ETTC'2003 - SEE

message data. The scheme of PCM demodulation is shown in fig.10, which has a reference of
Gardner’s model [3] .
f '( n)
interpolator
τ n
f '(
nT + τ )
f'(nT + T/ 2 + τ )
n
n
Timing error
detector
Fig.11 Scheme of PCM demodulation
decide
The data rate of f’(n) is 8Msps, therefore there would be four sample points in each symbol
and they perform cubic spline interpolation to get values on the two instants nT1+τn and
nT1+T1/2+τn , namely f’(nT1+τn) and f’(nT1+T1/2+τn), the former is the strobe value and is
used to make decisions, the latter is the value on the middle way of each symbol.τn is the timing
error for the nth step and is measured with Gardner’s timing error detector, which is a recursive
tracking algorithm and can be depicted in Eq.16~18, it has advantages of with little amount of
computation and with fast convergence speed. r and βare the properly chosen positive step-sizes,
and equals to 0.99 and 0.0001 respectively.
u
v
n
n
n
= −ru
= −rv
τ = τ
n−1
n−1
n−1
+ f '(
mT + τ )
+ β re{
u v
n
1
+ f '(
mT + τ + T
1
n
*}
n
n
1
/ 2)
(16)
(17)
(18)
Fig.12 plots the tracking curve of timing error with a supposed channel delay of 0.15T, from
which it can seen the timing error measured are correct and converges quickly. Fig.13 plots the
pre-filtered signal g(t) and interpolated signal of f’(n), from which it can be seen the results of
demodulation is better. However, detailed BER performance needs to be carefully analyzed in the
future.
Fig.12 Tracking curve of timing error Fig.13 g (t) and the interpolated results
3 Conclusion
In this paper, a Matlab model of digital PCM/FM receiver is established and discussed in
detail including ADC, DDC, FM demodulation as well as PCM demodulation, simulation results
reflects the presented scheme work well, however, detailed BER performance needs to be
carefully analyzed in the future.
7
a n
)