| PLL synthesized 70cm CW beacon exciter
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|Assembly source code (42 kB - Text file)||PCB V1.0 - Gerber files (252 kB - zip)|
The exciter is built arround a slightly more advanced ADF-4351 single chip PLL synthesizer rather then (among amateurs) more popular Si-570 or Si-5351 integrated circuis and is capable of providing up to about 250 mW of RF output power, depending on the supply voltage applied to the KON8. However according to the PCB design and it's heat dissipation capabilities it is recommended to stay within the 150 mW power limits. This way the exciter will operate reliably for a long period of time even under the permanent "CW key down" conditions (= 100% TX duty cycle).
The RF amplifier chain utilises BFG198 and BFG135 transistors and has been inspired by a similar circuit used in a low power 10 MBPS 70cm BPSK transceiver designed by Matjaz S53MV. The biasing circuit of the final transistor has been slightly modified to increase the efficiency and decrease the power dissipation. See the page 9 of the article written by the author for more details:http://lea.hamradio.si/~s53mv/nbp/rtx/MalaPSK70.pdf http://lea.hamradio.si/~s53mv/nbp/rtx.html
There should be no obstacles to adapt the exciter circuitry to operate at lower HAM bands as well. Although not verified yet, I guess, the same circuit could be easily used at frequencies down to 50 MHz only by adjusting the values of RF coils and it's corresponding capacitors used in the passband frequency response determining LC circuitry.
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