What is the value of the lowest Q?
What harmonic suppression are you aiming for? It should be 43dB + 10 log P,
without exceeding 50dB. If there's a tuner on the antenna, that may well mean
you don't need the low pass filter.
73
Peter G3RZP
========================================
Message Received: Nov 11 2012, 07:46 AM
From: "Carel, pc5m"
To: "'AMPS'"
Cc:
Subject: [Amps] low Q output matching
Hi, I was wondering what else, besides bad harmonic suppression, will happen
when I design my (single band) output matching network (PI-L) with low Q
(lowest Q needed to get the match done) ?
When using low Q design I will get
- much smaller (variable) capacitors, especially for the bigger load C, so
easier to get components
- broader no-tune range
-smaller RF stress on the components
As an example for a 160mtrs amp, with Rload of 1700Ohm and intermediate
impedance around 330Ohm:
With Q of 10:
CLoad =1200pF
L-l=10uH
Ctune=293pF
Lpi=33uH
With lowest Q:
CLoad=400pF
L-l=10uH
Ctune=132pF
Lpi=33uH
When you plot both matching networks in a Smith diagram you see immediate
the difference. Low Q is much more straightforward.
To get the needed harmonic suppression a lowpass filter with fixed
components is easy to construct (first harmonic at 3,8MHz needs 25dB and
second one 20dB to get the same values as higher Q design). But maybe I have
overlooked something . Thanks for your ideas
73's Carel, pc5m
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