TSH512CF datasheet(25/31 Pages) STMICROELECTRONICS

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TSH512

General description

Doc ID 8120 Rev 7

25/31

5.7

VCO section: voltage-controlled oscillator

Each TSH512 transmitter has its own oscillator to generate the carrier. The audio signal is

applied to the varicap diode to perform the frequency modulation. Thanks to the VCO-BIAS

voltage reference, the DC bias of the varicap is stabilized. The high power supply rejection

ratio (PSRR) of the VCO-BIAS ensures good immunity with the noise of the power supply.

Figure 33.

VCO schematics

The generated frequency can be set from 400 kHz to 11 MHz by external components.

Refer to Table 1 for the usual frequencies in infrared audio.

The working frequency is:

where Ct is the total capacity of CL, Cp, Cs and Cv:

Ct = 1/(1/Cc+1/CL) with Cc = Cp+1/(1/Cv+1/Cs)

It is possible to use varicap diodes SMV1212 (Alpha Ind.) or ZC833 (Zetex).

The output level of the VCO can be reduced by adding the resistor RVCO between pin 19

and pin 20 or between pin 36 and pin 37 for TX1 and TX2 respectively.

Table 5.

Usual infrared frequencies

IR frequency in MHz

Applications

1.6

AM mono

1.7

FM mono

2.3

FM right channel

2.8

FM left channel or mono

VCO

π LCt

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