MICRF218 datasheet(13/24 Pages) MICREL

13 / 24 page

Micrel

MICRF218

September 2007

M9999-090607

(408) 944-0800

Single Crystal Operation for Dual

Frequency Operation

When using a single crystal, the IF_BW function may

be used to switch between two operating frequencies.

Bandwidth will scale directly with operating frequency

(equation 3). Higher operating frequency will have the

wider IF bandwidth.

Given one operating frequency, the other frequency

can be determined.:

2.178)

(384

1.198)

(384

Freq1

Freq2

Bandwidth

Wide

Bandwidth

Narrow

(4)

1.198)

(384

2.178)

(384

Freq1

Freq2

Bandwidth

Narrow

Bandwidth

Wide

(5)

OOK Demodulator

The following section discusses the Demodulator

which is comprised of Detector, Programmable Low

Pass Filter, Slicer, and AGC comparator.

Detector and Programmable Low-Pass Filter

The demodulation starts with the detector removing

the carrier from the IF signal.

Post detection, the

signal

becomes

baseband

information.

The

programmable low-pass filter further enhances the

baseband information through the use of SEL0 and

SEL1. There are four programmable low-pass filter

BW settings for 433.92MHz operation, see Table 1.

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:

433.92

Freq)

(Operating

@433.92MHz

Freq

Operating

(6)

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 and Slicing Level

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 is at 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.

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, the

threshold 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 two components in Reference and Control

sub-block: 1) Reference Oscillator and 2) Control

Logic through parallel Inputs: SEL0, SEL1, SHDN and

IF_BW.

Reference Oscillator

Figure 6. Reference Oscillator Circuit

The reference oscillator in the MICRF218 (Figure 6)

uses a basic Colpitts crystal oscillator configuration

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