[hpsdr] DDC Noise Figure

[Edson Pereira]

Hi John,

Thanks for the comments and clarifications. My confusion was Equation 8 in
this Analog Devices paper:

http://www.analog.com/static/imported-files/tutorials/MT-006.pdf

NF = 10log10F = PFS(dBm) + 174 dBm – SNR – 10 log10[fs/2B] – 10 log10 B

But the equation is a little misleading as pointed out by Ulrich, N1UL. It
seems that, similarly to an analog receiver, the NF at the first stage will
dominate and the total NF will not be changed throughout the receiver. The
NF will degrade the signal at each stage. The SNR however can be improved
with processing gain. Would this be  a correct statement?

73,

-- Edson



On Sun, Mar 27, 2011 at 8:01 PM, John Miles <jmiles at pop.net> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
>
>  And yes, process gain will improve the SNR by 10*log(BW ratio), but
> that's not something you'd ordinarily associate with the term 'noise
> figure'.
>
> -- john, KE5FX
>
> -----Original Message-----
> *From:* hpsdr-bounces at lists.openhpsdr.org [mailto:
> hpsdr-bounces at lists.openhpsdr.org]*On Behalf Of *John Miles
> *Sent:* Sunday, March 27, 2011 3:54 PM
> *To:* hpsdr at openhpsdr.org
> *Subject:* Re: [hpsdr] DDC Noise Figure
>
> A good question.  Multiplication by saturated diodes in a conventional DBM
> yields a best-case conversion loss of 3.9 dB -- 3 dB for the opposite
> sideband which is unused, and 0.9 more dB lost in various distortion
> products arising from square-wave switching.   I'd expect the "noise figure"
> of an ideal DDC to be simply 3 dB, because sine-wave multiplication
> wouldn't be accompanied by all the distortion.
>
> In real life, the multiplier is presumably followed by one or more CIC
> filters, and the picture gets murkier there due to bit allocation.   You can
> lose up to ~6 dB (almost a full bit) if the MSB's precision isn't
> fully utilized.  In other words, if the MSB is toggled by only the very
> strongest signals, it will contribute less than the usual 6.02 dB of dynamic
> range, and that deficit is effectively part of the conversion loss.
>
> Another concern might be DC offset from the ADCs.  If the full dynamic
> range isn't available going into the DDC it won't be available at baseband,
> either.
>
> -- john, KE5FX
>
>
> -----Original Message-----
> *From:* hpsdr-bounces at lists.openhpsdr.org [mailto:
> hpsdr-bounces at lists.openhpsdr.org]*On Behalf Of *Edson Pereira
> *Sent:* Sunday, March 27, 2011 12:30 PM
> *To:* hpsdr at openhpsdr.org
> *Subject:* [hpsdr] DDC Noise Figure
>
>
> Hello,
>
> I have been trying to educate myself about SDR using direct sampling and am
> having some difficulty in understanding how to calculate the noise figure of
> a DDC based SDR system. Since the noise figure takes into account the
> bandwidth, would it be correct to say that even though the noise figure of
> the ADC is high, the total noise figure of the receiver system can be
> improved (by the various decimation stages)? Putting a pre-amp in front of
> the ADC would seems to help considerably, at the cost of lower dynamic
> range, but if my understanding is correct, the total noise figure will be
> much better than that of the ADC alone. Could anyone shed some light? Some
> math will be ok.
>
> 73,
>
> -- Edson, pu1jte, n1vtn, jf1afn, pu2mwd�
>
>
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