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Audio distortion on the 8 FET 75 meter rig




 
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Author Topic: Audio distortion on the 8 FET 75 meter rig  (Read 911 times)
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ka1tdq
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« on: October 22, 2016, 10:32:39 PM »

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


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ka1tdq
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« Reply #1 on: October 22, 2016, 10:47:35 PM »

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

Jon


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ka1tdq
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« Reply #2 on: October 22, 2016, 11:14:01 PM »

...and here's both drains.

Jon


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ka1tdq
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« Reply #3 on: October 23, 2016, 12:24:52 AM »

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
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