Jon says:
>Why can't the bandpass filter knock out the amps harmonics?
Firstly, if the amplifier is more than 25 watts output and meets the FCC
requirements [Part97.307(e)] it's already got harmonics at least 60dB down,
and not exceeding 25 microwatts. So how much practical benefit comes from an
extra filter? Harmonic removal is best done with a low pass filter, since
the working Qs of the branches is generally less, leading to lower
circulating currents and thus losses. (Not necessarily applicable in a
Cauer-Cchebycheff [aka elliptic function] filter, or certain m-derived
filters where there are resonators at harmonic frequencies to produce poles
in the response) However, where there's already a filter in place, as there
should be in a brick, the amount of harmonic power is low enough to allow a
Cauer-Cchebycheff, at least at amateur power levels, without worrying about
circulating currents in resonators. However, you do need to watch the
variety of filter since the ones that give maximum harmonic attenuation for
any particular number of poles have the highest SWR in the passband.
Running up the Q runs up the circulating current, and thus the losses. Every
time Q doubles, the circulating current doubles, and the losses go up 4
times. However, you need to run up the Q to narrow the response. If the
trouble is radiated close in phase noise and spurs, then you need a bandpass
filter between exciter and PA (at low power level) while a bandpass filter
after the PA helps with wide band PA noise and AM to PM conversion products.
Also if you've intermodulation products generated in the PA from a nearby
transmitter - which is incidentally, more of a problem with solid state than
tube PAs.
A bandpass filter should give you a nice warm feeling that you're squeaky
clean - even if it's not strictly necessary.
73
Peter G3RZP
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