[hpsdr] HPSDR Signal Overload ( In the digital processing???)

[Joe Martin K5SO]

Ah, okay; point taken.  However, there is more to this issue than  
simply "cancelling the gain" of the LTC6400-20 in this.  Adding an  
attenuator in front of the LTC6400-20 absolutely trashes the noise  
figure of the receiver, of course.

Indeed, adding a 20 dB attenuator in front of the LTC6400-20 to  
"cancel" its gain results in a significantly worse noise figure than a  
straight wire connection to the ADC would!  When the attenuator is in  
place, the noise figure of the receiver is effectively 20dB, which is  
a horrible noise figure.

The only reason I can see to ever insert the attenuator is if you have  
such strong signals that you can afford to throw away your hard earned  
receiver sensitivity; which, given my inherent weak-signal viewpoint  
seems like a terrible idea, hihi!

Anyway, thanks for the clarification, Greg.

73,  Joe K5SO


On Apr 6, 2011, at 11:42 AM, Greg - ZL3IX wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> Not quite accurate, Joe.  An LTC6400-20 provides 20 dB of gain in  
> front of the ADC.   I think one would have to call this a preamp.
>
> The switchable attenuator is in front of the preamp, and cancels its  
> gain when switched in.
>
> 73, Greg ZL3IX
>
> On 2011-04-07 01:05, Joe Martin K5SO wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> There is already a 20 dB switchable attenuator on the Mercury  
>> board.  The attenuator is switched in when the "Pre-amp" selection  
>> is "OFF".  Direct connection (no attenuation) occurs when "Pre-amp"  
>> is "ON" (there is no pre-amp on Mercury, as was discussed on the  
>> reflector recently).
>>
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