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MAX7036

300MHz to 450MHz ASK Receiver with Internal IF Filter

Maxim Integrated 

MAX7036

300MHz to 450MHz ASK Receiver

with Internal IF Filter

frequency, introducing an error in the reference frequency.

A crystal designed to operate at a higher load capacitance

than the value specified for the oscillator is always pulled

higher in frequency. Adding capacitance to increase the

load capacitance on the crystal increases the start-up

time and may prevent oscillation altogether.

In actuality, the oscillator pulls every crystal. The crystal’s

natural frequency is really below its specified frequency,

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 pull-

ing is given by:

= f P

CM

2







C

CASE

1

+

CLOAD

−

C CASE

1

+

C SPEC





×

10

6



where:

fp is the amount the crystal frequency is 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 XTAL1 with a 1000pF capacitor. Drive

XTAL1 with a signal level of approximately -10dBm.

AC-couple XTAL2 to ground with a 1000pF capacitor.

IF Filter

The IF filter is a 2nd-order Butterworth lowpass filter pre-

ceded by a low-frequency DC block. The lowpass filter is

implemented as a Sallen-Key filter using an internal op

amp and two on-chip 22kΩ resistors. The pole locations

are set by the combination of the on-chip resistors and

two external capacitors (C9 and C10, Figure 1). The val-

ues of these two capacitors for a 3dB cutoff frequency of

400kHz are given below:

= C9 (= 1.414)(1R)(π)(fc) (1.414)(22kΩ)= (13.14)(400kHz) 26pF

= C10 (= 2.828)(1R)(π)(fc) (2.828)(22kΩ= )1(3.14)(400kHz) 13pF

Because the stray shunt capacitance at each of the pins

(IFC1 and IFC2) on a typical PCB is approximately 2pF,

choose the value of the external capacitors to be approxi-

mately 2pF lower than the desired total capacitance.

Therefore, the practical values for C9 and C10 are 22pF

and 10pF, respectively.

MAX7036

22kΩ

22kΩ

10

9

11

IFC1

IFC2

IFC3

C10

C9

Figure 1. Sallen-Key Lowpass IF Filter

Data Filter

The data filter is implemented as a 2nd-order lowpass

Sallen-Key filter. The pole locations are set by the

combination of two on-chip resistors and two external

capacitors. Adjusting the value of the external capacitors

changes the corner frequency to optimize for different

data rates. Set the corner frequency to approximately 1.5

times the fastest Manchester expected data rate from the

transmitter. Keeping the corner frequency near the data

rate rejects any noise at higher frequencies, 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

with the coefficients in Table 1.

C5

=

b

a(100k)(π)(f c )

C6

=

a

4(100k)(π)(f c )

where fC is the desired corner frequency.

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