MICRF211 datasheet(7/18 Pages) MICREL

7 / 18 page

Micrel

MICRF211

March 2007

M9999-030107

(408) 944-0800

carrier from the IF signal.

Post detection, the signal

becomes base band information.

The programmable

low-pass

filter

further

enhances

the

base

band

information.

There are four programmable low-pass

filter BW settings: 1625Hz, 3250Hz, 6500Hz, 13000Hz

for 433.92MHz operation. Low pass filter BW will vary

with RF Operating Frequency. Filter BW values can be

easily calculated by direct scaling. See equation below

for filter BW calculation:

BWOperating Freq = BW@433.92MHz*

⎟

⎠

⎞

⎜

⎝

⎛

433.92

(MHz)

Freq

Operating

It is very important to choose the filter setting that best

fits the intended data rate to minimize data distortion.

Demod BW is set at 13000Hz @ 433.92MHz as default

(assuming both SEL0 and SEL1 pins are floating). The

low pass filter can be hardware set by external pins

SEL0 and SEL1.

SEL0

SEL1

Demod BW (@ 434MHz)

1625Hz

3250Hz

6500Hz

13000Hz

- default

Table 1: Demodulation BW Selection

Slicer, Slicing Level and Squelch

The signal prior to slicer is still linear demodulated AM.

Data slicer converts this signal into digital “1”s and “0”s

by comparing with the threshold voltage built up on the

CTH capacitor.

This threshold is determined by

detecting the positive and negative peaks of the data

signal and storing the mean value. Slicing threshold

default is 50%. After the slicer the signal is now digital

OOK data.

During long periods of “0”s or no data period, threshold

voltage on the CTH capacitor may be very low. Large

random noise spikes during this time may cause

erroneous “1”s at DO pin. Squelch pin when pull down

low will suppress these errors.

AGC Comparator

The AGC comparator monitors the signal amplitude

from the output of the programmable low-pass filter.

When the output signal is less than 750mV thresh-hold,

1.5µA current is sourced into the external CAGC

capacitor. When the output signal is greater than

750mV, a 15µA current sink discharges the CAGC

capacitor.

The voltage developed on the CAGC

capacitor acts to adjust the gain of the mixer and the IF

amplifier to compensate for RF input signal level

variation.

Reference Control

There are 2 components in Reference and Control sub-

block: 1) Reference Oscillator and 2) Control Logic

through parallel Inputs: SEL0, SEL1, SHDN

Reference Oscillator

Figure 2: Reference Oscillator Circuit

The reference oscillator in the MICRF211 (Figure 2)

uses a basic Colpitts crystal oscillator configuration with

MOS transconductor to provide negative resistance. All

capacitors shown in Figure 2 are integrated inside

MICRF211.

R01 and R02 are external pins of

MICRF211.

User only needs to connect reference

oscillation crystal.

Reference oscillator crystal frequency can be calculated:

FREF OSC = FRF/(32 + 1.1/12)

For 433.92 MHz, FREF OSC = 13.52127 MHz.

To operate the MICRF211 with minimum offset, crystal

frequencies should be specified with 10pF loading

capacitance.

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