[hpsdr] ALEX - Call for Comments - II

[S. Nestra]

Why not using Sub-Octave filters like they do in pro. receivers?
This will give you coverage of the full HF spectrum and if you use them 
for transmitting, then your harmonics will also be attenuated.

Just a thought of mine.

Anyway, keep all the good work going! Can't wait to receive my Janus and 
Ozy board and start playing.

Greets,

Stijn Nestra

Graham Haddock schreef:
> ***** 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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