All-Analog HF Transmitter

I'm going to Puerto Rico soon, and it just so happens we're staying on a beach facing the continental US. Since we won't have internet nor cell service while there, I thought it'd be fun to send tweets via HF as it would be a good excuse to building some simple analog radios. If I use slow synchronous CW driven and detected by FTDI dev boards and a few lines of C, all I need to do is hack up the radios themselves.

Starting at the TX end, I nabbed a gross piece of PCB from the pile and went to town dead-bugging the transmit chain. Yes, it looks like crap, but if it works as planned I'll design a proper board and cut it on the CNC mill. First I added the crystal oscillator, then its output buffer:



Nice! We can see from the spectrum analyzer that we are indeed getting waves, albeit with some fairly serious harmonics. Now to add a low pass filter:




In the first image, you can see that I added connectors between the previous stage's output and the filter's input so I can sweep the filter independently. In the next image, you can see the filter's response on the spectrum analyzer with the tracking generator providing a flat source. Finally, the last image shows our frequency response of the whole system (osc, buffer driver, and filter) - the filter successfully brought the first harmonic down to -54 dBc. Time to add another gain stage and filter:



In the first pic, I'm again using the tracking generator to sweep only the response of this amplifier and filter (note that I removed the BNC connector I tacked onto the input of the first filter, and connected it directly to the output of the osc buffer). In the second, the whole chain is hooked up end-to-end. It's putting out 27 dBm (1/2W) at ~7.2 MHz, but the last amplifier transistor get warm quickly - looks like I'm going to need to replace that with something beefier.

Update: Unfortunately I got wicked busy with work prior to leaving for PR and wasn't able to finish this. I do hope to pick it up again soon...