General description

TSH512

18/31

Doc ID 8120 Rev 7

5

General description

The TSH512 is a 0.4- to 11-MHz dual FM analog transmitter. This circuit offers the functions

needed for an advanced infrared STEREO transmitter. The access pins for each section

allow high versatility and therefore a lot of applications: mono infrared transmitter, stereo

transmitter, mono/stereo sub-carrier generator for video transmissions (for example the

popular 2.4 GHz video links). The block diagram for the TSH512 is shown in Figure 1 on

page 4.

Each audio input is amplified with a low noise amplifier (LNA section) allowing connection

to line level sources or directly to a microphone. Built-in MIC BIAS voltage references

provide bias for electret condenser microphones (ECM) with a high power supply rejection

ratio.

Each audio path also includes an automatic level control (ALC) to limit the over-

modulation and the distortion on very high signal amplitudes. The following operational

amplifier (PEA) allows a pre-emphasis transfer function before modulating the varicap

diode.

Built-in voltage references (VCO-BIAS) offer a regulated voltage to bias the varicap diodes.

The voltage controlled oscillator (VCO) is an integrated oscillator giving typically 600 mV

peak-to-peak at 2.8 MHz.

The output buffer section linearly amplifies the FM carrier to provide a sinusoidal output.

This sinusoidal signal reduces the inter-modulation products between the carriers,

especially in two-way or in multi-carrier systems (see Section 4: Application information on

page 14).

The voice operated transmit function (VOX) automatically detects when an audio signal

appears over the background noise.

The standby of the second transmitter reduces consumption in mono operation.

5.1

LNA section: low noise amplifier

For each transmitter, the audio source is connected to the LNA. The LNA stage is a low

noise operational amplifier typically usable with a gain from 0 to 40 dB.