MAX7033EUI+T Maxim Integrated Products, MAX7033EUI+T Datasheet - Page 12

• Manufacturer Part Number: MAX7033EUI+T
• Description: RF Receiver IC RX 315MHZ/433MHZ ASK
• Manufacturer: Maxim Integrated Products
• Type: Receiverr

Specifications of MAX7033EUI+T

• Package / Case: TSSOP-28
• Operating Frequency: 450 MHz
• Operating Supply Voltage: 3.3 V, 5 V
• Maximum Operating Temperature: + 105 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

315MHz/433MHz ASK Superheterodyne
Receiver with AGC Lock
designed to operate with higher differential load capac-
itance always pull the reference frequency higher. For
example, a 4.7547MHz crystal designed to operate
with a 10pF load capacitance oscillates at 4.7563MHz
with the MAX7033, causing the receiver to be tuned to
315.1MHz rather than 315.0MHz, an error of about
100kHz, or 320ppm.
In actuality, the oscillator pulls every crystal. The crys-
tal’s natural frequency is really below its specified fre-
quency, but when loaded with the specified load
capacitance, the crystal is pulled and oscillates at its
specified frequency. This pulling is already accounted
for in the specification of the load capacitance.
Additional pulling can be calculated if the electrical
parameters of the crystal are known. The frequency
pulling is given by:
where:
f
C
C
C
C
When the crystal is loaded as specified, i.e., C
C
It is possible to use an external reference oscillator in
place of a crystal to drive the VCO. AC-couple the exter-
nal oscillator to XTAL2 with a 1000pF capacitor. Drive
XTAL2 with a signal level of approximately -10dBm. AC-
couple XTAL1 to ground with a 1000pF capacitor.
The data filter is implemented as a 2nd-order lowpass
Sallen-Key filter. The pole locations are set by the com-
bination of two on-chip resistors and two external
capacitors. Adjusting the value of the external capaci-
tors changes the corner frequency to optimize for dif-
ferent data rates. The corner frequency should be set
to approximately 1.5 times the fastest expected data
rate from the transmitter. Keeping the corner frequency
near the data rate rejects any noise at higher frequen-
cies, resulting in an increase in receiver sensitivity.
The configuration shown in Figure 2 can create a
Butterworth or Bessel response. The Butterworth filter
offers a very flat amplitude response in the passband
and a rolloff rate of 40dB/decade for the two-pole filter.
12
P
M
CASE
SPEC
LOAD
SPEC
f
is the amount the crystal frequency pulled in ppm.
P
is the motional capacitance of the crystal.
______________________________________________________________________________________
=
, the frequency pulling equals zero.
C
is the case capacitance.
is the specified load capacitance.
is the actual load capacitance.
2
M
⎛
⎜
⎝
C
CASE
+
1
C
LOAD
-
C
CASE
1
+
C
LOAD
Data Filter
⎞
⎟ ×
⎠
LOAD
10
6
=
The Bessel filter has a linear phase response, which
works well for filtering digital data. To calculate the
value of C5 and C6, use the following equations, along
with the coefficients in Table 2:
where f
For example, to choose a Butterworth filter response
with a corner frequency of 5kHz:
Choosing standard capacitor values changes C5 to
470pF and C6 to 220pF, as shown in the Typical
Application Circuit .
The data slicer takes the analog output of the data filter
and converts it to a digital signal. This is achieved by
using a comparator and comparing the analog input to
a threshold voltage. One input is supplied by the data
Table 2. Coefficents to Calculate C5 and C6
Figure 2. Sallen-Key Lowpass Data Filter
Butterworth (Q = 0.707)
Bessel (Q = 0.577)
C
C
FILTER TYPE
C
5
6
19
DFO
is the desired 3dB corner frequency.
=
=
(
( )(
1 414 100
4 100
.
C
C
5
6
)(
C6
k
=
=
Ω
1 414
.
)(
1 000
21
OPP
a
4 100
k
3 14 5
.
(
(
Ω
.
100
100kΩ
)(
R
DF2
MAX7033
3 14 5
)(
k
k
.
b
a
1.3617
)( )( )
)( )( )
1.414
C5
kHz
π
π
a
)(
f
f
)
22
DFFB
C
C
kHz
≈
RSSI
100kΩ
R
225
DF1
)
≈
Data Slicer
pF
450
pF
1.000
0.618
b