[hpsdr] PING Preselector

[George Medakovich]

Oh, Graham, now I am REALLY embarrassed.  You have gone to the trouble of
summarizing the ALEX thread, but my question was really a LOT more naive.

In responding to the request for a ping, I just thought I would screw up my
courage and ask what I suspected was a really stupid question, which is basically,
"when will it be possible to digitize about 30MHZ of bandwidth right up front
and do all of the filtering in software (and get this type of performance)".
I was thinking the answer would be something like "whenever we get
gazillion-bit dac's and teraHz microprocessors", although perhaps a bit more refined.

I've been following the hpsdr listserv like hawk, becuase it's such a great learning
experience and provides the vicarious thrill of being able to watch something really
neat developing and participating by at least buying or building and being a "user".

Thanks so much for your explanation.

73,
George

Graham Haddock wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> 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
> 
> 
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at hpsdr.org
> Subscription help: http://lists.hpsdr.org/listinfo.cgi/hpsdr-hpsdr.org
> HPSDR web page: http://hpsdr.org
> Archives: http://lists.hpsdr.org/pipermail/hpsdr-hpsdr.org/
> 
> 


 1176767681.0