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Introducing the FAT5

A Solid State Class E, Series modulated AM Transmitter

Hernia-free AM for the thermionically impoverished

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FAT5 is an easy to build solid state AM transmitter for 80m and 160m.

Depending on components used, the design is easily scalable from a few watts to well over 100W carrier / 400W PEP. The 'official' supported version is shown below, handling power levels to around 50W carrier (absolute limit not tested!).

click for processor circuitryClick for RF section circuitsClick for Multi-rock pagesclick for Modulator circuits
[Click on the part of the diagram you wish to see]

Latest news


Second prototype in final stages of build - (GW8LJJ)


Demonstration FAT5 built and working - see picture below

Overall view

The picture above shows  a recently constructed FAT5 using the Multi-Rock PCB, the speech processor PCB and the mod driver PCB. The modulator is on the left, the RF section on the right. This is the 'developed' version of the FAT5 lash-up mentioned in the 15/12 update below. Extra mod transistor, EQ added, and different FETs used.

RF Section

                            Multi-Rock PCB, drivers and PA FETs                       Heatsink, ferrite and PA tuning                                     

The RF section is very simple to build. Start with a Multi-Rock PCB, feed the output into two FET driver chips, then connect the drivers to the PA FETs. For this design I chose some FETs I'd not tried before - IRF640s They have a very low on resistance (Rds-on) of 190 milliohms and seem to work very efficiently (efficiency around 84%). There are two drivers and four FETs  in parallel push-pull.(that's the six TO220 cases you can see to the right of the Multi-Rock PCB). There's a minimal heatsink for the FETs and drivers. I found a small fan to waft some air over it, but to be honest it works fine with natural cooling as there's very little heat to get rid of when properly tuned. RF from the FETs is combined via the ferrite transformer (a pair of Maplin ferrites) then into the PA tuning.


Emitter resistors, TDA2050  PCB                                    Speech processing

The speech processor PCB feeds audio to the TDA2050 driver PCB which in turn drives three power transistors in parallel as the main series modulator. In this version I used three 0.86 Ohm resistors to balance the current through the three  transistors (resistors bottom left), but probably a lower value would be ok. The power transistors (not visible) are bolted to the larger heatsink  which is fan cooled. The fan wouldn't be needed if the heatsink was mounted vertically although it's a good idea to use a fan anyway to keep the junction temperatures as low as possible. The small veroboard to the right of the mic socket is a Baxandall tone control I added to give a bit of  EQ.

This design is scalable from a watt or two to over 50W merely by altering the voltage to the modulator. No component changes are needed (although the mod pot needs adjusting for the power level used). The original idea was to make a solid state AT5 with comparable power levels but working off 13.8 Volts. This version of FAT5 achieves this. With 13.8V fed to the modulator the RF output is around 10W.

This transmitter has run continuously for several hours into a dummy load with an audio feed from Radio 4 driving it to full mod with no problems.


Prototype PCB testing going ok-ish. (I managed to cross the power leads of U1 and U2 on the PCB - doh!)
Also some issues with the electret power feed causing instability (solved). Spectrum looks good  - see Perseus RX screen grab below. Modulation envelope width in grey is 9kHz . Actual power level for the test was 10W carrier/ 40 PEP, using a pair of el cheapo IRF740 FETs (subsequently changed for IRF640s).

Processor PCB assembled

FAT5 test lash-up

Measuring is believing!

Prototype PCBs have been designed for the MK1 version of the low level audio processing stages (a self contained PCB that can be used on any transmitter), and two versions of the main modulator - one for medium power (up to 25W) and one for 100W transmitters. The breadboard versions work - hopefully the PCB versions will as well.

                     Processor PCB                        TDA2050 modulator PCB      LM3876 modulator PCB

Once any bugs are ironed out (as if...), the FAT5 project will then contain PCBs for all the parts that warrant a circuit board. The rest of the project is more about bolting things to metal, winding PA coils and the like.

The power supply side hasn't been addressed yet - I seem to have PSUs coming out of my ears, but not everyone is a sucker for anything big and heavy with 'Farnell' written on the side, so I may do something here as well. If you need power supply info please ask.

In the fullness of time there will be a manual for FAT5 much akin to the Multi-Rock manual, but maybe briefer!