Scott Myers <[email protected]> writes: > My first question is on the uplink/downlink frequencies. It simply states > 433 MHz. I assume this to be an approximate frequency in the 70 cm band and > not the actual exact frequencies. Rarely is something right at > 433.000 MHz.
Right, the radios are designed to operate in the neighborhood of 435MHz, but you can configure whatever frequency you like around that. > Are there separate uplink and downlink frequencies (full duplex) or does it > switch on/off transmit/receive at each transceiver (Telmetrum and > Teledongle) using the same exact frequency. During flight, it's a unidirectional transmission from flight computer to the ground. This lets you have multiple ground stations tracking the same flight, and makes the communication work better over long distances. For bi-directional communication in 'idle' mode, it's a half-duplex channel on a single frequency. Doing full-duplex communication would be very difficult in a single band, and would require separate transmit and receive radios which we don't have. > How wide is the data bandwidth in KHz? We're using 38.4kbps and 20.5kHz deviation GFSK, yielding a bandwidth of about 100kHz. > (I think they are all coded and using "telegraph" messaging > handshaking to avoid conflict with another Telemetrum, but I could be > wrong.) No, telemetry is uni-directional, so any receiver tuned to the right frequency will see the data. > I see there is a starter "kit" that uses the Teledongle. But there is also > the TeleBT, which seems to do everything the Teledongle does, PLUS adds > bluetooth for use with the Android App. It seems that starting with the > TeleBT would the better choice and don't even buy the Teledongle, but I > wanted to make sure I wasn't missing something on this. Agreed. The TeleDongle is a bit cheaper and smaller, but that's about it. > I saw the history on older versions with the problems with pyro events and > battery rail sag and that it is supposed to be solved now. (An age-old > problem on many avionics packages.) We have added a comparator to the pyro circuit which monitors the voltage coming in to the electronics power supply. This cuts off the pyro firing circuit when the voltage sags below that necessary to keep the flight computer processor running. There's a ballast capacitor at that point in the circuit, so the circuit ends up pulsing current into the pyro device. With a short across the pyro terminals, we end up hitting about a 90% duty cycle. This requires using a lipo without a current limiting circuit, which is what we provide in our store. > I think I'd rather put a second LiPo on the Telemetrum and give the > pyro events their own rail and simply avoid any potential rail sag > issue. I'd encourage you to check out what we've done and see if you still think this is necessary. You increase the complexity of the system as you now rely on both batteries working correctly for the pyro circuits to fire. > Just a small LiPo should do the job. I'm guessing others have done > this and I read where that is an option on the board. I'd like > feedback on that. I have seen lots of people using two batteries, and in fact, you can use a higher voltage if you like -- the pyro circuits are designed to support up to 15V or so. > Of course downside is mass of the connector soldered to the board and > having to support the feed line to account for high-G conditions so the > connector isn't broken from the board. Our new boards have metal on both sides of the board so that the edge-launch SMA is at least better supported, but yes, we've seen multiple boards snapped at the SMA connector due to high G loads. > I'd like some feedback and discussion on the use of the SMA connector. > I can't figure out how to order the Telemetrum with that option > anyway. Perhaps it is a field mod? We used to have it on the price list; I'm not sure how Bdale manages it these days. But, the boards are designed to work well with a simple wire whip, so unless you're going to mount it in an airframe that requires an external antenna, it really is nice to just use it as designed. I've run the wire through the ebay bulkhead and up alongside recovery systems in several airframes with good results. -- -keith
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