[Xylo-SDR] Architecture review

["Phil Harman" <pvharman@arach.net.au>]

I'd suggest we take a little time and review the architecture we are 
proposing so far.

I think that Phil C summed it up very well as follows:

1. We have a BUS/USB COMMs board to handle communications over the bus
and to/from USB.  The main function of this board is to control the
bus and handle communications over the bus.  This would be a Cyclone
II - Cypress FX2 based board like what Leon is designing.  It could
take on other roles as LE space permits.

2. Since the Cyclone II can do LVDS, maybe we should be looking at
having only LVDS signals on the motherboard bus.  This would require
that we either use a LVDS capable FPGA/CPLD or a LVDS
serializer/deserializer chip on each plug in board to talk to this
bus.

3.  The only clock signals on the bus are for driving the bus
communications.  All other clocks needed for the plug-in boards are
generated locally on that board or use SMA/SMB connectors to jump to
other plug-in boards.

4. In certain applications the requirement for a FPGA/CPLD or LVDS
serializer/deserializer could be relaxed and the BUS/USB COMMs board
could be reconfigured (after all it is a FPGA) to give single ended
signals... such as the I2S signals for comms with the audio ADCs and
DACs.

My comments.

The research that I have been doing on NCO's (DDS) are for use inside an 
FPGA interfaced to the LTC2208. I was not intending adding a D/A to the 
output to give us a Tx signal. Phil C suggested using a AD9954 for this and 
any analogue front ends we may use.   I agree that the level of spurs we get 
from the NCO are likely to be similar to the AD9954, the latter will be much 
easier to implement.

I also support the idea of multiple FPGA's. One that talks to  the outside 
world (USB, PTT, key etc) and  the backplane - i.e. just what Leon is 
designing now.  If we need others then lets add them on a board-by-board 
basis  e.g.  the LTC2208 should have its own dedicated FPGA and decimate 
down to low frequency signals to pass over the backplane.

Multiple high frequecy clocks and signals over the backplane seems like 
asking for trouble.

At the moment we have two boards plus the backplane thought throught.

1. USB etc and backplane interface board with Cyclone II FPGA.
2. Sound card replacement board - Wolfson  A/D (or TI etc) for Rx I/Q 
signals and TVL320 microphone/line input.  Dual  D/A converters, 
speakers/headset and I/Q to  QSE.  Looks like this board could also do  CW 
Tx  ( I/Q signals  and keyer plus sidetone ) if wanted.

Future boards
3. LTC 2208 with FPGA for  Rx. NCO inside FPGA with  D/A converter to 
provide Tx local oscillator.
4. Ham band  BPF board, use bus switches so can be switched into Rx or Tx 
chain. HPF's, LPF's, switchable attenuators and preamp(s).
5. Rf board  - QSD/QSE with AD9954 DDS  and say 1W output  ( only use QSD if 
we solve  NF problems). PIN diode or bus switches for fast Tx/Rx swithcing.

We need to be able to phase modulate the NCO/DDS on Tx at audio rates so we 
can use a high efficiency PA design later on.

The LTC board could form the basis of a nice digital scope or spectrum 
analyser.

How are we doing?

73's Phil...VK6APH







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