Well they finally built a radio that's beyond understanding. I can see
the new Extra Class exam:
Q: What is the function of the DSP stage?
1. Makes the radio sound better
2. Everything George says about it
3. Everything Doug says about it
4. No one knows but it sorta works and is fun to play with.
N4LQ
-----Original Message-----
From: "George, W5YR" <w5yr@att.net>
To: <tentec@contesting.com>, "Jim Reid" <jimr.reid@verizon.net>, "Robert
& Linda McGraw K4TAX" <RMcGraw@blomand.net>
To: <tentec@contesting.com>
Date: Fri, 3 Jan 2003 12:17:07 -0600
Subject: Re: [TenTec] Orion vs. Jupiter Audio vs. DSP
> Bob and Jim raise an interesting real-world aspect of A-D conversion.
> Perhaps a brief overview might be of interest to those encountering
> these concepts for the first time.
>
> Theoretically, an N-bit A/D converter can provide 2^N discrete
> quantization levels or steps or numbers corresponding to some
> specified analog input signal amplitude range. Since each quantization
> level represents a change in magnitude of 2, we equate each bit with 6
> dB of available dynamic range for the converter.
>
> Example: a 4-bit ADC can output the binary numbers 0000 through 1111,
> or 0 through 15 decimal. These 16 levels would all be usable except
> for the effects of indecision in the conversion process which results
> in the least significant bit (LSB) sometimes being correct and
> sometimes being incorrect. On average, the correct digital value
> "falls in the crack between levels" and produces quantization noise in
> the ADC output. That is, the numbers differ from sample to sample
> from the "exact values" by being forced to take on only the available
> values from the converter. This can be thought of as the true sampled
> signal accompanied by essentially uniformly distributed noise across
> the spectrum of the "real" signal.
>
> Thus, we regard the LSB as being "lost" to the effects of quantization
> noise even under the best of conditions. So, our N-bit ADC functions
> at best as an (N-1)-bit converter, and we have lost 6 dB of its
> theoretical dynamic range. The working dynamic range thus is reduced
> from 4 bits at 6 dB per bit or 24 dB to 3 bits or 18 dB. Effectively,
> we disregard the LSB and use only the remaining higher-order bits.
>
> Realistically, more than just the LSB is lost to quantization noise in
> the real world of A/D conversion. Common techniques such as sample
> clock dithering to control the effects of quantization noise can "use
> up" several low-order bits and therefore further reduce the effective
> number of quantization levels based on the "6 dB per bit" viewpoint.
>
> Apart from A/D issues, there are a number of other factors at work in
> a digitally controlled receiver which influence overall dynamic range.
> Chief among these is the analog response function of the circuitry
> preceding the A/D converter. Non-linearities in circuit operation can
> produce aggregate ADC input signal components of larger amplitude than
> the simple combination of signal amplitudes that would result from
> purely linear circuit operation ahead of the converter. Thus, more ADC
> bits are required to represent the input signal than otherwise would
> be needed with purely linear operation.
>
> Effectively, then, about 100 dB of dynamic range "could" be associated
> with about 16 or 17 active bits in the A/D conversion process, but
> this may or may not accurately reflect the actual number of "working"
> bit levels in the conversion process itself. Realistically, some
> equivalent "bits" are lost to front-end non-linearities and some to
> dithering and related ADC operational schemes.
>
> Conclusion: it can be confusing and lead to misunderstanding to think
> of the dynamic range of a digitally controlled receiver as being
> determined solely and simplistically by the number of bit levels
> provided by the ADC. Typically, a 24-bit ADC, theoretically capable of
> 24*6 = 144 dB overall numerical signal range, may produce a usable
> dynamic range of only 100 dB as previously reported. The element of
> design superiority plays into achieving as much of that theoretical
> 144 dB as possible in a production system; i.e., making the most of
> the available 24 bits.
>
> Doug Smith's very excellent chapter on DSP in the ARRL Handbook is
> required reading for anyone interested in this subject area. Seldom
> have so many complex topics been compressed into such small space and
> explained so clearly. An outstanding contribution . . .
>
> 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 IC-765 #2349 IC-756 PRO #2121 IC-756 PRO2 #3235
>
>
> ----- Original Message -----
> From: "Robert & Linda McGraw K4TAX" <RMcGraw@blomand.net>
> To: "Jim Reid" <jimr.reid@verizon.net>; <tentec@contesting.com>
> Sent: Friday, January 03, 2003 7:09 AM
> Subject: Re: [TenTec] Orion vs. Jupiter Audio vs. DSP
>
>
> > Jim brings up a very good point that most folks seem to overlook.
> >
> > That is "............ design seems to "only" realize some 18 or
> maybe 19
> > bits of "effective" A/D conversion as just over 100 dB dynamic range
> is
> > suggested by Doug as what is being realized in tests. Even if 32
> bit A/D
> > chips were available, it does not mean that that much resolution of
> the
> > analog conversion would occur. "
> >
> > State-of-the-art 24-bit IF-DSP converters, such as those used in the
> Ten-Tec
> > Orion, produce about 100 dB of dynamic range.
> >
> > 73
> > Bob, K4TAX
> >
>
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