SA614A

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Product data sheet

Rev. 4 — 14 February 2014

12 of 28

NXP Semiconductors

SA614A

Low power FM IF system

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 455 kHz. The basic phenomenon

is shown in Figure 10. 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 is high with no signal input (should nominally be 250 mV or lower)

a. Terminating HIGH impedance filters with transformation to LOW impedance

b. LOW impedance termination and gain reduction

Fig 11. Practical termination

Fig 12. Crystal input filter with ceramic interstage filter

aaa-009764

BPF

high impedance

high impedance

low impedance

BPF

aaa-009765

BPF

resistive loss into BPF

A

BPF

aaa-009766

12

3

SA614A

430 Ω

430 Ω

45678

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