Jim,
On the comparison between a PI and a PI-L network:
PI needs to be run at a higher Q than the PI-L for the same stopband
attenuation resulting in sharper tuning as you QSY.
PI needs 1 fewer poles on the bandswitch (obvious).
PI has slightly lower inductance value and slightly higher capacitance
values. This becomes more pronounced when comparing a Q=12 PI to a Q=8
PI-L (which has better stopband rolloff than the PI).
The Q=12 PI has about 75% higher current in the primary inductor than
the Q=8 PI-L.
Both have the save Vpk on the tuning cap. but the PI-L has about double
the voltage on the loading cap. (which is still a fairly voltage).
To me the need to retune more often on the PI network is a big deal but
based on how you operate it might not be.
73,
Larry, W0QE
On 6/18/2012 9:56 AM, Jim Garland wrote:
> I've been noodling around various tank circuit possibilities for a 160m
> monoband amplifier. A pi-L network isn't very practical for an 8877 because
> the relatively high plate impedance mandates very large values of C1 and C2.
> For an 8877, a simple pi-network is preferred. On the other hand, a Pi-L
> makes a lot of sense for, e.g., three 3CX800a7s or GU-74Bs, because the
> combined plate impedance is under 1000 ohms and the required capacitance and
> inductance values are quite reasonable.
>
> I know that good design practice is to orient L1 and L2 in a Pi-L at right
> angles to each other (or to use a self-shielding toroid for L2), to minimize
> mutual inductance between the two coils. However, I don't really understand
> why this is so important. In principle, it would be very convenient to use a
> single coil whose inductance is L1+L2, with a tap for connecting C2 at the
> junction point between L1 and L2. I'm sure that the mutual inductance would
> invalidate this approach, but I don't understand why. Can somebody who's
> thought about this issue clarify it for me. Thanks!
> 73,
> Jim W8ZR
>
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