SA624 Datasheet, PDF (8/15 Pages) NXP Semiconductors – High performance low power FM IF system with high-speed RSSI

Philips Semiconductors
High performance low power FM IF system with
high-speed RSSI
Product specification
SA624
42k
9
11
12
10
40k
V+
40k
8
80k
SR00446
Figure 7. Second Limiter and Quadrature Detector
BPF
HIGH IMPEDANCE
BPF
BPF
Figure 8. Feedback Paths
SR00447
BPF
HIGH IMPEDANCE
LOW IMPEDANCE
a. Terminating High Impedance Filters with Transformation to Low Impedance
BPF
BPF
RESISTIVE LOSS INTO BPF
b. Low Impedance Termination and Gain Reduction
Figure 9. Practical Termination
430
SR00448
16 15 14 13 12 11 10 9
NE 624
430
1 2 3 45 6 7 8
Figure 10. Crystal Input Filter with Ceramic Interstage Filter
SR00449
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
1997 Nov 07
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