SA614A Philips Semiconductors (Acquired by NXP), SA614A Datasheet - Page 8

• Manufacturer Part Number: SA614A
• Description: SA614A; Low Power FM if System
• Manufacturer: Philips Semiconductors (Acquired by NXP)

Philips Semiconductors
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
1997 Nov 07
12
Low power FM IF system
11
10
Figure 7. Second Limiter and Quadrature Detector
42k
40k
BPF
a. Terminating High Impedance Filters with Transformation to Low Impedance
BPF
HIGH IMPEDANCE
Figure 10. Crystal Input Filter with Ceramic Interstage Filter
b. Low Impedance Termination and Gain Reduction
9
Figure 9. Practical Termination
V+
RESISTIVE LOSS INTO BPF
80k
SR00329
40k
LOW IMPEDANCE
8
A
430
8
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
16
1
BPF
BPF
15
2
14
3
13
4
HIGH IMPEDANCE
614A
Figure 8. Feedback Paths
12
5
BPF
430
11
6
10
SR00332
7
9
8
BPF
SR00331
Product specification
SA614A
SR00330