[hpsdr] PING Preselector

[Graham Haddock]

George Medakovich wrote:
> I'm not complaining at all.
> It was just an expression of curiosity.
> I thought maybe somebody would chime in with the specifics ;-)
>
> George
George:
Current status of ALEX is block level diagram on the ALEX wiki page.
All (50 Ohm) RF switches shown are solid state.

1.8 to 30 MHz tunable filter being evaluated is two resonator,
capacitor tuned, bottom inductor coupled.  Phil H. is preparing to
evaluate the size inductor needed to get the IP3 of the filter up
where the filter won't degrade Mercury's IP3 performance.

Most likely topology has solid state switching of the two sets of tuning
capacitors in some kind of binary value tree.  Multi-tapped
(tuning) inductor switched by relays, and bottom coupling
inductors switched by relays.  The reason for using the
relays is that the ON resistance of either the Hittite switches
or the alternate FETs we will evaluate is in the 1.5 Ohm range,
and that is too high for the coupling inductors, and the max
voltage of the Hittite switches is not high enough for some
of the voltages that might be seen in a multi-tapped
tuning inductor when it works as an auto-transformer when
certain taps are selected, at maximum signal level.  So,
for tuning, something like 16 solid state switches and
12 relays.  I promise that we won't use a relay where
a solid state switch can be made to work.

Depending on the thickness of the toroids chosen to meet
IP3, we still may be able to fit into the ATLAS card spacing.
Lots of things still to learn.

I'll post some sample filter passband shapes and schematics as soon as we
choose the filter inductors, and switches for the capacitors, and things
are a little less fluid.

--- Graham



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