[hpsdr] Single chip SDRs

[Robert McGwier]

Actually, so long as you prevent overload,  the act of filtering and
downsampling increases the dynamic range so your statement needs to be
clarified by the TARGET sample rate after the Filtering/Decimation/DDC
stages and whether or not the design(s) allow for the processing gain.

Consider:

75,000,000/192,000   (for example) is quite a bit of processing gain  <<<
 SO LONG AS THE ACCUMULATORS IN THE FILTERS IN THE DOWNSAMPLING STAGES ARE
LARGE ENOUGH TO INCLUDE THE INCREASED DYNAMIC RANGE AT EACH STAGE OF
FILTERING AND DECIMATION >>>

Your comments on alias are spot on.

Bob
N4HY


On Thu, Jan 19, 2012 at 9:25 PM, Phil Harman <phil at pharman.org> wrote:

> ***** High Performance Software Defined Radio Discussion List *****
>
> There are a number of 'single chip' DDC SDR designs starting to appear on
> the market.  These incorporate the digital front end of a DDC in a single
> IC and follow the
>
> ADC > NCO > CIC > FIR
>
> architecture that we use in Mercury and Hermes.
>
> Examples of these ICs are the LM97593 and AFE8220-Q1. Both these chips
> incorporate dual DDCs which is a very nice feature.
>
> These chips are intended to be used primarily in Cell site base stations
> which have lesser requirements in relation to dynamic range and alias
> rejection than a high performance HF receiver.
>
> For example the LM97593 uses a 4th order CIC filter. This means that the
> first alias will only be 45dB down if a 64.512MHz clock is used for a
> final sampling rate of 192kHz. Once an alias signal has been generated it
> can't be removed using the subsequent FIR filters.
>
> Recent measurements on an SDR using this chip confirm this figure.
>
> The AFE8220-Q1 uses a 5 stage CIC filter and can sample at 75MHz which
> will improve the alias rejection.  However, it only provides a 16 bit
> output which limits the dynamic range to 96dB. This figure has to include
> any band noise so will be 10 to 20dB less in practice.
>
> There will be applications where such chips are very useful but just be
> aware of the limitations before building using these chips or purchasing
> ready built boards on eBay.
>
> Hopefully future generations of these ICs will be more suited to high
> performance HF receiver construction.
>
> On a more positive note, the new AD9253 Quad ADC could form the basis of a
> 4 receiver version of Mercury.
>
> 73 Phil...VK6APH
>
>
>
>
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-- 
Bob McGwier
Facebook: N4HYBob
ARS: N4HY
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