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[Xylo-SDR] Architecture review

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

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

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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