MICRF004 Просмотр технического описания (PDF) - Micrel

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MICRF004

QwikRadio™ Low-Power VHF Receiver

Micrel 

MICRF004/RF044

Functional Diagram

Micrel

CAGC

CAGC

ANT

VDD

VSS

RF

Amp

fRX

fIF IF

Amp

fLO

Programmable

Synthesizer

5th Order

Band-Pass Filter

500kHz

IF

Amp

AGC

Control

Peak

Detector

2nd Order

Programmable

Low-Pass Filter

Switched-

Capacitor

Resistor

RSC

Compa-

rator

VHF Downconverter

OOK Demodulator

DO

CTH

CTH

SEL0

SEL1

SWEN

SHUT

Control

Logic

REFOSC

CR

Ceramic

Resonator

MICRF004

Reference

Oscillator

Reference and Control

Resettable

Counter

Wakeup

WAKEB

Functional Description

Refer to “MICRF004 Block Diagram”. Identified in the block

diagram are the four sections of the IC: UHF Downconverter,

OOK Demodulator, Reference and Control, and Wakeup.

Also shown in the figure are two capacitors (CTH, CAGC) and

one timing component (CR), usually a ceramic resonator.

With the exception of a supply decoupling capacitor, these

are the only external components needed by the MICRF004

to assemble a complete UHF receiver. Four control inputs are

shown in the block diagram: SEL0, SEL1, SWEN, and SHUT.

Using these logic inputs, the user can control the operating

mode and selectable features of the IC. These inputs are

CMOS compatible, and are pulled-up on the IC.

Sweep-Mode Enable

Logic-input SWEN selects either fixed-mode or sweep-mode

operation. When SWEN is low, the IC is in fixed mode, and

functions as a conventional superheterodyne receiver. When

SWEN is high, the IC is in sweep mode.

Fixed-Mode Operation

For applications where the transmit frequency must be accu-

rately set (that is, applications where a SAW transmitter is

used for its mechanical stability), the MICRF004 may be

configured as a standard superheterodyne receiver (fixed

mode). Fixed-mode operation receives a narrower band-

width making it less susceptable to competing signals. Fixed

mode is selected by connecting SWEN to ground which

forces the on-chip LO frequency to a fixed value. In fixed

mode a crystal (higher frequency tolerance) must be used

instead of a ceramic resonator (lower frequency tolerance).

Data rates beyond 10kb/s are possible in fixed mode.

Sweep-Mode Operation

In sweep mode, while the topology is still superheterodyne,

the LO (local oscillator) is deterministically swept over a

range of frequencies at rates greater than the data rate. When

coupled with a peak-detecting demodulator, this technique

effectively increases the RF bandwidth of the MICRF004,

allowing the device to operate in applications where signifi-

cant transmitter-receiver frequency misalignment may exist.

The swept-LO technique does not affect the IF bandwidth,

therefore noise performance is not degraded relative to fixed

mode. The IF bandwidth is 500kHz whether the device is

operating in fixed or sweep mode.

Due to limitations imposed by the LO sweeping process, the

upper limit on data rate in sweep mode is approximately

2.5kb/s.

Examples of sweep-mode operation include applications

utilizing low-cost LC-based transmitters, where the transmit

frequency may vary up to ±0.5% over initial tolerance, aging,

and temperature. In sweep mode, the LO frequency is varied

in a defined fashion which results in downconversion of all

signals in a band approximately 1.5% around the nominal

transmit frequency. The transmitter may drift up to ±0.5%

without the need to retune the receiver and without impacting

system performance. Similar performance is not currently

available with crystal-based superheterodyne receivers which

can operate only with SAW- or crystal-based transmitters.

In sweep mode only, a range reduction will occur in installa-

tions where there is an undesired competing signal of suffi-

cient strength within of 2% to 3% around the transmit fre-

quency. This is because the process indiscriminately in-

February 9, 2000

7

MICRF004/RF044

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