I'm implementing demodulation schemes in digital form and I'm
currently trying to establish what the advantages and disadvantages
are of each domain. I've written down what I think are the most
important, and would be interested in hearing if I've covered all the
bases and if the information is accurate.
* Flexibility – different demodulation schemes for different
applications and configurations
-- Analog alternative requires numerous circuits. Different schemes
require the use of PLLs, and only one PLL function is required for
* Wide Dynamic Range - linear and non-linear math operations work over
a wide range of signal.
* Higher Order Filters - relatively low incremental cost. Additional
memory and computations only.
* Adaptive Filters - become practical.
* In Phase Mixing – local oscillators exactly in phase.
-- many heterodyne demodulation schemes require that two in phase
local osillators of differing frequency (10,000Hz and 20,000Hz for
example) are needed for mixing with incoming signals. In analog, a
signal osillator is often divided down. A slight phase difference will
distort the signal.
* Cheaper - in an environment where a computer is already present.
-- on an airplane for example, processors are readily available.
* Control – easy integration with a control system.
-- process the demodulated signal and sound an alarm if neccesary.
* Logging - Additional memory.
* Processor Speed - Limited by the speed of the DSP.
* Quantization Noise - inherent in any digital demodulation technique.
Resolution – 16 or 32 bit?
Frequency Range - limited by the sampling rate of the ADCs and DACs.
Posted by Randy Yates●June 23, 2003
> Hi all,
> I'm implementing demodulation schemes in digital form and I'm
> currently trying to establish what the advantages and disadvantages
> are of each domain. I've written down what I think are the most
> important, and would be interested in hearing if I've covered all the
> bases and if the information is accurate.
> * Flexibility – different demodulation schemes for different
> applications and configurations
> -- Analog alternative requires numerous circuits. Different schemes
> require the use of PLLs, and only one PLL function is required for
> * Wide Dynamic Range - linear and non-linear math operations work over
> a wide range of signal.
Actually, with regards to non-linear operations, I would put this
in the disadvantage category for two reasons:
1. If you're using a fixed-point DSP, you usually *don't* have a
wide dynamic range. Doing a nonlinearity such as a high-order polynomial
can be a real pain in such a situation because you will have to examine
the polynomial for various ranges of input to determine how to scale.
2. Performing a non-linearity such as an "absolute value" of the
signal produces harmonics which, if it were analog, would simply
go out towards infinity, but in a digital system these harmonics
get folded back into the passband of the signal. Not good. One
way to mitigate this is to oversample (upsample) before doing the
nonlinearity and then filter and downsample when you're done, but
that cost MIPS.
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <email@example.com> % 'Big Wheels', *Out of the Blue*, ELO