[hpsdr] ALEX - Call for comments

[pvharman at arach.net.au]

Hi Lyle,

Thanks for the feedback. 

Both you and Graham make a good case for using SPI to control the board. Graham 
also is concerned about the ~1MHz clock that some I2C chips use being so close 
to the filters. 

What SPI chip do you recommend - Graham suggested an 74HC594.

With Mercury or a QSD (with the NF problem fixed) we are not going to need a 
preamp on Alex for HF band work but we may need one on 6m. Not being a 6m 
operator I wanted feedback from those operators before adding that to the 
circuit. If we do want a 6m preamp then it would be a narrow band one optimized 
for this band and not a GP wide band amp.  Perhaps one of the 6m operators 
could recommend a high performance design for us to consider.

Alex is intended to be inserted between the TxRx relay and the Rx front end 
since it can take about 2W before the RF switches compress. It is not intended 
to take the roll of the main antenna relay. 

I like the idea of a second (or more) antenna input and perhaps an separate 6m 
antenna input (via a preamp if considered necessary).

Again, great feedback - keep it coming folks!


73's Phil...VK6APH 




Quoting Lyle Johnson <kk7p at wavecable.com>:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hello Graham!
> 
>   > 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.
> 
> Hmm, is it possible to also include SPI as an alternative selection 
> method?  Looking ahead to Sasquatch, and not sure how the mix of SPI and 
> I2C peripheral pins will work out, so I'd like to have both options if 
> feasible.  SPI is very easy to implement in a CPLD or FPGA, and requires 
> few resources.
> 
> 
> > 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?
> 
> Penelope may even be 185 mm :-/
> 
> > 2.) Does anyone care if I put SMT chip components on the
> > back side of the board, and perhaps a shield?
> 
> I suggest that the back side of the board be used for routing control 
> signals to the Hittite switches, and the front side of the board only, 
> or mostly, carry the RF paths.  It is easy to get stray coupling to the 
> control lines and really limit the filter performance.
> 
> > 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.
> 
> Sounds good.  Also, "TMP" connectors maybe suitable for between-board 
> signals, and sch connectors and cables, while less common, are quite 
> inexpensive.
> 
> > 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.
> 
> Hmm, I wonder if we should keep the board to mostly an internal 
> component as opposed to a complete RF front end.  OTOH, if it is like 
> the RFE board that Flex made, we'd probably want a strong preamp in the 
> receive direction and a bypass in the Tx direction.
> 
> > 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?
> 
> If a preamp is included, it should work with all the filter sections, if 
> possible, and be bypassable.  It also needs to be very strong so it 
> doesn't degrade the IMD3 and BDR3 (or the DDC-based Rx equivalent).  If 
> the filters are used to drive a QSD rather than Mercury, having a 
> controlled, known source impedance may be helpful (thinking of the RFE 
> again).
> 
> Thanks for taking this on, Graham, and welcome aboard!
> 
> 73,
> 
> Lyle KK7P
> 
> 
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