[hpsdr] ALEX - Call for Comments - II

Tue Mar 6 14:44:36 PST 2007

how about 2 cards one for ham bands and the second for what ever.  use one 
or both, which means we need a way to couple two atlas cards for expansion 
space. hope this is good........
tom w0kgw at citlink.net

----- Original Message ----- 
From: "Graham Haddock" <grahamh at verizon.net>
To: "'High Performance Software Defined Radio Discussion List'" 
<hpsdr at hpsdr.org>
Sent: Tuesday, March 06, 2007 3:27 PM
Subject: [hpsdr] ALEX - Call for Comments - II

> ***** High Performance Software Defined Radio Discussion List *****
>
> Hello all:
>
> Well, the ALEX "Call for Comments" sure stirred up the
> email reflector.
>
> Thanks to all for the inputs, good comments and thoughts.
>
> There seems to be a few fundamental philosophy points that
> need to be cleaned up, before the design requirements can
> get locked down.
>
> I am laying out the philosophy behind what you see so far.
> I am not necessarily defending it, just letting you know
> what it is.  Feel free to rip it up.
>
>
> 1.) Name...
> The Greek spelling was chosen, because he was a Greek God.
> We can switch to the Latin spelling.
> I don't know if "Alex" would be offended or not.  ;-)
>
> As to the suggestion of Cerberus, I decided I would
> rather be the gatekeeper of Mount Olympus than Hell.  ;-)
>
>
> 2.) Control...
> This is a receiver front end preselector.  I would prefer
> to not have any in-band oscillators, clocks, or noisy high speed
> buses appear on the card.  For that reason I would propose that
> this be a "dumb" slave card, controlled by a three wire SPI bus,
> which would be filtered upon entrance.  I2C would also work.
> Phil H wanted I2C, I personally prefer SPI, Lyle seems to like SPI.
>
> It could be controlled by any other (one) CPLD on the Atlas bus.
>
> Space is also an issue. The universal CPLD bus interface takes a
> noticeable portion of a 100mm x 120mm card.  I am mostly worried
> about noise. (Read Henry's comments again.)
>
> From the great response to the "Call for Comments", this will be
> a much reworked card, so the mapping of filter selection to
> operating frequency will need to be flexible.  Some of the people
> making comments have CPLD/programming capability.  I suspect that
> the majority of the 500 users will not. (I don't.)  Therefore doing
> the frequency-to-switch-control mapping on the card seems like
> a potential problem.  My thoughts are that the RF switch-to-
> operating frequency mapping should occur in a user-editable
> (text file) table up in the application control software
> preferences, requiring no re-programming changes, anywhere.
> Or something like that.
>
> I humbly apologize in advance for proposing to put a card in
> a Software Defined Radio, which has no software on it.  :-)
>
>
> 3.) Filter Design and selection...
> This design is intended to be a ham-band preselector, with
> accommodations for other uses.  By swapping coils and
> capacitors, with ten or more filter sections on the card,
> all kinds of things are possible.
>
> I assume the majority of the users want good ham band performance
> and casual SWL, WWV, and experimental capability.  The
> native performance of the underlying SDR should be good
> enough for the "casual" and incidental uses with the preselectors
> bypassed. [ Phil H, please comment. ]  Any serious or focused
> other uses can put a dedicated filter section in the user
> defined spaces, or rework the whole filter pack.  The PCB
> layout will accommodate third order (three inductor)
> bandpass or low-pass filters in each section.
>
> In the next day or so, I will post on the Wiki a package of
> the values and filter performance plots for the ham band
> filter sections per the Pic-A-Star tunings.
> They were originally designed in ELSIE with settings:
>   Mesh capacitor coupled bandpass
>   Chebycheff, 3rd order, 0.01 dB ripple,
>   I assumed an inductor Q of 40 for my plots.
>
> Download your own copy of ELSIE and go for it.
> http://tonnesoftware.com/elsie.html
> The free "student version" will easily handle these filters.
>
> The Chebycheff gives best far-out-of-band rejection.
> (These single band filters are typically 50 to 60 dB down
> at the adjacent ham band, and keep going.)
> The Cauer design gives better rejection close in, but
> at the expense of far out of band rejection, (for the
> same number of inductors.)  Considering the
> ultra-broadband nature of the underlying SDR, I think
> the Chebycheff filter proposed might be appropriate. It
> will certainly be easier to align, with an all-peak-at-the-
> center-frequency design.  I see some wisdom in what the
> Pic-A-Star designers did.
>
> As far as smart tracking filters, that is certainly possible,
> and is included in one of the commercial SDRs just announced.
> They seem to operate like smart antenna tuners, with switchable
> binary trees of capacitors and inductors.  The one I saw
> looked like a single order bandpass filter that took the
> same physical space as the entire ten section third-order
> filter pack proposed here.  So no where near the performance
> in the same amount of space.  But, it would give the
> software guys a lot more to do designing tuning algorithms
> than this proposal, and guarantee that there was some software
> on the card. ;-)
>
> I propose we do something like this first, which should be
> relatively simple and fast.
>
> As far as transmitter power levels, the RF switches will
> limit power to something below 1 watt (0.1 dB compression point).
> The inductors might limit below that power level.
> Some measurements of a built design will be necessary.
> Don't count on this to be more than a low level exciter
> filter.
>
>
> 4.) Physical design...
> If we go with SPI or I2C control, I intend to try to make
> this a two sided PCB design. The inductors are thru-hole
> shielded cans, about 1/2 inch square, three per filter
> section.  Ground plane, inductors and control on the top side.
> RF interconnect, and tuning chip caps on the bottom side.
> Option to put a shield (0.031 in. one sided PCB) over the entire
> bottom, if necessary to control noise pick-up.
> I'll see how many filter sections I can fit on a
> 100 mm x 160 mm card.
>
>
> 5.) Preamp...
> I heard no demand for adding a preamp to this card.
>
> --- Graham / KE9H
>
> ==
>
>
>
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