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


Thank you for stirring... Whatever the other design decisions, at least the low level audio signals MUST be differential and well balanced.

73, Ahti OH2RZ

----- Original Message ----- From: "Lyle Johnson" <kk7p@wavecable.com>
To: "Xylo-SDR Discussion" <xylo-sdr@lists.ae5k.us>
Sent: Friday, December 16, 2005 8:48 AM
Subject: Re: [Xylo-SDR] Clocks for USB FX2, TI PCM4202, etc.

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.


Lyle KK7P

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