1993_Motorola_Linear_Interface_ICs_Vol_2.pdf

MC3356
FIGURE 7 - APPLICATION WITH SELF·ADJUSTING BIAS ON DATA SHAPER
Data
Oul
APPLICATION NOTES (continued)
+5.0 V Car. Del. Out
MC3356
Depending on the external circuit, inverted or non·
inverted data is available at Pin lB. Inverted data makes
the higher frequency in the FSK signal a 'one' when Ihe
local oscillator is above the incoming RF. Figure 6 sche·
matic shows the comparalor wilh hysteresis. In this cir·
cuit 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 sys·
tems where the data rates can drop to zero. Some sys·
tems have a low frequency limit on the data rate, such
as syslems using the MC3B50 ACIA that has a start or
stop bit. This defines the low frequency limit that can
appear in the data stream. Figure 6 circuit can then be
OVor4.0V
130 k 3.3 k
MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA
8-63
f '" 10.7
150 pF
changed to a circuit configuralion as shown in Figure
7. In Figure 7 the reference voltage for the comparator
is derived from the demodulalor outpul through a low
pass circuit where T 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. Hys·
teresis is added to the circuit externally by the resistance
from Pin 14 to Pin 15.