APPLICATION INFORMATION (continued)

FM Section

IF Amplifier and limiter

The 10.7 MHz IF signal from the ceramic filter is amplified and limited by a chain of four differential stages.

Pin 16 is the amplifier input and has a typical input impedance of 6.5 KW in parallel with 14 pF at 10.7 MHz.

Bias for the first stage is available at pin 14 and provides 100% DC feedback for stable operating conditions.

Pin 15 is the second input to the amplifier and is decoupled to pin 14, which is grounded by a 20 nF capacitor.

An RLC network is connected to the amplifier output and gives a 90

° phase shift (at the IF centre frequency)

between pins 13 and 12. The signal level at pin 13 is about 150 mV rms

FM Detector

The circuit uses a quadrature detector and the choise of component values is determined by the acceptable

level of distortion at a given recovered audio level.

With a double tuned network the linearity improves (distortion is reduced) and the phase shift can be

optimized; however this leads to a reduction in the level of the recovered audio. A satisfactory compromise

for most FM receiver applications is shown in the test circuit.

Care shoul be taken with the physical layout.

The main recommandations are:

AM-FM Switching

AM-FM switching is achieved by applying a DC voltage at pin 13, to switch the internal reference.

Typical DC voltages (refer to the test circuit)

Pins

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Unit

AM

9

1.4

9

1.4

1.4

8.4

9

0.7

1.9

9

0

0.1

0.1

8.5

8.5

8.5

V

FM

9

0.02

9

0.02

0.02

8.5

9

0

1.7

90998

88

V

7/18

TDA1220B