[hpsdr] ALEX - Call for comments

[Graham Haddock]

All:

Hello, I am Graham, KE9H, the project leader and PCB layout
person for ALEX which is the RF preselector filter pack for use
with Mercury or other receivers.    Phil, VK6APH, is the designer.

ALEX is a multi-band receiver preselector intended to
reduce the level of out-of-band signals at the input of a
receiver, but it can also be used as a low level transmitter
filter.

ALEX is an Atlas bus compatible card.  The Atlas bus is used
for power and control, and RF in and out will be on coax
connectors on the card, separate from the Atlas bus.

The card is relatively "dumb" compared to the other HPSDR cards.
It will set the switches as it is told, via an I2C command
coming from the bus.

Switching will be via Hittite HMC226 RF switches, rather than
relays.

If I try to stay with a 100mm by 120mm card, (same size as
Janus) then I suspect that we will be limited to about ten
filter sections.

I propose that these ten filter sections would be:
160 Meters
 80 Meters
 40 Meters
 30 Meters
 20 Meters
 17/15 Meters
 12/10 Meters
  6 Meters
User Defined #1
User Defined #2

There is also a bypass feature, providing no
filter or frequency selectivity.

Each "user defined" option would be an un-populated filter area,
with circuit runners laid out to use the same family
of inductors and capacitors as the others, and could be
populated by the user to tune the 60 Meters ham band, Medium
Wave, AM Broadcast, 160 KHz or whatever. A three section
bandpass or lowpass could be placed in each "user defined" area.

A block diagram of Alex and other information is on the Wiki.  
The current plan is to use an I2C I/O expander chip to control
the RF switches, not the full CPLD bus interface as shown.

http://hpsdr.org/wiki/index.php?title=ALEXAIRES


Specific questions for the group.

1.) Is ten filter sections enough?  If more is needed, will
there be any issues going to a longer card, such as the 160 mm
length card proposed for Penelope?

2.) Does anyone care if I put SMT chip components on the
back side of the board, and perhaps a shield?

3.) Any preference for SMA verus BNC connectors?  I propose
to use BNC for "external" connections such as antenna(s), and
SMA for "internal" or card to card connections.

4.) Should we allow for multiple antenna inputs? - we could
select each one on a Tx/Rx and band basis using a GUI on the PC.
 
5.) Using Mercury as a base line, we have an MDS of -140dBm in
500Hz without Alex in circuit. Alex will degrade this by say
2.5dB on 6m due to the loss through the filter and RF
switches.  Is an MDS of -137.5dBm acceptable on 6m or should we
consider adding a low noise preamp after the 6m filter?

6.) Any comments you want to make?

Best regards,
--- Graham

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