MICRF221 datasheet(8/26 Pages) MICREL

8 / 26 page

Micrel Inc.

MICRF221

October 2008

M9999-100108

(408) 955-1690

Mixers and Synthesizer

The LO port of the mixers are driven by quadrature

local oscillators of the synthesizer block.

The

synthesizer block produces the local oscillator signal

on the low side of the desired RF signal with

suppression of the image frequency, at twice the IF

frequency below the wanted signal. The local

oscillator is set to 64 times the crystal reference

frequency by way of a phase locked loop synthesizer

with a fully integrated loop filter.

Image Reject Filter and Band-Pass Filter

The IF ports of the mixer produce quadrature down-

converted IF signals. The IF signal is filtered by the

image reject filter to remove image frequency

components and then follow up with a third order

band-pass filter.

The IF center frequency is

1.285MHz. The IF BW is 380kHz @ 915 MHz, and

the IF BW varies with RF operating frequency. The IF

BW can be calculated via direct scaling.

BW IF = BW IF@ 915MHz*(Operating Freq (MHz)/

915)

These filters are fully integrated inside the MICRF221.

After filtering, four active gain controlled amplifier

stages enhance the IF signal to proper level for

demodulation.

OOK Demodulator

The demodulator section is comprised of detector,

programmable low pass filter, slicer, AGC and

Desense.

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.

There are four settings for

programmable low-pass filter BW options: 1625Hz,

3250Hz, 6500Hz, 13000Hz. for 915MHz operation.

Low pass filter BW will vary with RF Operating

Frequency. Filter BW values can easily calculated by

direct scaling.

See equation below for filter BW

calculation:

BW Operating Freq = BW @ 915MHz*(Operating Freq

(MHz)/ 915)

It is very important to choose filter setting that fits best

for the intended data rate to minimize data distortion.

Demod BW is set at 13000Hz @ 915MHz as default

(assuming both SEL0 and SEL1 pins are floating).

The low pass filter can be hardware set by external

pins SEL0 and SEL1, or via serial programming

through register D3 and D4

SEL0

SEL1

Demod BW (@ 915MHz)

1625Hz

3250Hz

6500Hz

13000Hz

- default

Slicer and Slicing Level

The signal prior to the slicer is still AM. The data slicer

converts the AM signal into ones and zeros based

upon the threshold voltage built up in the CTH

capacitor. After the slicer, the signal is ASK or OOK

digital data.

The slicing threshold defaults at 50%. The slicing

threshold can be set via serial programming through

Slicing Level

Slice Level 30%

Slice Level 40%

Slice Level 50%

- default

Slice Level 60%

AGC

AGC monitors the signal amplitude from the output of

the programmable low-pass filter. When the output

signal is less than 750mV thresh-hold, AGC increases

the gain of the mixer and the IF amplifier. When the

output signal is greater than 750mV, the AGC lowers

the gain of the mixer and the IF amplifier.

Desense

Desense is a function designed to reduce the

sensitivity of the MICRF221 receiver to a maximum of

45dB for training the MICRF221 receiver to recognize

an intended transmitter. Very often, a receiver needs

to learn how to recognize a particular transmitter. It is

important for the receiver not to learn the signal of a

stray transmitter near by. The simplest solution is to

turn down the receiver gain, so the receiver only

recognizes the transmitter at close range.

The desense function is accessible only through serial

programming.

MICRF221 Datasheet(PDF) 8 Page - Micrel Semiconductor

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