Intermediate Frequency and RSSI

The IF section presents a differential 330

Ω load to pro-

vide matching for the off-chip ceramic filter. The six

internal AC-coupled limiting amplifiers produce an

overall gain of approximately 65dB, with a bandpass-fil-

ter-type response centered near the 10.7MHz IF fre-

quency with a 3dB bandwidth of approximately 10MHz.

The RSSI circuit demodulates the IF by producing a DC

output proportional to the log of the IF signal level, with

a slope of approximately 14.2mV/dB (see the

Typical

Operating Characteristics).

Applications Information

Crystal Oscillator

The crystal oscillator in the MAX7033 is designed to

present a capacitance of approximately 3pF between

the XTAL1 and XTAL2. If a crystal designed to oscillate

with a different load capacitance is used, the crystal is

pulled away from its stated operating frequency, intro-

ducing an error in the reference frequency. Crystals

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:

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:

C

b

ak

f

C

a

kf

C

C

5

100

6

4 100

=

()( )( )

π

π

f

C

CC

CC

P

M

CASE

LOAD

CASE

SPEC

=

++

⎛

⎝⎜

⎞

⎠⎟

×

2

11

10

6

-

315MHz/433MHz ASK Superheterodyne

Receiver with AGC Lock

12

______________________________________________________________________________________

RSSI

100k

Ω

100k

Ω

C5

19

DFO

21

OPP

22

DFFB

C6

MAX7033

FILTER TYPE

a

b

Butterworth (Q = 0.707)

1.414

1.000

Bessel (Q = 0.577)

1.3617

0.618

Figure 2. Sallen-Key Lowpass Data Filter

Table 2. Coefficents to Calculate C5 and C6