I believe that the architecture of an SDR should begin by looking at the most basic hardware required to have a top tier transceiver. I would list these as:
1. Power Supply
2. Antenna connector(s) to a multi band receiving RF deck containing pre amp(s), mixer, and IF transformer.
3. Antenna connector(s) to a multi band transmitting RF deck containing IF transformer, mixer and power amplifier(s).
4. High stability oscillator(s) feeding the mixers.
5. Very High speed A/D converter capable of digitizing the output of the receiving IF transformer
6 Very High speed D/A converter capable of providing the analog drive voltage needed by the IF Transformer on the Transmitting deck.
7. Connector to Good quality audio output amplifier
8. Microphone and other inputs connector.
9. Reasonably fast D/A converter to provide processed analog audio to the output audio amplifier.
10. Reasonably fast A/D converter to digitize the rig's input voice, data and imaging sources.
11. Metal box to provide shielding, component protection and cooling, and (BEAUTY?)
12 And finally appropriate interfacing connectors to assorted attachments like PCs, 2 KW Linears, Antenna tuners and antennas, Down converters and Transverters, etc, and controls to function in logical ways that the operator is used to having in an upper tier Transceiver.
13. The most difficult to define component is the very complex DSP hardware that handles all this software defined magic. I think that it is best implemented as a super DSP system consisting of several reasonably performing subsystem DSP units.
Activities performed by the DSP super block (The Transmitting DSP is very minor in computational intensity compared with the extremely intense Receiving DSP.)
Receiving DSP IF filtering, image rejection, and mode definition and implementation with maximal quantization levels.
Transmitting DSP wave shaping, image rejection and mode definition. implemented with a high bit resolution DSP. For instance, it could provide dynamically controlled Doppler frequency and time correction when operating over satellites or during periods of severe multipathing.and linked with the Receiving DSP correctly.
Receiving post detection DSP for final DSP filtering for optimal filtering as needed by the operator, such as variable bandwidth and reception enhancements like QRM fitters,QRN filters, notchers and output audio enhancers.
Transmitting initial DSP to define bandwidths, audio shaping and general mode wave shaping. For instance, specifying keying waveshapes to eliminate key clicks or low frequency boosting to better transmit Digital SSTV.
Display management. Provide basic user defined appearance of the redefinable rig, and ability to select only those features desired. Provide knobs sliders, mousings as the operator desires ( and NOT ONLY AS THE SDR MANUFACTURER PROVIDES!) Some modes such as PSK-31, JT-65 and Very Low Bandwidth Digital Voice require dynamically defineable spectral displays.
Configuration Management. Memories to hold this complex array of activities, accessible in understandable and minimal keystrokes. Provides the ability to download new software enhancements and corrections and also has the ability to optimize the hardware structure for an improved functionality. For instance it may save considerable electricity to dynamically reconfigure the hardware and eliminate certain unneeded functions
Diagnostic hardware and firmware to detect problems and suggest solutions or at least workarounds by the operator without having to make a number of frustrated phone calls
The all important idea of this hardware is that it must be defined identical at some point so that with a minimum of board or component substitutions the hardware can be upgraded with a reasonable array of future components invented in the future, or, specialty hardware that will provide new levels of functionality, unneeded by the operator at the time of initial purchase. In PC computers it is the power plugs, I/O data cabling and interface sockets and connectors that permit a a major upgrade by merely connecting a newer,faster, and better in general Motherboard and CPU to the existing ATX case, power supply, memory drives, video systems and monitors, keyboard, mouse and assorted inputs, and modems, ethernet's, and cooling subsystems.