[hpsdr] Odyssey Siren Design

[Lyle Johnson]

> 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

It will only have an output swing of 2V (+/- 1V about the center) with a 
single 5V supply, and each amplifier consumes 12 to 18 mA.

A Linear Tech LT6231 is 1.1 nV-rt-Hz (at 10 kHz), stable at unity gain, 
draws 3.5 mA/amplifier, rail-to-rail output.

Anyway, I like the idea of a low-noise buffer to reduce the equivalent 
noise figure of the ADC input!

73,

Lyle KK7P


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