[hpsdr] Fwd: ALEX - Call for comments

[Larry Gadallah]

Forwarding this because I forgot to CC the list :-(

---------- Forwarded message ----------
From: Larry Gadallah <lgadallah at gmail.com>
Date: Mar 6, 2007 1:39 PM
Subject: Re: [hpsdr] ALEX - Call for comments
To: Graham Haddock <grahamh at verizon.net>


My comments on Alex are below

On 3/5/07, Graham Haddock <grahamh at verizon.net> wrote:
> 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?

I feel that only covering the (existing) ham bands is a big loss.
There should be "N" (where N >= 10) filters with overlapping coverage
from LW to VHF (60 Mhz). Doing anything else simply reduces the target
audience for the whole system. If an outboard box is needed, no
problem, all that is needed is a clever/generic way to bring out the
control signals from the Alex card. Perhaps enough control/addressing
lines can be brought out to allow an auto-tuner like functionality
with an external box if someone wants more front-end selectivity than
can be fit into the current form factor.

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

No.

>
> 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.

This makes sense to me.

>
> 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.

Yes!

>
> 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?

How about adding some "Rx in/Rx out" and control lines to allow an
outboard preamp or other device to be put in line as needed?

>
> 6.) Any comments you want to make?

With respect to Rx protection, I think that the tried and true Neon
bulb and static bleed resistor are proven, cheap, and linear for
almost all conditions.

Cheers,
--
Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B


-- 
Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B

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