Re: [Xylo-SDR] Clocks for USB FX2, TI PCM4202, etc.

[Lyle Johnson <kk7p@wavecable.com>]

Just to stir the pot...

1) Think about putting the default ADC and DAC on the main board with 
the FPGA.  Add connector for hooks to tie in external ones.  This will 
minimize RFI, of some importance if this is going inside a radio.

2) The ADC and DAC may have I2C or SPI, but that is just for control. 
Typically, there is a bit clock and frame sync signals, with frame sync 
in the 48 kHz to 192 kHz range and the bit clock typically 32 to 64 
times faster.  You can expect a 12.288 MHz bit clock in a 192 kHz sample 
rate system using the PCM4202, for example.  Harmonics of this can make 
interesting spurs and birdies, so you might want to keep the leads 
really short, which is a large part of the reason to put the ADC and DAC 
on the same board with the FPGA.

3) Consider putting the ADC buffer amplifier on the same board, and make 
it differential. Like the OPA1632 for the TI ADC.  This could be on a 
plug-in board, but then you are making a requirement to use the plug-in 
if you intend to use the ADC at all.

4) For ADCs of this caliber, think differential signaling.

5) If power consumption is a consideration, consider avoiding the 
Cyclone II or Spartan 3 or any of the 90 nm process FPGAs.  They are 
denser and cost a little less for given functionality, but the static 
current drain is dramatically higher than the older, 120 nm process 
parts.  If power consumption is not a consideration, then go for it and 
use the latest 90 nm process parts.

6) The analog section needs its own power supply regulators, well 
decoupled from the digital stuff.

7) You'll likely need negative voltage at several mA for the analog 
circuitry.  Avoid charge pumps such as those used in RS232 converter 
chips if RF noise is a concern.

73,

Lyle KK7P