[Amps] new 2200m/630m amplifier - distorted waveform output

[Jim Brown]

On 5/26/2018 9:10 AM, Roger Graves wrote:
> 2) The output waveform is close to a sinewave (with no LPF) when driving a 50 Ohm dummy load. However, when driving my antenna, especially on 137 kHz, the waveform is distorted. On 137 kHz the antenna load is 50 Ohm and close to resistive (at 137 kHz) as best as I can measure.

Exactly how did you measure it?   If, for example, the coax is grounded 
at the feedpoint and grounded in the shack, did you measure it with 
those connections in place?

> I tried inserting a 1:1 balun at the output of the output transformer, which had no effect.

Define what you mean by "balun." The word is used to define at least ten 
very different things.

> A LPF cleans up what comes out of the filter nicely. However, the input to the filter (amp output) still looks distorted in the same way. I am using a T format LPF which I believe is the correct configuration for the amp output stage which is (correct me if I am wrong) voltage feed b/c of the center tapped choke DC connection. The input drive to the amplifier looks like a nice sinewave in all conditions.
>
> Do you see a problem operating the amp into a T (inductor input and output) LPF with the distorted output (voltage) waveform? It seems to be running reasonably cool so I don’t see an overheating problem. I would prefer to see a clean output from the amplifier and would like to operate with linear modes occasionally so the distorted waveform leads me worry about IMD (I have not run IMD tests).

A properly operating filter removes the harmonic at the output, but, 
depending on the form of the filter, the harmonic may still be present 
at the input. As Manfred has noted, a Pi section (or Pi-L) is commonly 
used in power amps.

On 5/27/2018 11:58 AM, Manfred Mornhinweg wrote:
>> I tried inserting a 1:1 balun at the output of the
>> output transformer, which had no effect.
>
> No surprise. A balun shouldn't be frequency-selective. 

WRONG -- again, it depends on what you mean by the word "balun," which 
is used to describe at least ten very different things.  A properly 
designed common mode choke wound on a ferrite core, commonly called a 
"current balun," SHOULD be VERY frequency sensitive, in that it should 
present a very high resistive impedance at the frequency of interest. To 
accomplish that, the number of turns and the core material must be 
carefully chosen to put the self resonance at the frequency of 
interest.  I've recently studied and found suitable designs for the new 
480 kHz band, but not for 137 kHz.

The need for resistive choking Z is simple and fundamental, but not as 
well known as it should be. A choke whose impedance is primarily 
reactive, whether L or C, can cancel the common mode impedance of the 
feedline that is of opposite sign. The common mode Z of the line is 
simply it's behavior as an antenna, and it's electrical length is that 
of the conductor(s) with their outer insulation, typically 0.98 -0.99, 
depending their diameter.  If the Rs of the choke is small as compared 
to Xs, cancellation causes common mode current in increase rather than 
decrease.

73, Jim K9YC