[hpsdr] ALEX - Call for Comments - II - Clarification

[Graham Haddock]

Clarification - Under Filter selection and design
I am proposing that we do ALEX first, not the
smart tracking filter. --- Graham

=====================================================

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 ALEX 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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