MC3356_1 Datasheet, PDF (5/8 Pages) Motorola, Inc – Wideband FSK Receiver

Freescale Se_Mm_C3i₃c₅o₆nductor, Inc.
Figure 6. Application with Self–Adjusting Bias on Data Shaper
RF In
Data
5.0 V
Out
1
Car. Det. Out
0 V or 4.0 V
130 k 3.3 k
15 k
0.01
1:2
47 k
47 k
470
10 k
0.1
pF
18 k
470 pF
3.3 k
0.1
470 k
20
19
18
17
16
15
14
13
12 11
RF Input Ground Data Comp(+) Comp(–) Squelch Squelch Demod Demod Quad
Output
Status Control Out
Filter Input
f = 10.7
150 pF
1.5 µH
APPLICATION NOTES (continued)
Depending on the external circuit, inverted or
noninverted data is available at Pin 18. Inverted data makes
the higher frequency in the FSK signal a “one” when the local
oscillator is above the incoming RF. Figure 5 schematic
shows the comparator with hysteresis. In this circuit the dc
reference voltage at Pin 17 is about the same as the
demodulated output voltage (Pin 13) when no signal is
present. This type circuit is preferred for systems where the
data rates can drop to zero. Some systems have a low
frequency limit on the data rate, such as systems using the
MC3850 ACIA that has a start or stop bit. This defines the
low frequency limit that can appear in the data stream.
Figure 5 circuit can then be changed to a circuit configuration
as shown in Figure 6. In Figure 6 the reference voltage for
the comparator is derived from the demodulator output
through a low pass circuit where τ is much lower than the
lowest frequency data rate. This and similar circuits will
compensate for small tuning changes (or drift) in the
quadrature detector.
Squelch status (Pin 15) goes high (squelch off) when the
input signal becomes greater than some preset level set by
the resistance between Pin 14 and ground. Hysteresis is
added to the circuit externally by the resistance from Pin 14 to
Pin 15.
MOTOROLA
SOLUTIONS
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