homebrew mept – part 1

I’m waiting on parts to finish up my bitx 20 project so I started working on a home brew mept (manned experimental propagation transmitter).

At first, I was going to build a simple crystal controlled transmitter, but I had an extra AD9850 module laying around and decided to go with that.

This is the AD9850 module that I started with.

In researching the module, I found that the module, as designed, is not a good fit for what I am trying to do, but with a few modifications, it should work okay. This site has a lot of good info on the AD9850. The modifications and filter that I am using came from there.

First, I removed R12. The module feeds the sine wave back into the chip through R12 to create the square wave outputs. I’m not using them so I removed R12 to help with any noise that may have been generated from the square wave outputs.

Next, I removed some of the filter components. R4, C1, L1, C2, L3, C6, R5, C7, and R9 all need to be removed.

Next, solder a jumper wire from the hot side of R4 to the hot side of R5 to bypass the rest of the filter.

Once the modifications were complete, I started building a new filter. I used this filter from here.

I built the new filter on a piece of perfboard and added a small amplifier with two 2n2222s.

As it stands now, the transmitter is working. I can do CW, FSK CW, and QRSS.

Next, I will be adding more digital modes and GPS/RTC timekeeping. I will probably make a few plug in filters so I can switch bands easily and then build some sort of enclosure.