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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

For example, to choose a Butterworth filter response with

a corner frequency of 6kHz:

C5 (= 1.414)(1001k.Ω00)(03.14)(6kHz) 375pF

= C6 (= 4)(100kΩ1.)4(31.414)(6kHz) 186pF

Choosing standard capacitor values changes C5 to 390pF

and C6 to 180pF, as shown in the Typical Application

Circuit.

Table 1. Coefficients 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

MAX7036

RSSI

RDF2

100kΩ

RDF1

100kΩ

16

14

15

DSP

OPP

DFFB

C6

C5

Figure 2. Sallen-Key Lowpass Data Filter

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 thresh-

old voltage. One input is supplied by the data-filter output.

Both comparator inputs are accessible off chip to allow

for different methods of generating the slicing threshold,

which is applied to the second comparator input.

The suggested data-slicer configuration uses a resistor

(R1) connected between DSN and DSP with a capaci-

tor (C4) from DSN to GND (Figure 3). This configuration

averages the analog output of the filter and sets the

threshold to approximately 50% of that amplitude. With

this configuration, the threshold automatically adjusts as

the analog signal varies, minimizing the possibility for

errors in the digital data. The values of R1 and C4 affect

how fast the threshold tracks to the analog amplitude. Be

sure to keep the corner frequency of the RC circuit much

lower than the lowest expected data rate.

MAX7036

DATA

SLICER

20

DATAOUT

DATA

FILTER

17

16

DSN

DSP

R1

C4

Figure 3. Generating Data-Slicer Threshold

Note that a long string of zeros or ones can cause the

threshold to drift. This configuration works best if a coding

scheme (e.g., Manchester coding, which has an equal

number of zeros and ones) is used.

Peak Detector

The peak-detector output (PDOUT), in conjunction with

an external RC filter, creates a DC output voltage equal

to the peak value of the data signal. The resistor provides

a path for the capacitor to discharge, allowing the peak

detector to dynamically follow peak changes of the data-

filter output voltage. The peak detector can be used for at

least two functions. First, it can serve as an RSSI for ASK

modulation. Second, it can be used for faster data-slicer

response by adding it to the threshold pin (DSN) on the

data-slicer comparator (Figure 4). The two capacitors in

this circuit should be equal, and the peak detector resistor

should be approximately 10 times larger than the resistor

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