MAX7033(2004) データシートの表示（PDF） - Maxim Integrated

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MAX7033
(Rev.:2004)

315MHz/433MHz ASK Superheterodyne Receiver with AGC Lock

Maxim Integrated 

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:

fP

=

CM

2



 CCASE

1

+ CLOAD

-

CCASE

1



+ CSPEC 

× 106

where:

fP is the amount the crystal frequency pulled in ppm.

CM is the motional capacitance of the crystal.

CCASE is the case capacitance.

CSPEC is the specified load capacitance.

CLOAD is the actual load capacitance.

When the crystal is loaded as specified, i.e., CLOAD =

CSPEC, the frequency pulling equals zero.

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.

Data Filter

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.

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:

C5

=

b

a(100k)(π)(fC)

C6

=

a

4(100k)(π)(fC)

where fC is the desired 3dB corner frequency.

For example, to choose a Butterworth filter response

with a corner frequency of 5kHz:

C5

=

1.000

(1.414)(100kΩ)(3.14)(5kHz)

≈

450pF

C6

=

1.414

(4)(100kΩ)(3.14)(5kHz)

≈

225pF

Choosing standard capacitor values changes C5 to

470pF and C6 to 220pF, as shown in the Typical

Application Circuit.

Data Slicer

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

FILTER TYPE

Butterworth (Q = 0.707)

Bessel (Q = 0.577)

a

1.414

1.3617

b

1.000

0.618

MAX7033

RSSI

RDF2

100kΩ

RDF1

100kΩ

19

21

22

DFO

OPP

DFFB

C6

C5

Figure 2. Sallen-Key Lowpass Data Filter

12 ______________________________________________________________________________________

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