ETTC'2003 - SEE

f
f
p
IF
= R
b
= 4R
b
∆f
= 0.
7R
b
With the quick developments of ADCs as well as VLSI technologies nowadays, it becomes
increasingly attractive to design IF digital receivers, which are less expensive, more accurate, and
flexible while easier to be manufactured compared with traditional analog counterparts. For that
reason, we present a scheme of digital PCM/FM receiver which directly samples the 2 nd IF
PCM/FM signal and all demodulations will be implemented with all digital methods, which is
plotted in fig.2.
antenna
LNA
Ist
local
osc
1st IF
amplifier
2nd
local
osc
BP
filter
B
IF
AGC
(1)
(2)
(3)
2nd IF
amplifier
Digital receiver
ADC
Fixed
Fig.2 Block diagram of digital PCM/FM telemetry receiver
2 Design of PCM/FM digital receiver
Digital
demodulators
data
&clk
A Matlab simulation model is founded to help us design and simulate PCM/FM digital
receiver for Matlab is a popular and effective tool to design DSP systems nowadays, as can be
seen in fig.3.
an
Prefilter
fixed
r (nT)
ADC DDC
g(t) x(t) y(t)
Fm
modulate
I(n)
Q(n)
n(t)
FM
demodulation
f(n)
BP
filter
r (t)
PCM
demodulation
Fig.3 Simulation model for digital PCM/FM receiver
In the simulation model, PCM data an are bipolar NRZ codes whose rate is supposed to be
2Mbps. The carrier of PCM/FM is 10MHz, and the peak modulate frequency deviation is 700KHz.
PCM data are pre-filtered to be g(t) and further frequency modulates the carrier and forms
PCM/FM signal x(t), it is added by a gauss white noise signal n(t) which simulates the channel
noises, the sum of y(t) is bandpass filtered to suppress noises and then feeds to ADC. r(t) is
sampled by the ADC at fixed rate of 64Msps and becomes discrete r(nT), the DDC module (digital
down conversion) translates r(nT) from IF to baseband and forms two quadrature components of I
(n) and Q (n), they are filtered and decimated to great reduce the data rate and feeds into the FM
2
a n
)