Re: New FT8 Digital Mode Easier on IRF510?

Adam Rong
 

You don't have to worry about IRF510 itself because it has a good heat sink of the bottom case. It is the saturation of the final transformer T3 that causes the problem if the transmit time is too long and power keeps too high. When the transformer is no longer functioning as a transformer, the load of IRF510 will change and it might cause its melting. This is why our recommendation is that you change the ferrite toroid from FT37-43 to FT50-43 or even bigger.

SSB voice will be fine as the power will vary all the time based on the voice, so there is no problem found.

Thanks,
Adam

于 2017-8-12 1:47, landrjoh@... [CHINA_QRP] 写道:

 

Hey gang,


In looking over the characteristics of the new weak signal FT8 digital mode, I noticed it has a sustained TX time of only 15 seconds (versus nearly 50 seconds for JT9 and JT65).  Presumably this is followed by a minimum 15 second RX period... probably a bit longer.


Wouldn't this new FT8 digital mode be a lot easier on the IRF510 final amplifier found in our KN-7QA and CS-series transceivers compared to JT9 and JT65?  It should cause a lot less heat buildup right?


The following excerpt is from the version 1.8.0 release candidate WJST-X software:


Brief Description of the FT8 Protocol
		-------------------------------------

WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN
and K1JT.  The mode name "FT8" stands for "Franke and Taylor, 8-FSK
modulation".  FT8 uses 15-second T/R sequences, provides 50% or
better decoding probability down to -20 dB on an AWGN channel, and 
maintains good performance on Doppler-spread fading channels.  An
auto-sequencing facility includes an option to respond automatically
to the first decoded reply to your CQ.  FT8 QSOs are 4 times faster
than those made with JT65 or JT9.  FT8 is an excellent mode for HF
DXing and for situations like multi-hop E_s on 6 meters, where deep
QSB may make fast and reliable completion of QSOs desirable.

Some important characteristics of FT8:

  - T/R sequence length: 15 s
  - Message length: 75 bits + 12-bit CRC
  - FEC code: LDPC(174,87)
  - Modulation: 8-FSK, tone spacing 6.25 Hz
  - Constant-envelope waveform
  - Occupied bandwidth: 50 Hz
  - Synchronization: 7x7 Costas arrays at start, middle, and end
  - Transmission duration: 79*1920/12000 = 12.64 s
  - Decoding threshold: -20 dB; several dB lower with AP decoding
  - Multi-decoder finds and decodes all FT8 signals in passband
  - Optional auto-sequencing and auto-reply to a CQ response
  - Operational behavior similar to JT9, JT65

We plan to implement signal subtraction, two-pass decoding, and use of
a priori (AP) information in the decoder.  These features are not yet
activated in v1.8.0.

We haven't yet finalized what the three extra bits in the message
payload will be used for.  Suggestions are welcome!

	-- Joe, K1JT, for the WSJT Development Team



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