The AM Forum

I was having a problem with the Chinese DDS VFO on my 75 meter 8 FET-er.  When switching from transmit to receive, I had to turn down both drive pots on it so that I wouldn't hear the oscillator running on the receiver.  It developed a problem in that the amplitude got stuck on full even when set to zero.  Cheap Chinese pots.  Anyway, to get around this I would just simply switch the VFO on and off.  

I decided to feed over the output of a new VFO that makes things much simpler.  I just hit a button to turn off the output and I can change frequency easier.  My problem now though is that I can't hit the 150% modulation like I did before, my audio is a little distorted according to the playback of my REA modulation monitor, and my efficiency went down.

According to my calculation of efficiency under the old VFO, it was 99%.  It was too good to be true, but it was high nonetheless.  Now, my efficiency is 88%.  

I know this discussion will turn to waveforms, but I do have about a 40% duty cycle going into the gates (as before) and it's tuned about the same as before.

Just a shot in the dark for all you experienced guys out there, what's my problem?  I'm also using Heising modulation.

Jon

...here's the drain and gate for side 'a' at 400 watts carrier.

Jon

...and here's both drains.

Jon

Ah, never mind... I think I got it.

There's 10 feet of coax in between the VFO and the chassis of the transmitter.  From there, there's a little less than two feet of twisted pair feeding the IXDD bus.  I had just attached the twisted pair to the BNC center and ground. 

That must've made things unhappy going from unbalanced to balanced feedline.  I put a 1.2k resistor on each BNC pin to ground and the positive peaks came back.  Yaye!  I felt inadequate there for a few hours with wimpy 100% modulation. 

Jon