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[AMPS] Band Pass Filtering

To: <amps@contesting.com>
Subject: [AMPS] Band Pass Filtering
From: notawc@juno.com (notawc@juno.com)
Date: Thu, 29 Apr 1999 11:44:12 -0400



I doubt that a bandpass filter between the rig and the amp or after the
amp is going to do much for most distortion caused by solid state amps. 

}   No, a harmonic filter will not  clean up IMD. But a harmonic filter
WILL help clean up an amp that is driven into Class C by its poor bias
design.
A simple test is to monitor the bias as the drive increases. A true AB
bias is approximately  -0.6VDC. Once the bias voltage starts to head
towards zero the linearity is gone. Most ham VHF bricks, with the
exception of Teletec which uses active bias, are at class C at roughly
30% of rated output. 



A bandpass filter may reject some LO leakthru (from band selection
oscillators, not from sideband generation oscillators) or other mixer
products that get into the transmitter output. It will also reject
harmonics of the transmitter or amplifier but not the in-band products of
distortions that are generated by the typical over driven solid state
amplifier. Bandpass filters just aren't narrow enough for this sort of
filtering. 


}   A DCI or similar BP filter will go a long way in improving RX
performance in RF polluted areas and in contesting, roving, etc.


That's my knee jerk reaction. There are plenty of experts on this list
that can straighten me out and I am eager to learn.


73, Harry, W3IIT

=====================================

At 01:27 PM 4/28/99 -0700, you wrote:

>

>Hello Jon et al,

>

>Please permit me to join the chorus of those disdaining the suggestion
of

>placing the BPF IN BETWEEN the exciter and the brick amplifier.  Bricks

>are notorious for their non-linearity and no matter how good yours may

>be, I doubt that it is as linear as your exciter. Read that distortion,

>resulting in out of band energy. However, even IF its linearity is equal

>to your exciter, the resulting degradation will be about 3dB.

>

>Therefore, placing the filter at the input  the amplifier will probably

>defeat much of the effectiveness of it...... this also assumes a flat 50

>ohm system for the filter to perform as it is presumably designed.

>

>GL!

>

>Marv, W6FR

>

>___________________________________________________________________

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You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
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