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Old Jul 09, 2012, 04:54 PM
SimonChambers is offline
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Radio? Screwdriver!
United Kingdom, England, Bristol
Joined Aug 2011
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The Futaba 8J S-FHSS is better than Futaba 18MZ S-FHSS! The 8J may get Telemetry...

While I was browsing through the FCC OET archive at the new T8J RF circuit (possibly my next protocol reverse engineering project), I found something very surprising. It looks like the T8J could may be firmware upgradable in the future to have telemetry.
Now a picture is worth a thousand words: (I've highlighted the RF paths in red so everyone can follow)


Now someone chime in if I'm wrong, but if you follow the RF path from the CC2500 (RF Chipset), it first goes to a RF switch (lets call it SW1) which then splits the signal to either two ways. Now this is where I am assuming here, but I reckon the bottom device is a Power Amplifier (PA - increases the transmit power of the signal - the CC2500 on its own doesn't transmit powerful enough to be full-range), and the top is a Low Noise Amplifier (amplifies the incoming RF signal for the CC2500 to listen to). After that it those two outputs then go back into another RF switch (call it SW2), and then into a final RF switch (SW3) - which selects the antenna to be used.

Now SW1 and SW2 would be switched together - depending on if the module is transmitting or receiving. As the CC2500 can't transmit a signal into a LNA to be transmitted out the antenna - and likewise the CC2500 can't receive a signal from a PA.

I've seen this RF layout countless times on telemetry receivers and transmitters. However the interesting thing is that the T8J is not supposed to have telemetry! Maybe Futaba is future-proofing itself this time? Question is, why doesn't it have telemetry out of the box? All I can say is ....

Now for contrast, lets look at the RF layout of the 18MZ:

At the top, you can see the FASST transmit circuit - I haven't gone into detail here, as we're not that interested in it. However you can see the output from that into the RF Switch.

Now looking at the bottom half, you can see that once out of the CC2500 (and the Balun - which takes two balanced outputs and balances them two one unbalanced signal), it goes straight into an IC. This has to be a Power Amplifer (PA - looks the same IC as used above on the 8J) - as after that, it goes straight into a RF Switch and then out to the antenna. So the 18MZ in S-FHSS mode, has no way of by-passing the power amplifier to allow a signal to be received! Hence it could never have Telemetry in S-FHSS mode.

Now I may be completely wrong (wouldn't be the first time for me ) - so if anyone see's a flaw with this theory, feel free to correct me! Without it in my hands to probe, I can't be certain.

However if it is true, its interesting eh?

If only I could be a fly on the wall at a Futaba design spec meeting....

Cheers,
Si.
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