[hpsdr] Mercury Noise/Sensitivity

[Greg - ZL3IX]

Hi Gerd,

Sorry for my late comment on this question, but I have been away on 
holiday.  I did not see a comment from any other list member, so let me 
share some thoughts with you.

You are correct that the quantisation step for this ADC is 22uV.  There 
is a formula that states that the RMS quantising noise is equal to the 
quantising step divided by root 12 (3.5), so in this case the quantising 
noise would be about 6.6 uV.  This however, is spread over the whole 
Nyquist bandwidth, or half the sampling rate.  To find the noise floor 
in uV per root Hz, we would therefore divide by about 8000, giving just 
less than 1 nV/root Hz.

There is a catch to this theory though.  The ADC is not a perfect 
converter, and other noise sources have to be taken into account.  These 
noise generators are in the front end of the ADC.  Some manufacturers 
use a parameter called ENOB or Equivalent Number of Bits.  This factor 
is calculated using the actual observed noise figure of the device, and 
is always smaller than the resolution of the device.  For example, a 
16-bit device may have an ENOB value of 12.5 bits.

The practical noise figure of the LTC2208 is around 29 dB, which means 
that about 17 dB of gain is needed in front of it to achieve the 12 dB 
noise figure normally considered adequate for HF operation.  This limits 
the dynamic range at the top end of course, as strong signals are 17 dB 
closer to causing an overflow.

ADC's suitable for HF direct conversion have had this kind of noise 
figure for about 10 years now, little progress having been made in this 
area.  Any improvement that will be achieved in Rx BDR will need an 
equivalent improvement in noise figure, or in the ENOB parameter if you 
prefer to think of it that way.  What has improved quite considerably in 
the last 10 years, is the Spurious Free Dynamic Range (SFDR).  The 
LTC2208 is the best of the bunch for SFDR, leading to its choice for the 
Mercury project.

I hope that goes some way to answering your question.

73, Greg, ZL3IX

Gerd Loch wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi,
> there is a fundmental calculation for the receive sensitivity in the Mercury
> Wiki based on noise figures for the ADC LTC2208. There is no dependency on
> the bit resolution of the ADC in the calculation.
>
> I wonder which influence the bit resolution has on the sensitivity. 16 bit
> resolution of a full scale signal of 1,5V means 22uV. Does this mean that
> the minimal discernible voltage is 22uV?
>
> What would be the result if the ADC had only i.e. 10 bit resolution?
>
> Gerd, DJ8AY
>
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