TSH512C datasheet(15/19 Pages) STMICROELECTRONICS

15 / 19 page

TSH512

15/19

Electret Condenser Microphone source

When a Electret Condenser Microphone (ECM) is

used, a high gain LNA is recommanded, but low

frequencies have to be attenuated. The ECM has

to be biased with a stable and clean reference

voltage.The TSH512 offers you the LNA and the

MIC-BIAS sections to perform this functions. (see

MIC-BIAS chapter).

Figure 3 : Electret Condenser Microphone source

The capacitor C in serie with the microphone

stops the DC coming from MIC-BIAS.

The resistor R provides the DC from MIC-BIAS to

supply the ECM.

Thanks to the ALC (Automatic Level control), the

great variations of amplitude will not overmodulate

the transmitter (refer to the chapter on ALC).

The self-adaptative VOX (Voice Operated Trans-

mit) offers an automatic transmitting with a good

discrimination of the background noise (see the

chapter on VOX).

MIC-BIAS section: microphone bias voltage

The MIC-BIAS bias voltages are dedicated to the

bias of Electret Condenser Microphones. These

bias voltages on pin 10 for TX1 and pin 2 for TX2,

exhibit a low voltage noise density of 22nV/

SQR(Hz). This allows more than 55 dB S/N con-

sidering a bandwith of 7 kHz. (see the figure in the

’Electret Condenser Microphone source’ chapter).

The MIC-BIAS voltage is related with VCC as fol-

low (with I MIC-BIAS= 2.5 mA):

VMIC-BIAS = 0.844.VCC-0.140 (Volts)

Moreover, the supply rejection ratio is guaranteed

better than 50 dB without any decoupling capaci-

tor. To address biasing of most of the micro-

phones, the current drive capability is 2.5 mA. The

MIC-BIAS voltage depend linearly on the supply

voltage Vcc (refer to the curve ’MIC-BIAS vs.

VCC’).

ALC section: Automatic Level Control

Both transmitters of the TSH512 are including Au-

tomatic Level Control (ALC). When the level of

the audio signal is too high, the ALC compress the

signal in order to avoid overmodulation of the FM

VCO. Therefore, the ALC reduces the distorsion

and keep a reduced transmit spectrum with very

high amplitude signals.

Figure 4 : Automatic Level Control Schematic

The ALC features a 20dB gain and an output sig-

nal regulated to 700 mVpp in compression.

The attack time is the response time of the ALC to

go from the linear amplification to the compression

region. The attack time mainly depends on CALC

capacitor value. A typical value of CALC is 1µF with

music as audio signal (refer to the ’application

schematic’).

The decay time is the response time of the ALC to

recover a full gain amplifying mode from a com-

pression mode. The decay time depends mainly

on the RALC resistor value. A typical value of RALC

is 470k with music as audio signal (refer to the ’ap-

plication schematic’).

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