On Wed, 28 Apr 1999 13:27:36 -0700 marv gonsior <w6frmarv@juno.com>
writes:
>
> 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.
And here comes another explanation.
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.
Explain that please? The output devices usually come from the same
company, Motorola, and except for the power gain are darn near identical.
Dont blame the device if its due to poor execution.
The REAL problem with most brick amps is that the bias sucks. Typical
over the counter bricks use passive diode bias. Take apart most bricks (
Teletec is an exception) and monitor the bias line; I'll bet that it
drops from the static level of .6V or so to zero at about 20-30 % of
rated drive/power.
Changing most bricks over to active bias is real simple and cheap. A pair
of transistors an a few other Mouser or RatShack parts and away you go.
This has been a hot topic on VHF and above ever since N1DPM published the
article in the NE Conference Proceedings back in '96.
What works on VHF and above is applicable to HF, CB, etc.
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.
Huh?
I cant make much sense of the above statement but if a filter with equal
50 Ohm in and out impedence is placed at the input of an amp it will not
only tend to improve the input match but will also perform its intended
function.
>
> GL!
>
> Marv, W6FR
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