HT9015 datasheet(15/23 Pages) HOLTEK

15 / 23 page

HT9015

April 10, 2000

Preliminary

Intermediate frequency (IF) decoder part

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:

· 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.

· 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.

· 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 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.

· Mixers

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.

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.

P r es el ec t o r

RF - a m p l i f i e r

Re c e i v e

ant e nna

I n t e r nal

1s t Lo_os c

( R X_ VC O )

2nd Lo_os c

(C ry s t a l )

2nd I F

IF _ A M P

1 s t IF

2nd M i x e r

1s t M i x e r

Q uad r a t u r e

De t e c t o r

RS S I

RS S I C O M P

No i s e F i l t e r

Da t a C O M P

D_ CO M P _ I N

D_ CO M P _ O UT

RS S I

LP F

No i s e CO M P

Figure 5 Intermediate frequency decoder block

2nd Lo_os c

(C ry s t a l )

C e r a m i c F ilt e r

( 10. 7M H z )

( 455k H z )

Ce r a m i c F i l t e r

1 s t IF

2 n d IF

1s t Lo_os c

(R X _ V C O )

Figure 4 The illustration of mixers

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