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[linrad] 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 ?