Philips Semiconductors

Product specification

SA614A

Low power FM IF system

1997 Nov 07

8

11

12

10

40k

80k

8

40k

V+

9

42k

SR00329

Figure 7. Second Limiter and Quadrature Detector

BPF

BPF

SR00330

Figure 8. Feedback Paths

a. Terminating High Impedance Filters with Transformation to Low Impedance

b. Low Impedance Termination and Gain Reduction

BPF

HIGH IMPEDANCE

LOW IMPEDANCE

HIGH IMPEDANCE

BPF

BPF

RESISTIVE LOSS INTO BPF

BPF

A

SR00331

Figure 9. Practical Termination

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

614A

430

430

SR00332

Figure 10. Crystal Input Filter with Ceramic Interstage Filter

Because of the very high gain, bandwidth and input impedance of

the limiters, there is a very real potential for instability at IF

frequencies above 455kHz. The basic phenomenon is shown in

Figure 8. Distributed feedback (capacitance, inductance and

radiated fields) forms a divider from the output of the limiters back to

the inputs (including RF input). If this feedback divider does not

cause attenuation greater than the gain of the forward path, then

oscillation or low level regeneration is likely. If regeneration occurs,

two symptoms may be present: (1)The RSSI output will be high with

no signal input (should nominally be 250mV or lower), and (2) the

demodulated output will demonstrate a threshold. Above a certain