MICRF002/RF022

Micrel

MICRF002/RF022

12

March 2003

Power Supply Bypass Capacitors

VDDBB and VDDRF should be connected together directly at

the IC pins. Supply bypass capacitors are strongly

recommended. They should be connected to VDDBB and

VDDRF and should have the shortest possible lead lengths.

For best performance, connect VSSRF to VSSBB at the

power supply only (that is, keep V

through the V

Increasing Selectivity With an Optional BandPass

Filter

For applications located in high ambient noise environments,

a fixed value band-pass network may be connected between

the ANT pin and VSSRF to provide additional receive selec-

tivity and input overload protection. A minimum input configu-

ration is included in figure 7a. it provides some filtering and

necessary overload protection.

Data Squelching

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

transitions randomly with noise. Most decoders can

descriminate between this random noise and actual data but

for some system it does present a problem. There are three

possible approaches to reducing this output noise:

1). Analog squelch to raise the demodulator threshold

2). Digital squelch to disable the output when data is not

present

3). Output filter to filter the (high frequency) noise glitches on

the data output pin.

The simplest solution is add analog squelch by introducing a

small offset, or squelch voltage, on the CTH pin so that noise

does not trigger the internal comparator. Usually 20mV to

30mV is sufficient, and may be achieved by connecting a

several-megohm resistor from the CTH pin to either V

V

MICRF002 has receiver AGC noise at the internal compara-

tor input is always the same, set by the AGC. The squelch

offset requirement does not change as the local noise strength

changes from installation to installation. Introducing squelch

will reduce sensitivity and also reduce range. Only introduce

an amount of offset sufficient to quiet the output. Typical

squelch resistor values range from 6.8M

Ω to 10MΩ.

Wake-Up Function

The WAKEB output signal can be used to reduce system

power consumption by enabling the rest of a system when an

RF signal is present. The WAKEB is an output logic signal

which goes active low when the IC detects a constant RF

carrier. The wake-up function is unavailable when the IC is in

shutdown mode.

To activate the Wake-Up function, a received constant RF

carrier must be present for 128 counts or the internal system

clock. The internal system clock is derived from the reference

oscillator and is 1/256 the reference oscillator frequency. For

example:

f

f

P

128 counts x 0.04ms = 5.12ms

where:

f

f

P

The Wake-Up counter will reset immediately after a detected

RF carrier drops. The duration of the Wake-Up signal output

is then determined by the required wake up time plus an

additional RF carrier on time interval to create a wake up

pulse output.

WAKEB Output Pulse Time = T

Carrier On Time

For designers who wish to use the wakeup function while

squelching the output, a positive squelching offset voltage

must be used. This simply requires that the squelch resistor

be connected to a voltage more positive than the quiescent

voltage on the CTH pin so that the data output is low in

absence of a transmission.

I/O Pin Interface Circuitry

Interface circuitry for the various I/O pins of the MICRF002

are diagrammed in Figures 1 through 6. The ESD protection

diodes at all input and output pins are not shown.

CTH Pin

6.9pF

PHI2B

PHI1B

PHI1

PHI2

CTH

Demodulator

Signal

2.85Vdc

VDDBB

VSSBB

VSSBB

Figure 2. CTH Pin

Figure 2 illustrates the CTH-pin interface circuit. The CTH pin

is driven from a P-channel MOSFET source-follower with

approximately 10

µA of bias. Transmission gates TG1 and

TG2 isolate the 6.9pF capacitor. Internal control signals

PHI1/PHI2 are related in a manner such that the impedance

across the transmission gates looks like a “resistance” of

approximately 100k

Ω. The dc potential at the CTH pin is

approximately 1.6V