QwikRadio

tm

8

July 1999

MICRF002

MICRF002

Micrel

Functional Description

Please refer to “MICRF002 Block Diagram”. Identified in the

block diagram are the four principal functional blocks of the

IC, namely (1) UHF Downconverter, (2) OOK Demodulator,

(3) Reference and Control, and (4) 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 all the

external

components

needed

with

the

MICRF002

to

construct a complete UHF receiver. Four control inputs are

shown in the block diagram, SEL0, SEL1, SWEN, and

SHUT. Through 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.

Input SWEN selects the operating mode of the IC (FIXED

mode or SWP mode). When low, the IC is in FIXED mode,

and functions as a conventional superheterodyne receiver.

When SWEN is high, the IC is in SWP mode. In this mode,

while the topology is still superheterodyne, the local

oscillator (LO) 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 MICRF002, so

the device can operate in applications where significant

Transmitter/Receiver frequency misalignment may exist.

[Note:

The swept LO technique does not affect the IF

bandwidth, so noise performance is not impacted relative to

FIXED mode. In other words, the IF bandwidth is the same

(500kHz) whether the device is in FIXED or SWP mode.]

Due to limitations imposed by the LO sweeping process, the

upper limit on data rate in SWP mode is approximately

2.5kbps. Data rates beyond 10kbps are possible in FIXED

mode however.

Examples of SWP mode operation include applications

which utilize low-cost LC-based transmitters, whose transmit

frequency may vary up to

± 0.5% over initial tolerance,

aging, and temperature. In this (patent-pending) mode, the

LO frequency is varied in a prescribed fashion which results

in downconversion of all signals in a band approximately

1.5% around the transmit frequency.

So the Transmitter

may drift up to

± 0.5% without the need to retune the

Receiver, and without impacting system performance. Such

performance is not achieved with currently available crystal-

based superheterodyne receivers, which can operate only

with SAW or crystal based transmitters.

[Note:

In SWP mode only, a range penalty will occur in

installations where there exists a competing signal of

sufficient strength in this small frequency band of 1.5%

around the transmit frequency.

This results from the fact

that sweeping the LO indiscriminately “sweeps” all signals

within the sweep range down into the IF band. This same

penalty also exists with super-regenerative type receivers,

as their RF bandwidth is also generally 1.5%.

So any

application for a super-regenerative receiver is also an

application for the MICRF002 in SWP mode.]

For applications where the transmit frequency is accurately

set for other reasons (e.g., applications where a SAW

transmitter is used for its mechanical stability), the user may

choose

to

configure

the

MICRF002

as

a

standard

superheterodyne receiver (FIXED mode), mitigating the

aforementioned problem of a competing close-in signal.

This can be accomplished by tying SWEN to ground. Doing

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

In

FIXED mode, the ceramic resonator would be replaced with

a crystal.

Generally, however,

the MICRF002 can be

operated in SWP mode, using a ceramic resonator , with

either

LC or CRYSTAL/SAW based transmitters, without

any significant range difference.

The inputs SEL0 and SEL1 control the Demodulator filter

bandwidth in four binary steps (625Hz—5000Hz in SWP,

1250Hz—10000Hz in FIXED mode), and the user must

select the bandwidth appropriate to his needs.

Rolloff response of the IF Filter is 5

th

order, while the

demodulator data filter exhibits a 2

nd

order response.

Multiplication factor between the REFOSC frequency ft and

the internal Local Oscillator (LO) is 64.5X for FIXED mode,

and 64.25X for SWP mode (i.e., for ft = 6.00MHz in FIXED

mode, LO frequency = 6.00MHz * 64.5 = 387MHz).

Slicing Level and the CTH Capacitor

Extraction of the DC value of the demodulated signal for

purposes of logic-level data slicing is accomplished by

external capacitor CTH and the on-chip switched-cap

“resistor” RSC, indicated in the block diagram. The effective

resistance of RSC is 118kohms. The value of capacitor CTH

is easily calculated, once the slicing level time-constant is

chosen. Slicing Level time constant values vary somewhat

with decoder type, data pattern, and data rate, but typical

values range 5-50msec. Optimization of the CTH value is

required to maximize range, as discussed in “Application

Note TBD”.

During quiet periods (i.e., no signal transmissions) the Data

Output (DO pin) transitions randomly based on noise. This

may present problems for some decoders.

The most

common solution is to introduce a small offset (“Squelch”)

on the CTH pin so that noise does not trigger the internal

comparator.

Usually 20-30mV is sufficient, and may be

introduced by connecting a several-Megohm resistor from

the CTH pin to either VSS or VDD, depending on the desired

offset polarity. Since the MICRF002 is an AGC’d receiver,

noise at the internal comparator input is always the same,

set by the AGC. So the squelch offset requirement does not

change as the local “ether” noise changes from installation

to installation.

Note that introducing squelch will reduce

range modestly, so only introduce an amount sufficient to

“quiet” the output.