[Elecraft] K3 & ADAT ADT-200A by HB9CBU

[Philip Covington]

On Mon, Dec 1, 2008 at 4:48 PM, Kok Chen <chen at mac.com> wrote:
>
> On Monday, December 01, 2008, at 01:16PM, "Bill W4ZV" <btippett at alum.mit.edu> wrote:
>
>>...which is why the hybrid approach of a narrow roofing filter before the
>>ADC works so well in Orion and the K3.
>
> Bill is correct.
>
> Think of this this way: the clipping level of a codec does not change, but the noise floor of the processed passband falls.
>
> You gain nothing anymore only after the Nyquist rate of the signal of interest has exceeded (think energy density) the decimated sampling rate.
>
> 73
> Chen, W7AY
>

Chen,

You must also realize that placing analog components in front of the
ADC introduces non-linearity and distortion.  In the case of DSP IF
radios like the K3, there is *a lot* of analog components in front of
the ADC, each degrading the signal to some extent.  There is also the
problem of the phase noise of the LO.   Even QSD based SDRs which use
a DDS for the LO have worse phase noise (and spurs) than the crystal
oscillator LO in direct sampling receivers like the Perseus or QS1R.
In the DDS, the DAC only approximates an analog signal resulting in
spurs, and depending on whether the internal PLL is used in the DDS,
added phase noise.  In direct sampling receivers such as Perseus or
QS1R, the LO is not converted back to an analog signal as in the DDS
case - the quality of the LO is then pretty much determined by the
quality of the sampling clock (very good) and how many bits you use in
your NCO in the FPGA.  PLL based LOs (like the K3) have much higher
phase noise than this.

-- 
Phil Covington
Software Radio Laboratory LLC
Columbus, Ohio
http://www.srl-llc.com