Preliminary
HT9015
·
Intermediate frequency (IF) decoder part
Mixers
The figure below shows a simplified block dia-
gram for a double-conversion super heterodyne
FM receiver. Heterodyne means to mix two fre-
quencies together in a non linear device or to
translate one frequency to another using non
linear mixing. The super heterodyne receiver is
an improvement over other receiver in gain, se-
lectivity, and sensitivity characteristics.
The super heterodyne receiver is divided into
five parts:
A Mixer is a non linear device whose purpose
is to convert radio frequencies (RF) to inter-
mediate frequency (IF) (RF-to-IF frequency
translation). In the frequency conversion pro-
cess, RF signals are combined with the local
oscillator frequency in a non linear device.
The output of the mixer contains an infinite
number of harmonic and cross-product fre-
quencies which include the sum and the dif-
ference between the desired RF carrier and
the local oscillator frequencies. The IF
band-pass filter are tuned to the different fre-
quencies. Therefore, the IF signal is filtered
out by the BPF. In the HT9015 the double con-
version method (Figure 4) is applied to pro-
duce the lower IF.
·
RF section. Generally consists of a preselector
and an amplifier stage.
·
Mixer conversion section. Includes a ra-
dio-frequency oscillator stage (commonly
known as a local oscillator) and a mixer con-
version stage (commonly known as a fre-
quency detector) to produce the IF signal.
(
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The IF section. Generally consists of a series of
IF amplifiers and bandpass filters and known
as the IF strip. Most of the receiver gain and se-
lectivity is achieved in the IF section.
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Figure 4 The illustration of mixers
·
The FM demodulator section: The quadrature
FM demodulator uses a 90° phase shift, a sin-
gle tuned circuit, and a product detector to de-
modulate the FM signals.
The first IF is a relatively high frequency
(10.7MHz), for good image-frequency rejec-
tion, while the second IF is a relatively low
frequency (455kHz) that allows the IF ampli-
fiers to have a relatively high gain and still
not be susceptible to oscillations.
·
The audio amplifier section. The audio section
comprises several cascaded audio amplifiers
and one or more speakers. The number of am-
plifiers used depends on the audio signal
power desired.
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Figure 5 Intermediate frequency decoder block
15
April 10, 2000
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
