APPLICATIONS INFORMATION

FSD270 in Digital Audio Broadcasting (DAB)

Circuits

Digital audio is set to dominate car radio and other

mobile radio applications. Most of the required

functions can be achieved with a single integrated

circuit. One parts of the circuit that are not integrated

are the Voltage Controlled Oscillator (VCO) used in the

Phase-Locked Loop (PLL) channel selection circuit and

the tracking filters. Figure 1 shows the FSD270 dual

variable capacitance diode (also known as varactor or

tuner diode) in a DAB application.

Most textbook circuits show a single variable

capacitance diode in place of the FSD270. In practice

radio frequency oscillations in the tracking filters can

drive the single diode into conduction on peaks

increasing the bias voltage and giving rise to

undesirable harmonics. Common-cathode variable

capacitance diodes are used to overcome this problem.

The FSD270 dual common cathode device in the space

saving SOT23 package has been optimized for this

application. Zetex application note AN9 covers our

range of variable capacitance diodes and their

applications in detail.

FSD270

ISSUE 1 - AUGUST 2002

3

Figure 1.