October 2001

5

MICRF005

MICRF005

Micrel

Functional Diagram

Peak

Detector

AGC

Control

2nd Order

Low-Pass Filter

5th Order

Band-Pass Filter

Programmable

Synthesizer

Control

Logic

Reference

Oscillator

Crystal

ANT

SHUT

REFOSC

Switched-

Capacitor

Resistor

DO

MICRF005

RF

Amp

IF

Amp

IF

Amp

Compa-

rator

Reference and Control

UHF Downconverter

OOK Demodulator

T/R

MICRF005 Block Diagram

Functional Description

Refer to “MICRF005 Block Diagram”. Identified in the block

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

OOK Demodulator and Reference and Control. Also shown

in the figure are two capacitors (C

component (CR), usually a crystal. With the exception of a

supply decoupling capacitor, these are the only external

components needed by the MICRF005 to construct a com-

plete UHF receiver. Two control inputs are shown in the block

diagram: T/R and SHUT. Through these logic inputs, the user

can control the operation of the IC. These inputs are CMOS

compatible, and are pulled-up on the IC.

IF Bandpass Filter

Rolloff response of the IF Filter is 7th order, while the

demodulator data filter exhibits a 2nd order response.

Slicing Level

Extraction of the dc value of the demodulated signal for

purposes of logic-level data slicing is accomplished using the

external threshold capacitor C

capacitor “resistor” R

effective resistance of R

Slicing level time constant values vary somewhat with de-

coder type, data pattern, and data rate, but typical values

range from 5ms to 50ms. Optimization of the value of C

required to maximize range.

Squelch

During quiet periods (no signal) the data output (DO pin)

transitions randomly with noise, presenting problems for

some decoders. A simple solution is to introduce a small

offset, or squelch voltage, on the C

not trigger the internal comparator. Usually 20mV to 30mV is

sufficient, and may be introduced by connecting a several-

M

Ω resistor from the C

on the desired offset polarity. Since the MICRF005 has

receiver AGC, noise at the internal comparator input is

always the same, set by the AGC. The squelch offset require-

ment does not change as the local noise strength changes

from installation to installation. Introducing squelch will re-

duce range modestly. Only introduce an amount of offset

sufficient to quiet the output.

Automatic Gain Control

The signal path has AGC (automatic gain control) to increase

input dynamic range. An external capacitor, C

connected to the C

to-attack time-constant is fixed at 10:1 (that is, the attack time

constant is 1/10th of the decay time constant). However, the

attack time constant is set externally by choosing a value for

C

By adding resistance from the C

in parallel with the AGC capacitor, the ratio of decay-to-attack

time constant may be varied, although the value of such

adjustments must be studied on a per-application basis.

Generally the design value of 10:1 is adequate for the vast

majority of applications.

To maximize system range, it is important to keep the AGC

control voltage ripple low, preferably under 10mVpp once the

control voltage has attained its quiescent value. For this

reason capacitor values of at least 0.47

µF are recommended.

The AGC control voltage is carefully managed on-chip to

allow duty-cycle operation of the MICRF005 in excess of

10:1. When the device is placed into shutdown mode (SHUT

pin pulled high), the AGC capacitor floats, to retain the

voltage. When operation is resumed, only the voltage droop

on the capacitor due to leakage must be replenished, there-

fore a relatively low-leakage capacitor is recommended for