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New FT8 Digital Mode Easier on IRF510?


AA7US
 

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



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




AA7US
 

Adam,

Is it acceptable to use a heavier gauge (larger diameter) wire when winding a larger toroid such as the FT50-43?

Thank you,

John
AA7US

---In CHINA_QRP@..., <rongxh@...> wrote :

This is why our recommendation is that you change the ferrite toroid from FT37-43 to FT50-43 or even bigger.


Robert Hudson
 

Hi John and all

While replacing the final  FET I forgot to put the insulator under the nut holding it to the heat sink case so it was shorted s soon as power was applied and burn’t out the winding on T3.   I rewound it with 0.63mm wire on the original toroid and the wires fitted the centre hole without any problems.    I saw the info  about changing the toroid.    I wonder if it is becoming saturated when using digital modes.   I also learnt to put a 2 amp in the power supply lead , much easier to replace than the FET.    Also my local supplier did not have the original FET but a higher current rating and seems to work well. (IFR-540)  This was not my radio but will purchase one for myself.   Great project kit thanks Adam.


On 13 Aug 2017, at 9:08 am, landrjoh@... [CHINA_QRP] <CHINA_QRP@...> wrote:

Adam,


Is it acceptable to use a heavier gauge (larger diameter) wire when winding a larger toroid such as the FT50-43?

Thank you,

John
AA7US

---In CHINA_QRP@..., wrote :

This is why our recommendation is that you change the ferrite toroid from FT37-43 to FT50-43 or even bigger.


Bob Hudson








Adam Rong
 

It is true that the wire gauge has little impact to the performance of the transformer. Heavier gauge will have a bit less loss but harder to wind.

Thanks,
Adam

于 2017-8-13 8:08, Robert Hudson vk2aor@... [CHINA_QRP] 写道:

 

Hi John and all


While replacing the final  FET I forgot to put the insulator under the nut holding it to the heat sink case so it was shorted s soon as power was applied and burn’t out the winding on T3.   I rewound it with 0.63mm wire on the original toroid and the wires fitted the centre hole without any problems.    I saw the info  about changing the toroid.    I wonder if it is becoming saturated when using digital modes.   I also learnt to put a 2 amp in the power supply lead , much easier to replace than the FET.    Also my local supplier did not have the original FET but a higher current rating and seems to work well. (IFR-540)  This was not my radio but will purchase one for myself.   Great project kit thanks Adam.


On 13 Aug 2017, at 9:08 am, landrjoh@... [CHINA_QRP] <CHINA_QRP@...> wrote:

Adam,


Is it acceptable to use a heavier gauge (larger diameter) wire when winding a larger toroid such as the FT50-43?

Thank you,

John
AA7US

---In CHINA_QRP@..., wrote :

This is why our recommendation is that you change the ferrite toroid from FT37-43 to FT50-43 or even bigger.


Bob Hudson









AA7US
 

Hi Bob,

I'm surprised that the IRF540 is working as a good replacement for the original IRF510.

The IRF540 has a significantly higher input capacitance (1700 pF) compared to the IRF510 (180 pF).  From what I've read, higher MOSFET input capacitance requires higher drive levels.  Also, the IRF540 is slower to respond than a IRF510.

I wonder if the IRF540 has decreased the transmit power?

Maybe Adam can comment on this.

73,

John
AA7US

---In CHINA_QRP@..., <vk2aor@...> wrote :

Hi John and all

Also my local supplier did not have the original FET but a higher current rating and seems to work well. (IFR-540)  


Robert Hudson
 

Thanks Adam

The instruction text says to twist the the wires together and the photo of the toroids shows them in parallel.   I rewound the T3 coil using the method in the photo.   Having come from a motor winding background you need to identify the start and finish of the coils and as such I have seen biflar windings done in 2 different colours which overcomes the problem of getting 2 starts together and hence to finishes.      They transceiver works very well.   

Thanks Bob
On 13 Aug 2017, at 11:20 am, Adam Rong rongxh@... [CHINA_QRP] <CHINA_QRP@...> wrote:


It is true that the wire gauge has little impact to the performance of the transformer. Heavier gauge will have a bit less loss but harder to wind.

Thanks,
Adam

于 2017-8-13 8:08, Robert Hudson vk2aor@... [CHINA_QRP] 写道:
 

Hi John and all


While replacing the final  FET I forgot to put the insulator under the nut holding it to the heat sink case so it was shorted s soon as power was applied and burn’t out the winding on T3.   I rewound it with 0.63mm wire on the original toroid and the wires fitted the centre hole without any problems.    I saw the info  about changing the toroid.    I wonder if it is becoming saturated when using digital modes.   I also learnt to put a 2 amp in the power supply lead , much easier to replace than the FET.    Also my local supplier did not have the original FET but a higher current rating and seems to work well. (IFR-540)  This was not my radio but will purchase one for myself.   Great project kit thanks Adam.


On 13 Aug 2017, at 9:08 am, landrjoh@... [CHINA_QRP] <CHINA_QRP@...> wrote:

Adam,


Is it acceptable to use a heavier gauge (larger diameter) wire when winding a larger toroid such as the FT50-43?

Thank you,

John
AA7US

---In CHINA_QRP@...,  wrote :

This is why our recommendation is that you change the ferrite toroid from FT37-43 to FT50-43 or even bigger.


Bob Hudson











Bob Hudson  VK2AOR
President CCARC
0415 969 308
Lecture Co-ordinator and Publicity
Wyong Field Day  25thFebruary 2018.







Robert Hudson
 

Hi John

Thanks for the info on the FET , I was probably a bit impatient to get it going and had not considered that aspect.    Will get some IFR510’s and replace it.  


Best Regards  Bob
On 13 Aug 2017, at 11:42 am, landrjoh@... [CHINA_QRP] <CHINA_QRP@...> wrote:

Hi Bob,


I'm surprised that the IRF540 is working as a good replacement for the original IRF510.

The IRF540 has a significantly higher input capacitance (1700 pF) compared to the IRF510 (180 pF).  From what I've read, higher MOSFET input capacitance requires higher drive levels.  Also, the IRF540 is slower to respond than a IRF510.

I wonder if the IRF540 has decreased the transmit power?

Maybe Adam can comment on this.

73,

John
AA7US

---In CHINA_QRP@..., wrote :

Hi John and all

Also my local supplier did not have the original FET but a higher current rating and seems to work well. (IFR-540)  


Bob Hudson








AA7US
 

I definitely prefer winding bifilar and trifiar toroids using different colored wires.

Not only is it easier to keep track of the wires, but it makes the toroid look pretty... and a pretty toroid must improve performance! 

73,

John
AA7US


---In CHINA_QRP@..., <vk2aor@...> wrote :

Thanks Adam

Having come from a motor winding background you need to identify the start and finish of the coils and as such I have seen biflar windings done in 2 different colours which overcomes the problem of getting 2 starts together and hence to finishes.