ETTC'2003 - SEE

demodulator, where the instantaneous phase and frequency are estimated. The FM demodulated
signal f (n) inputs to the PCM demodulator to do timing error extraction and correction as well as
decisions, at last regenerated codes of a n
) are obtained.
2.1 Pre-filtering
In the simulation model, the signal pre-filtered signal g(t) can be expressed in Eq.4,
g = ∑
n
n I
∞
( t)
a h ( t − nT )
where an =±1,and T is the symbol duration, hI (t) is the pulse response of the pre-filter, it is a
RRC filter whose roll off coefficient is 0.5, fig.4 plots an and g(t).
2.2 Frequency modulate
(4)
Fig.4 message data and signal pre-filtered
g (t) frequency modulates the 10 MHz carrier, the instantaneous frequency f (t) and phase Ф
(t) are expressed in Eq. 5~6, where kf is the peak modulate frequency deviation of700KHz. In
Eq.7 , x (t) is the modulated PCM/FM signal, andФ0 is the original phase of the carrier. x (t) is
added with a gauss noise of n(t) whose mean is 0 and variation is 0.01. The summed y (t) is
bandpass filtered by a BP filter and r(t) is obtained which also feeds to the ADC.
f ( t)
= f 0 + k f ⋅ g(
t)
(5)
t
∫ f ( t)
dt = 2πf
t + 2 k f ⋅
−∞
∫−
∞
φ ( t ) = 2π
0 π g(
t)
dt (6)
x t)
= Acos(
φ ( t)
+ φ ) (7)
( 0
y ( t)
= x(
t)
+ n(
t)
(8)
r BP
BP
( t)
= y(
t)
* h = [ x(
t)
+ n(
t)]
* h = x'(
t)
+ n'(
t)
(9)
In equation (9) hBP is the pulse response of the BP filter, and the denotation ‘*’ represents the
operation of convolution. The FIR typed BP filter is centered at 10MHz, with passband of 4MHz
selected according to Eq.2. y (t) and its spectrum are figured in Fig.5 (a) while fig.5 (b) plots r (t)
and its spectrum.
3
t