-----Original Message-----
From: John Fielding <johnf@futurenet.co.za>
To: Steve Thompson <rfamps@ic24.net>; amps@contesting.com
<amps@contesting.com>
To: <amps@contesting.com>
Date: 24 March 2000 08:06
Subject: Re: [AMPS] 3/2 law, DAF, other musings
>Steve Thompson wrote:
>----------
>> Subject: [AMPS] 3/2 law, DAF, other musings
To: <amps@contesting.com>
>> Date: Wednesday, March 22, 2000 9:58 AM
>> Close in IMD is caused by odd order distortion an an amplifier
>> characteristic. Even order (eg square law) produces IMD at harmonic
>> frequencies, which get filtered out.
>>
>> Steve
>>
>However you need to ask yourself what happens to the even harmonic power
(energy) when it
>arrives at the harmonic filter?
>
>If the filter is an absorptive type then the vast majority gets dumped into
a resitive
>load and is safely dissipated.
>
>BUT very few harmonic filters are of the absorptive type.
>
>The type most often used is a "reflective type" i.e. the energy is
rejected by the
>filters reactive elements and returns to the source (the anode tank
circuit). Depending
>on the phase and amplitude of these harmonic energies they can either add
or subtract
>from the energy at the anode tank. It is quite possible to either increase
or decrease
>the levels of odd order IMD products by this process. A trick which I have
demonstrated
>to tech students many times!
>
>Sure to start another argument!
Not with me :-) I see a similar effect all the time, where maximum output
power changes whent he filters are added. It's especially a problem with
wideband solid state amps where the harmonic frequencies fall within the
bandwidth of the amp.
Choosing the right filter topology and selecting the cable length between
the amp and filters can make amps look very good on the bench.
Steve
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