General description

TSH512

20/31

Doc ID 8120 Rev 7

Figure 27.

Electret condenser microphone source

The capacitor C in series 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 automatic level control (ALC), the great variations of amplitude will not over-

modulate the transmitter (refer to the Section 5.4: ALC section: automatic level control).

The self-adaptive VOX (voice operated transmit) offers automatic transmitting with a good

discrimination of the background noise (see Section 5.5: VOX description: voice operated

transmit on page 21).

5.3

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 22 nV/

√Hz). This allows more than 55 dB S/N considering a bandwidth of 7 kHz

(Figure 27).

Moreover, the supply rejection ratio is guaranteed to be better than 50 dB without any

decoupling capacitor. To address biasing of most of the microphones, the current drive

(refer to Figure 11 on page 11).