I agree George with all your comments about the effects of linear vs
"less than linear" circuits.
In my work, we use DSP filters extensively. (Vibration Analysis).
We have some very expensive toys to play with that I am sure would
compare well against the military systems you mention.
Its all engineering tradeoffs. Stuff we all do every day.
Sure would be nice to have an infinite development budget and
an infinite unit manufacturing cost budget as well .
:)
73 de Gary, AA2IZ
----- Original Message -----
From: "George, W5YR" <w5yr@att.net>
To: "Gary Hoffman" <ghoffman@spacetech.com>
Cc: "Ten-Tec Reflector" <tentec@contesting.com>
Sent: Tuesday, October 29, 2002 7:31 PM
Subject: Re: [TenTec] Receivability
>
> Again, an interesting observation, Gary.
>
> Location of the filter(s) *should* have no effect on the overall
> performance of the IF chain, in fact of the entire receiver, *provided*
> that the entire system is operating linearly. In a cascade of linear
> systems, the overall transfer function is the product of the individual
> functions, etc. The trick, of course, is to maintain a cascade of truly
> linear sub-systems.
>
> Placing the filters up front, as you point out, can be an advantage if
> they prevent large signals, etc. which *could* drive one or more of the IF
> stages into non-linear operation, from progressing beyond a early stage
> that can handle their amplitudes linearly. This evidently is the situation
> to which you refer in stating a preference for having the filtering come
as
> early as possible, as in the Corsair IF design. This suggests that the
> filters are located "up front" in a deliberate effort to allow the use of
> subsequent stages with a lower overall dynamic range than would otherwise
> be required.
>
> This is the same situation that prevails with the entire RF/IF system in a
> receiver. Provided that the system from antenna connector to some
arbitrary
> point of delineation in the receiver, such as the demodulator stage input
> or the input to the ADC in an IF/DSP architecture, is operated linearly at
> all times, regardless of input signal levels, it matters not where the
> bandlimiting function is located. Hence, the modern approach with IF/DSP
> receivers to concentrate the selectivity in the DSP section subsequent to
> the ADC stage. This approach has powerfully attractive performance and
> economic ramifications.
>
> Almost every modern commercial and military radio in production today - in
> fact, I can think of no exceptions - follows this approach and employs
> nothing but DSP filtering for setting passband limits. The extraordinary
> and near-mathematicially perfect DSP filter shapes permit a level of
> performance essentially unobtainable with analog means. The Ten-Tec RX-340
> is a typical example of this architecture at the low end of the
> commercial/military receiver market.
>
> On the other hand, if a less than outstanding design and implementation of
> the receiver "front end" - i.e., from the antenna connector to, say, the
> ADC input for the IF/DSP section - is the case, then despite having near
> perfect filters in the DSP section, distortion products generated by
> overdriving various stages ahead of that point will be present within the
> ADC input and can conceivably not only contribute to signal distortion
> post-DSP but also could readily exceed the input signal limit of the ADC
> itself, thereby creating the worst possible distortion environment in the
> receiver, from which there is no recovery.
>
> It is in an effort to avoid these effects that we find that some receiver
> designs (which for whatever reasons - cost, complexity, design effort,
etc.
> - are unable to achieve the desired linearity ahead of the ADC input)
> employ varying levels of selectivity "along the way" as it were with the
> use of passive usually crystal-based ladder filters among the IF amplifier
> stages in an effort to minimize the forward progress of distortion
products
> that could drive subsequent stages into non-linearity.
>
> This is probably a reasonable engineering tradeoff between the cost of
> analog and software engineering, components, production and so on,
required
> for a truly "bullet-proof front end" that remains linear under all
> operating and signal conditions and the cost of one which is "good enough"
> when supported by additional relatively narrow passive filters.
>
> We find examples of both design approaches in the amateur marketplace
> today. It will be interesting to observe the direction and rate of
progress
> of these differing design philosophies as time passes.
>
> Thanks for the information, Gary, and a very enjoyable exchange of views.
>
> 73/72, George
> Amateur Radio W5YR - the Yellow Rose of Texas
> In the 57th year and it just keeps getting better!
> Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
> K2 #489 Icom IC-765 #2349 Icom IC-756 PRO #2121
>
> Gary Hoffman wrote:
> >
> > George,
> >
> > I think it is important to note that a big difference is made by where
> > in the receiver chain the filters are located.
> >
> > In the Pegasus, as you know, the DSP work is done at the end of the
chain.
> >
> > On the Corsair, and many other radios, the crystal filters are in the
early
> > stages of the chain.
> >
> > Since they eliminate much noise up front, before it gets fully amplified
by
> > the following stages, they can be much more effective than the same
quality
> > filter applied downstream. In fact, since they can help prevent
> > fundamental
> > overload, they can be dramatically helpfull.
> >
> > I don't mean to suggest that the IF DSP filter in the Pegasus is a poor
> > filter. I only note that it is at the end of the chain, and less
effective
> > because
> > of that fact.
> >
> > 73 de Gary, AA2IZ
> >
> > ----- Original Message -----
> > From: "George, W5YR" <w5yr@att.net>
> > To: "Gary Hoffman" <ghoffman@spacetech.com>
> > Cc: "Mike Hyder -N4NT-" <N4NT@charter.net>; "Scanandoah, Alan"
> > <ascanand@harris.com>; "Ten-Tec Reflector" <tentec@contesting.com>
> > Sent: Monday, October 28, 2002 11:19 PM
> > Subject: Re: [TenTec] Receivability
> >
> > > Very interesting, Gary, and thanks!
> > >
> > > I have made detailed spectral comparisons between the IF DSP filters
in
> > the
> > > ICom 756 PRO, the Kachina 505DSP of similar architecture and the Icom
765
> > > with conventional crystal filters. There is no comparison whatever
between
> > > the shape factors and overall shapes of the PRO filters compared to
any of
> > > the others. The PRO filters are mathemetically nigh perfect in shape
and
> > > performance. I think that you have found that the Pegasus IF DSP
filters
> > > are not the equal to the conventional INRAD filters - that is very
> > > surprising to me.
> >
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