, PTT, CORs, Audio out and MIC in are all available at the small header on the right. The other header is just for power. Power is 5V0, a diode knocks this down to 4V4 volts since the module uses 3.3-4.7Vdc. The header at the top is for connecting a 3.3V FDTI module to. This is the programming connection, and is a simple 3.3V serial connection. For programming the module all that is needed is a computer USB port, the FDTI module ( 3.3V) and the connection to TX/RX and a ground. For programming the module for your frequencies, W0ANM had a easy to use python program. Its available on his website, just goggle W0ANM.
Filtering:
The modules alone when powered up have terrible harmonics. The second harmonic usually shows up about -30dBc and the third and fourth arent far below that in power. This is totally unacceptable. At worst case, something on the order of -60dBc would be a starting point, and -70dBc would be better, heck no second or third harmonics would be great but this isnt the case.
On the web there are many great free Chebyshev low pass filter calculators. I opted for using a 5 pole Chebyshev for the first generation boards. This consisted of 2 5pf and a 11pf capacitor sandwiched with 2, 22nH coils. I chose in the first generation board to use chip (SMD) filter parts. I used 805 SMD chip parts, the next generation will use 1206 smd chips, and Im considering just chip caps and hand wound inductors, that has yet to be determined. In any case, the low pass filter worked really well. I measured the second harmonic at -67dBc, and the third and fourth harmonics were really not even hardly noticeable on the scope. Im pretty certain that this setup will not tolerate a huge amount of usage, but the first generation board is a prototype, not a final production model. Also note, the prototype board is designed for a 7 pole low pass filter. I added this in case I wanted more filtering or the 5 pole wasnt enough. In the end I bridge soldered the last coil pads together, and omitted the last smd cap.
There is a 1206 LED used for both power, and SQL, for visual indications only. These can also be omitted if you want to further reduce power consumption, its your choice. I like to see when things are working, and Im a big believer in smd LEDs for this purpose. You can troubleshoot a pieve of gear pretty fast simply by knowing the gear, and looking at the indicators.....its always the first place I start, that and meter readings.
Thats the transmitter for now, it will be improved. I have the receiver done, and more test on sensitivity and a preselector design are in the works. More on those, as well as the 30W amplifier design will be posted later.
In the meantime,
Happy Coding!
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