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

To: Amps Amps <amps@contesting.com>
Subject: Re: [Amps] new 2200m/630m amplifier - distorted waveform output
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sat, 2 Jun 2018 13:37:22 -0700
List-post: <mailto:amps@contesting.com>
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

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