MICRF213 datasheet(7/16 Pages) MICREL

7 / 16 page

Micrel, Inc.

MICRF213

May 2007

M9999-052307-A

(408) 944-0800

Detector and Programmable Low-Pass Filter

The demodulation starts with the detector removing the

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: 1180Hz, 2360Hz, 4270Hz, 9400Hz for

315MHz operation. Low pass filter BW will vary with RF

Operating Frequency. Filter BW values can be easily

calculated by direct scaling. See the equation below for

the filter BW calculation:

BWOperating Freq = BW@315MHz ×

⎟

⎠

⎞

⎜

⎝

⎛

315

(MHz)

Freq

Operating

It is very important to choose filter setting that best fits

the intended data rate as this will minimize data

distortion.

Demod BW is set at 9700Hz @ 315MHz 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 (@ 315MHz)

1180Hz

2360Hz

4270Hz

9400Hz

- 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

determined

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 becomes

digital OOK data.

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

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 threshold,

1.5µA current is then 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.

Reference Oscillator

BIAS

CC2

IBIAS

RO1

RO2

Normally on

Startup

Circuit

Figure 2. Reference Oscillator Circuit

The reference oscillator in the MICRF213 (reference

Figure 2) uses a basic Colpitts crystal oscillator

configuration with a MOS transconductor to provide

negative resistance. All capacitors shown in the figure

are integrated inside MICRF213. R01 and R02 are

external pins of MICRF213. The user only need connect

the reference oscillation crystal.

Reference oscillator crystal frequency can be calculated

thus as:

FREFOSC = FRF/(32 + 1.1/12)

For 315MHz, FREFOSC = 9.81563 MHz.

To operate the MICRF213 with minimum offset, crystal

frequencies should be specified with 10pF loading

capacitance.

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