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For general interest

Here are a few sites that might be useful

For comparing sound cards

The Skyworks SKY73001-11 (http://www.skyworksinc.com/) is an interesting device designed specifically for direct conversion I-Q receivers. Brief specs are
RF 2 -3500 MHz
IF 0 -100 MHz
IIP3 +26 dBm
IIP2 +67 dBm

I have been looking at the noise specifications of various DC regulators which could affect base-band noise levels, there is a wide range. In summary

At the high end the LM 317 and 337 series have RMS noise levels of 0.003% of Vout. The spec states this is over 10 Hz to 10 KHz

The best I could find from Motorola was the MC1723C that seems to have very low noise (2.5 uV) this is stated over the range 100 Hz to 10 kHz

The MC78L/79L 100 m/A series have noise levels 5V/40 uVrms, 12V/75 uVrms, 15V/90 uVrms stated to be in the 10-100 kHz range.

It is reasonable to expect these levels could be reduced by Tantalum capacitors at the device output but it seems smart to select low noise devices to begin with.

For further immunity the spec for the AD797 amplifier recommends 0.1 uF and 4.7uF capacitors at the + and - supply pins with <5 mm leads to ground.

Now a question.
In most I-Q demod designs I note the phase shift is applied to the LO. In the simple 40 kHz 2x SBL-1 design that Leif published in QEX the shift is applied to the RF. What is the reason for this approach ?