"Kevin Neilson" <[email protected]> wrote in message news:<8oOac.52137$K91.127528@attbi_s02>...
> > You are sampling two samples per symbol, so if the loop is locked and you
> are sampling at the symbol instants and midway between the samples, then
> x(r-1), x(r-1/2), and x(r) will be three successive samples. These three
> would comprise two strobes and one midway or one strobe and two midway
> samples.
>
> The TED can be used to adjust a PLL that controls the sample clock. Then no
> interpolation is required; the sample clock is just adjusted so that the ADC
> samples at the correct points. That's mostly an older method. The other
> type of loop uses an ADC that samples at a constant rate, and then a
> resampling filter controlled by the loop interpolates the value at the
> sampling instant, which will be between the ADC samples. Before the loop is
> locked, the interpolator will not be sampling at the symbol instants, but
> the error value will be large and will push the loop until the interpolated
> values occur at the symbol instants and the midway points.
> -Kevin

Why do you call tuned sampling an older method? It is really a matter
of circumstances, rather than age. If you are, say, building modems to
run in the PCM telephone network you have no control over the
sampling, and have to fiddle things around by filtering. If you have
control over the sampler, especially for very high speed channels,
tuning the sampling time to match the symbols simplies things quite a
bit.

Regards,
Steve