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

[Bob Henderson]

Hi Ken

Nothing to disagree with there, though a little to add.  Poor design at any
stage in the signal path is going to prejudice dynamic range.

More and more folks now accept, that it's close-in dynamic range which
really matters in the way we use our radios.  Measurement at 20 kHz tells us
very little of use.  We're better off with measurement at 5 kHz but that's
still a veritable football field so far as CW is concerned.  Roofing filters
place less demands on those stages following them where signal levels are
higher.  Selectable roofing filters such as those provided in Orion can
really enhance performance in this area.  Independent tests verify this.

http://www.sherweng.com/Dayton_2004/Dynamic_Range_Data.pdf

If tests were routinely carried out on radios at 2 kHz, 1 kHz and 500 Hz we
would really start to see an interesting picture develop.  I guess everyone
would then see the limitations inherrent in the design of tranceivers using
a vhf first IF, where narrow roofing filters aren't available.  Somehow, I
suspect Yaecomwood would be rather less than enthusiastic about that  :-))

Bob, 5B4AGN

----- Original Message ----- 
From: "Ken Brown" <ken.d.brown at verizon.net>
To: <tentec at contesting.com>
Sent: Friday, September 03, 2004 4:55 AM
Subject: Re: [TenTec] Re: [Ten-Tec] Alas, not for me - more


> 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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> >
>
>
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