Dual-Band Mini Transmitter

[NZ0I]

The dual-band transmitter project using the Receiver Development Platform boards has reached the point where it is ready to move on to a prototype design. Starting with the last experimental design, which worked reasonably well on the test bench, I have attempted to minimize the design. Parts count has been greatly reduced, and some features have been dropped to further reduce size and cost. It looks like the entire transmitter, except for the power supply, might fit on a single PCB designed to fit in the original watertight chassis box.

The schematic for the minimal transmitter design is available here: https://drive.google.com/open?id=1yk6tQTWNo6adcF94_UHlC6f1EtOyIl8k

Other than the FET used for the 80m power amp, there are no discrete transistors in the design.

Questions or comments are welcome.

[Grunfov x]

Very interesting project. I've few questions

- how do You key this minimal design transmitter since You omit TS5A3157 from minimal design

- what is the supply voltage of IRF for 1W power

thanks, and best wishes for Your project

[Charles Scharlau]

Hello Grunfov,

Thank you for the questions.

The TS5A3157 in the 80m transmitter design was there to provide a simple way to key RTTY. The new design does not support FSK modulation, at least not using that approach. The processor can turn the SI5351 oscillator off/on using I2C to implement CW. Theoretically, the processor might accomplish RTTY in a similar manner, but I haven't looked into whether the processor could change the frequency quickly and cleanly enough over I2C. Currently, I don't have plans to support RTTY.

The correct voltages for achieving transmit power will be determined experimentally. I've taken a few measurements with experimental hardware, and if I recall correctly it was around 9V for 1W out. But I have made some changes to help ensure the FET is driven with a perfectly symmetrical square wave - the flip flop will do that. That might help improve efficiency. Improvements to the LPF should also improve signal purity and matching to a 50-ohm antenna. That might also help matters.

73,

Charles

NZ0I

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[Charles Scharlau]

Also...

The PWR_FET_CTRL signal could be used to implement CW by configuring JP101 and JP102 appropriately. I'll see whether I2C or PWR_FET_CTRL is the best option.

[NZ0I]

The schematic for the transmitter power supply has been posted, along with BOMs for both the transmitter and the power supply.

A new approach for the schematics is being used: the schematics now contain all the project parts, including parts that don't go on the PCB and aren't even electronic parts, like the chassis box and screws. The reason for this is that it allows for complete BOMs to be generated directly from the schematics with a click of the mouse. Even better, CSV format BOMs can be generated just as easily and can be uploaded to Digi-Key to place an order. 

2-Band Transmitter design: https://drive.google.com/open?id=1yk6tQTWNo6adcF94_UHlC6f1EtOyIl8k

2-Band Transmitter BOM: https://drive.google.com/open?id=1No_7dxBfy28bRe8x3MDbI4ZtFgoUCT7w

Power Supply design: https://drive.google.com/open?id=1-MaFYPLmGoDe8K0oQFh0_NOGJtJ3i_w2

Power Supply BOM: https://drive.google.com/open?id=1w7QGjfE_yZBG3j9xHcF43AT6-T33CHuz

Note: the BOM documents are HTML files. They can be opened in any browser, but Google Docs doesn't handle them like HTML, instead displaying them as text. You can download them and double-click to view them in a browser. I'll see if I can find a better way to present them.