TSH512CF datasheet(17/19 Pages) STMICROELECTRONICS

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TSH512

17/19

PEA section: Pre-Emphasis

The amplitude regulated audio coming from the

ALC feeds the postive input of the Operational

Amplifier

called

PEA

(Pre-Emphasis).

The

pre-emphasis consist in a high-pass filter in order

to compensate the behavior of the FM transmis-

sion.

Figure 8 : Pre-Emphasis schematic

RPEA1 and CPEA1 set the time constant of the

pre-emphasis as:

τ = R

PEA1 . CPEA1

50 µs or 75µs time constant are generally used.

Choosing the gain of the PEA stage allows also to

set the right modulation level to the varicap diode.

The gain in the pass-band is:

GPEA = 1+ (RPEA2/RPEA1)

VCO section: Voltage Controlled Oscillator

Each TSH512’s transmitter has his own oscillator

to generate the carrier. The audio signal is applied

on the varicap diode to perform the Frequency

Modulation. Thanks to the VCO-BIAS voltage ref-

erence, the DC bias of the varicap is stabilized.

The high PSRR (Power Supply Rejection ratio) of

the VCO-BIAS insure good immunity with the

noise of the power supply.

Figure 9 : VCO schematic

The generated frequency can be set from 400 kHz

to 11 MHz by external components. Refer to the

table 1 for the usual frequencies in Infrared audio.

The working frequency is:

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’s possible to use varicap diodes SMV1212 (Al-

pha Ind.) or ZC833 (Zetex).

The output level of the VCO can be reduced by

adding the resistor RVCO beetween pin 19 and

pin 20 or beetween pin 36 and pin 37 for TX1 and

TX2 respectively.

Output Buffer section

The output buffers are able to deliver a sinusoidal

signal with 1.5Vpp amplitude in a 1K

Ω load. This

impedance is compatible with popular biasing cir-

cuitry of external transistor drivers of IR LEDs.

The VOX-MUTE logic signal can be used to con-

trol the external LED drivers. When the audio is

not present on the TX1 input, VOX-MUTE is at

’Low’ state, the TSH512’s internal buffers are mut-

ed, and external drivers can be switched off by

controlling their bias.

SBY pin: Standby for mono operation

A high state on the Standby pin (SBY) sets the

second transmitter TX2 in power-down. The SBY

pin is typically used when the TSH512 is used as a

mono transmitter (ie: infrared microphone trans-

mitter).

Usual Infrared frequencies

IR frequency

applications

1.6 MHz

AM mono

1.7 MHz

FM mono

2.3 MHz

FM right channel

2.8 MHz

FM left channel or mono

fVCO

LCt

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