MICRF007

Micrel

MICRF007

12

December 2000

Application Information

Transmitter Compatibility

Generally, best performance and range will be realized when

the MICRF007 is operated in a system using a SAW or crystal

based transmitter. The receiver reference oscillator requires

the use of a crystal.

Bypass Capacitors

The power supply bypass capacitors connected to VDD

should have the shortest possible lead lengths. For best

performance, connect directly to VSS

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 VSS to provide additional receive selectivity

and input overload protection. A typical filter is included in

Figure 7a.

Data Squelching

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 MICRF007

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.

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.

Crystal Selection

Selecting Reference Oscillator Frequency f

T

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

1.064

f

390

LO

TX

TX

=± 







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

64.5

T

LO

=

Frequency f

REFOSC on the MICRF007. Four-decimal-place accuracy

on the frequency is generally adequate. The following table

identifies f

t

i

m

s

n

a

r

T

y

c

n

e

u

q

e

r

F

f

X

T

r

o

t

a

l

l

i

c

s

O

e

c

n

e

r

e

f

e

R

y

c

n

e

u

q

e

r

F

f

T

z

H

M

5

1

3z

H

M

0

7

9

8

.

4

z

H

M

0

9

3z

H

M

0

3

6

0

.

6

z

H

M

8

1

4z

H

M

3

8

9

4

.

6

z

H

M

2

9

.

3

3

4z

H

M

8

5

4

7

.

6

Table 2. Common Transmitter Frequencies

External Timing Signals

Externally applied signals should be ac-coupled and the

amplitude must be limited to approximately 0.5Vpp.

Frequency and Capacitor Selection

Selection of the slicing level capacitor (C

capacitor (C

Selecting Capacitor C

TH

The first step in the process is selection of a data-slicing-level

time constant. This selection is strongly dependent on sys-

tem issues including system decode response time and data

code structure (that is, existence of data preamble, etc.). This

issue is covered in more detail in Application Note 22.

Source impedance of the CTH pin is given by equation (4),

where f

(4)

R1

k

4.90

f

SC

T

= 18 Ω

Assuming that a slicing level time constant

τ has been

established, capacitor C

(5)

C

R

TH

SC

= τ

A standard

±20% X7R ceramic capacitor is generally suffi-

cient.

Selecting C

Selection of C

AGC control voltage by using a sufficiently large capacitor.

Factory experience suggests that C

vicinity of 0.47

µF to 4.7µF. Large capacitor values should be

carefully considered as this determines the time required for

the AGC control voltage to settle from a completely dis-

charged condition. AGC settling time from a completely

discharged (zero-volt) state is given approximately by Equa-

tion 6: