A homebrew Class D TX with transformer coupled modulator
WO1U:
Hey all,
I just finished building a transmitter, and I thought folks might find the attached article interesting. It was a quick build, and not terribly expensive, and so far, works well.
I hope I am posting this in the right place...
73
Mike
WO1U
W4NEQ:
An inspired project! I always wondered about using certain flavors of power transformers for modulation, but, as you point out, an early experience showed there to be problems with frequency response. Magnetics is a weak area of mine, and I would never have guessed that the torroidal style core designs would be better. Are variacs wound on similar cores? So, what do you suppose is the difference? Is it a homogenous ferrite core?
I share your frustration locating good RF caps.
Chris
WO1U:
Hi Chris,
I have never looked at how the core is constructed in a variac, and I am unsure whether it is the tapewound type or some other construction. You could always take the armature off and wind another winding on it make it into an isolated transformer, load it up to see how the frequency response is. It would be interesting. If you do that please post your results.
Yes regarding the caps, high current RF caps seem to be a dying breed. I mean you can buy them new...for like ~$50 each, but it's not a $50 problem.
73
Mike
WA1GFZ:
wow an H bridge on 75 meters! That is cool.
Variac core is a tape core.
W3GMS:
Hi Mike,
Great work on your Class D rig using an H bridge topology. I have designed a lot of high power switching power supplies using the H bridge but never tried it as a class D RF final.
Some time ago, Stu-AB2EZ did quite a bit of study on using toridal power transformers as modulation transformers and they worked extremely well.
Here are some former post to Stu's study:
http://amfone.net/Amforum/index.php?topic=16683.0;wap2
Jay, W1VD has done a lot of work with Class D final amplifiers. If you have not seen his website, here it is:
http://www.w1vd.com/375wattclassD.html
Regards,
Joe, W3GMS