Once my family went to bed I started working on another prototype.
In this version I used the ClassE software from TonneSoftware to try to determine the proper values.
I used a variable cap for the shut capacitor so that I could try to tune the circuit a bit. The C2L2 tuned circuit had some challenging values so I had to get creative with the capacitors and the inductor was 33 turns on a T50 core. (I probably should have used magnet wire... but it worked ok as-is)
When I first tuned up the circuit it was clear that 10dBm was not enough drive to effectively turn on the FET as a switch. I add a voltage divider to the gate of the FET which improved the performance quite a bit. (A variable resistor would have been a better way to tweak the bias of the FET.)
10dBm of drive + ~1.7vdc gate bias + 13.5vdc supply voltage = ~160mW (boring)
10dBm of drive + ~2.5vdc gate bias + 19.5vdc supply voltage = ~1W (Interesting)
Here is the oscope image with 19.5vdc of supply voltage. The top (channel 1) is the input and the bottom (channel 2) is output with no additional filtering.
- While 1 watt is interesting it did not meet my goal. I had to use 19.5 volts and my goal was 13.5 volts.
- The gate voltage is going to require more investigation. I would like to find a no-tune option using standard value parts if possible. (2 volts looks to be approximate the gate voltage I need to target)
- Running 1W the FET was warm but did not burn my skin. I suspect that I am not getting the proper turn-on and turn-off for the proper Class E efficiency that I should expect.
- The current draw seemed to be ok (it could be better) for what will hopefully become a portal CW radio.
- I am not getting the gain that I am exepcting but I am not sure why.
- I took an hour to build this prototype and the blog entry took almost as long. :-)
73 de NG0R