Philips Semiconductors

Product specification

SA624

High performance low power FM IF system with

high-speed RSSI

1997 Nov 07

8

11

12

10

40k

80k

8

40k

V+

9

42k

SR00446

Figure 7. Second Limiter and Quadrature Detector

BPF

BPF

SR00447

Figure 8. Feedback Paths

a. Terminating High Impedance Filters with Transformation to Low Impedance

b. Low Impedance Termination and Gain Reduction

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HIGH IMPEDANCE

LOW IMPEDANCE

HIGH IMPEDANCE

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BPF

RESISTIVE LOSS INTO BPF

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SR00448

Figure 9. Practical Termination

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NE 624

430

430

SR00449

Figure 10. Crystal Input Filter with Ceramic Interstage Filter

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

input level, the limited signal will begin to dominate the regeneration,

and the demodulator will begin to operate in a “normal” manner.

There are three primary ways to deal with regeneration: (1)

Minimize the feedback by gain stage isolation, (2) lower the stage

input impedances, thus increasing the feedback attenuation factor,

and (3) reduce the gain. Gain reduction can effectively be