SL6652

Fig. 3 Internal schematic

GENERAL DESCRIPTION

The SL6652 is a very low power, high performance

integrated circuit intended for IF amplification and

demodulation in FM radio receivers. It comprises:

q

A mixer stage for use up to lOOMHz

q

An uncommitted transistor for use as an oscillator

q

A current sink for biasing this transistor

q

A limiting amplifier operating up to 1.5MHz

q

A quadrature detector with differential AF output

q

An RSSI (Received Signal Strength Indicator) output

Mixer

The mixer is single balanced with an active load. Gain is

set externally by the load resistor although the value is

normally determined by that required for matching into the

ceramic filter. It is possible to use a tuned circuit but an

increase in mixer gain will result in a corresponding reduction

of the mixer input intercept point.

The RF input is a diode-biased transistor with a bias

current of typically 300

µA. The oscillator input is differential

but would normally be driven single-ended. Special care

should be taken to avoid accidental overload of the oscillator

input.

Oscillator

The oscillator consists of an uncommitted transistor and a

separate current sink. The user should ensure that the design

of oscillator is suitable for the type of crystal and frequency

required; it may not always be adequate to duplicate the

design shown in this data sheet.

IF amplifier

The limiting amplifier is capable of operation to at least 1

MHz and the input impedance is set by an external resistor to

match the ceramic filter. Because of the high gain, pins 12 and

13 must be adequately bypassed.

Detector

A conventional quadrature detector is fed internally from

the IF amplifier; the quadrature input is fed externally using an

appropriate capacitor and phase shift network. A differential

output is provided to feed a comparator for digital use,

although it can also be used to provide AFC.

RSSI output

The RSSI output is a current source with value

proportional to the logarithm of the IF input signal amplitude.

There is a small residual current due to noise within the

amplifier (and mixer) but beyond this point there is a measured

and guaranteed 70dB dynamic range. The typical range

extends to 92dB, independent of frequency, and with

exceptionally good temperature and supply voltage stability.

18

16

17

15

9

20

10

19

7

4k

13

14

12

200k

200k

11

5

6

8

3

BAND-GAP

REFERENCE

4

2

1

40k

40k

BIAS