[hpsdr] Odyssey Siren Design

[Joseph Julicher]

I am about 80% finished in the schematic entry on Siren and ran into  
an interesting design idea that may help simplify the power supply  
for use in portable (or extraterrestrial) designs.

We are using the TI TLV320AIC31 Codec which has differential inputs  
and outputs.
The I/Q demodulator is the standard QSD circuit.
To improve the S/N ratio a good instrumentation amplifier is  
typically used after the QSD commutator and before the CODEC.
However, there are two problems:
1) the INAMP requires a bipolar power supply, and
2) the TI CODEC has a differential input and the INAMP will convert  
the differential QSD signal to a single ended signal.

I am proposing the following solution:
Put a low noise 10x buffer on each line beween the codec and the qsd.
The LMH6624 is a low noise opamp from National.  At 1MHz it has  
0.92nv/sqrt(hz) noise.  That is quite low.
http://www.national.com/pf/LM/LMH6624.html

The + and - signals are directly fed to the CODEC.  The circuit shown  
provides the following transfer function.

Vout = (Vin - 1.65)10 + 1.65

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