[TenTec] Re: [Ten-Tec] Alas, not for me - more

[Ken Brown]

Bob Henderson wrote:

>On the other hand Ken you are right.  Introducing filters ahead of the A to
>D convertor will not improve A-D convertor dynamic range.  The dynamic range
>of the A to D convertor will stay exactly as it was.  What will be improved
>is the dynamic range of the system.....and that's what we're interested
>in......the dynamic range of the receiver not its A to D convertor.
>
The dynamic range of the front end of a receiver (everything before the 
"roofing" filter) has to be good regardless of whether the following 
stages of the receiver are completely "traditional" analog or if they 
incorporate DSP. If strong signals cause "overload" (meaning distortion, 
and intermodulation of signals in the front end is producing new 
products) then the ability to hear weak signals amongst the strong 
signals is impaired, with either a traditional analog backend or a DSP 
backend.

Then when the signals that have passed through the roofing filter get to 
the backend, those stages also have to handle all of the signals that 
get through. If there is distortion from overdrive, of either analog 
circuitry or an analog to digital converter, the ability to hear weak 
signal amongst the strong signals (that have passed through the roofing 
filter) will be impaired.

Getting better performance by using a narrower roofing filter is not a 
unique characteristic to DSP radios. It can help with radios that do not 
have DSP at all. The dynamic range limitations of analog to digital 
converters in DSP radios may make it neccessary to use narrower roofing 
filters at lower signal levels than would be necessary in a traditional 
analog radio. That depends on the dynamic range of the traditional 
analog backend you are comparing the DSP backend with.

The character of the distortion products generated when an analog system 
is overdriven can be quite different from when a ADC in a DSP system is 
overdriven.

Ken , N6KB

>
>Bob, 5B4AGN
>
>
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