[hpsdr] ALEX - Call for Comments - II

[Philip Covington]

On 3/6/07, Graham Haddock <grahamh at verizon.net> wrote:
<snip>
> 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.

As soon as you run SPI or I2C to the board you have a clock signal on
your board.  You don't need an on board oscillator when using the CPLD
so it is equivalent to using a dedicated SPI or I2C expander chip in
that regard.  I believe the CPLD's output slew rate can be programmed
to reduce noise also.

> 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.)

Please be aware that you are not constrained to only a 100mm x 120mm
card - you can go longer like 160, 180 or 220 mm (or anything in
between).

If you put a dedicated SPI or I2C chip on board you will also have to
add inverters for each BYPASS and /BYPASS control line to the HMC226
pairs.  The combination of a I2C or SPI expander and a inverting
buffer will take up about the same room as a EPM240 CPLD.  One EPM240
CPLD will allow you to configure the interface to the board to be I2C,
SPI, RS232, line-per-band or whatever someone can dream up.

Bringing most or all of the CPLD pins to the DIN96 connector will
require some more room, I agree - but it is well worth it is the
future reconfigurability it offers.

>  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.)

Soon there will be the capability of programming the CPLDs on each
board with the OZY over USB, so the problem of programming cables will
be solved.  You can build a simple parallel programming cable for
under $5 right now.

> 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.

What if there is not really any "application control software" in the
case of a stand alone system?

> I humbly apologize in advance for proposing to put a card in
> a Software Defined Radio, which has no software on it.  :-)

That is normal since the FPGA and CPLDs do not really have any
software in them either.

>
> 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.

Even more things will be possible if a CPLD is used instead of a
dedicated SPI or I2C chip :-)

<snip>

> 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.

Are you *sure* you don't want to do it in four layers?   I'd rather
see a solid ground plane layer in there than just a top layer ground
plane pour of copper.

>
> 5.) Preamp...
> I heard no demand for adding a preamp to this card.
>
> --- Graham / KE9H
>
> ==

Phil N8VB

 1173222421.0