February 9, 2000

11

MICRF004/RF044

MICRF004/RF044

Micrel

Application Information

Transmitter Compatibility

Generally, the MICRF004 can be operated in sweep mode,

using a low-cost ceramic resonator. Sweep mode works with

LC-, crystal-, or SAW-based transmitters, without any signifi-

cant range difference. In fixed mode a SAW-based or crystal-

controlled transmitter must be used.

Bypass and Output Capacitors

The bypass and output capacitors connected to VSSBB

should have the shortest possible lead lengths. For best

performance, connect VSSRF to VSSBB at the power supply

only (that is, keep V

V

Crystal or Ceramic Resonator Selection

Do not use resonators with integral capacitors since capaci-

tors are included in the IC.

If operating in fixed mode, a crystal must be used. In sweep

mode, either a crystal or ceramic resonator may be used.

External Timing Signals

Externally applied signals should be ac-coupled and the

amplitude must be limited to approximately 0.5Vpp.

Bandwidth Programming

Bandwidth must be selected accoring to the application.

0

L

E

S1

L

E

S

h

t

d

i

w

d

n

a

B

r

o

t

a

l

u

d

o

m

e

D

e

d

o

M

p

e

e

w

Se

d

o

M

D

E

X

I

F

11

z

H

0

0

4

4z

H

0

0

8

8

01

z

H

0

0

2

2z

H

0

0

4

4

10

z

H

0

0

1

1z

H

0

0

2

2

00

z

H

0

5

5z

H

0

0

1

1

Table 1. Bandwidth Selection

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 typical filter is included

in Figure 7a.

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 CTH pin so that noise does

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

sufficient, and may be introduced by connecting a several-

megohm resistor from the CTH pin to either V

depending on the desired offset polarity. Since the MICRF004

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.

Utilizing Wake-Up

To utilize the wake-up function, a burst of RF carrier in excess

of 5.5ms must be received at the start of

each data code word

(preferred for best communication reliability) or a single

5.5ms RF carrier tone must be received at the start of the data

pattern. When this constant carrier is detected, without inter-

ruption, WAKEB will transition low and stay low until data

begins.

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.

AGC Configuration

By adding resistance from the CAGC pin to VDDBB or

VSSBB 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.

Frequency and Capacitor Selection

Selection of the reference oscillator frequency f

capacitor (C

marized in this section.

Selecting Reference Oscillator Frequency f

T

(Fixed Mode)

As with any superheterodyne receiver, the difference be-

tween the internal LO (local oscillator) frequency f

incoming transmit frequency f

center frequency. Equation 1 may be used to compute the

appropriate f

(1)

f

f

0.787

f

LO

TX

TX

=± 







150

Frequencies f

f

and “low-side mixing,” and there is generally no preference of

one over the other.

After choosing one of the two acceptable values of f

Equation 2 to compute the reference oscillator frequency f

(2)

f

f

32.5

T

LO

=

Frequency f

REFOSC on the MICRF004. Four-decimal-place accuracy

on the frequency is generally adequate. The following table

identifies f

MICRF004 is operated in fixed mode.