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

[Jim Thomson]

Date: Sun, 27 May 2018 18:58:47 +0000
From: Manfred Mornhinweg <manfred at ludens.cl>
To: amps at contesting.com
Subject: Re: [Amps] new 2200m/630m amplifier - distorted waveform
output

###  Manfred, what if he used a diplexer ?    IE:   HP filter, with its input
in parallel  with the input of his LP filter.  Output of LP filter goes to ant.
Output  of HP filter  goes to small 50 ohm dummy load.  Then all the harmonics
get terminated in 50 ohms...problem solved.   Without the HP filter, the harmonics 
will all get reflected back into his  200 watt  amp.   The LP filter wont absorb
harmonics, nor will it route then to ground. Instead the harmonics reflect back to the
output of the amp, degrading its imd..and distorting its waveform.  

##  That would be relatively ....simple to implement.   HP filter only has to be a low power
device..and ditto with its  mating small 50 ohm dummy load.   Flex radio does this on their
new  SS   1.5 kw....power genius XL.   7 x high power LP filters..and  7 x mating low power
HP filters... but only one small 50 ohm dummy load.  2nd, 3rd, 5th etc odd harmonics typ are
not suppressed very much, like  10-13 db.    2nd-4th-6th  etc, even harmonics are typ  suppressed
40-42 db.... hence the requirement for the LP filter.  Adding in the low powered HP filter + dl  
cleans up the entire mess.  In older books it was also referred to as an ..absorptive LP filter. 

Jim   VE7RF 




<Roger,

> 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.

FETs are inherently nonlinear, even more so the switching FETs, and 
specially when operated at low bias. So a distorted waveform is to be 
expected. If the distortion is moderate, you might not immediately see 
it on a scope, but it's still there.

FETs, just like bipolar transistors and pentodes, perform as current 
sources. If you have a small amount of distortion in the combined drain 
current, and apply this to a load resistor, the voltage you see on your 
scope only has the same small distortion, which might be too small to 
notice. But your antenna, despite offering a clean load on the operating 
frequency, surely has a high reactive impedance on the harmonics. So the 
small harmonic currents cause large voltage drops across the antenna, at 
harmonic frequencies, and they are phase-shifted too. And that's why the 
voltage waveform you see looks more distorted with the antenna than with 
a resistive load, and probably changes with frequency (varying phase 
shifts between fundamental and harmonics).