MICRF002/RF022

Micrel

MICRF002/RF022

8

March 2003

In sweep-mode, a range reduction will occur in installations

where there is a strong interferer in the swept RF band. This

is because the process indiscriminately includes all signals

within the sweep range. An MICRF002 may be used in place

of a superregenerative receiver in most applications.

Step 2: Selecting The Reference

Oscillator

All timing and tuning operations on the MICRF002 are de-

rived from the internal Colpitts reference oscillator. Timing

and tuning is controlled through the REFOSC pin in one of

three ways:

1. Connect a ceramic resonator

2. Connect a crystal

3. Drive this pin with an external timing signal

The specific reference frequency required is related to the

system transmit frequency and to the operating mode of the

receiver as set by the SWEN pin.

Crystal or Ceramic Resonator Selection

Do not use resonators with integral capacitors since capaci-

tors are included in the IC, also care should be taken to

ensure low ESR capacitors are selected. Application Hint 34

and Application Hint 35 provide additional information and

recommended sources for crystals and resonators.

If operating in fixed-mode, a crystal is recommended. In

sweep-mode either a crystal or ceramic resonator may be

used. When a crystal of ceramic resonator is used the

minimum voltage is 300mV

signal it should be AC-coupled and limited to the operating

range of 0.1V

Selecting Reference Oscillator Frequency f

T

(Fixed Mode)

As with any superheterodyne receiver, the mixing between

the internal LO (local oscillator) frequency f

ing transmit frequency f

frequency. Equation 1 may be used to compute the appropri-

ate f

(1)

f

f

0.86

f

315

LO

TX

TX

=± 





Frequencies f

f

and “low-side mixing.” High-side mixing results in an image

frequency above the frequency of interest and low-side

mixing results in a frequency below.

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 MICRF002. Four-decimal-place accuracy

on the frequency is generally adequate. The following table

identifies f

MICRF002 is operated in fixed mode.

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. Fixed Mode Recommended Reference

Oscillator Values For Typical Transmit Frequencies

(high-side mixing)

Selecting REFOSC Frequency f

T

(Sweep Mode)

Selection of the reference oscillator frequency f

mode is much simpler than in fixed mode due to the LO

sweeping process. Also, accuracy requirements of the fre-

quency reference component are significantly relaxed.

In sweep mode, f

(3)

f

f

64.25

T

LO

=

In SWEEP mode a reference oscillator with frequency accu-

rate to two-decimal-places is generally adequate. A crystal

may be used and may be necessary in some cases if the

transmit frequency is particularly imprecise.

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

8

8

.

4

z

H

M

0

9

3z

H

M

5

0

.

6

z

H

M

8

1

4z

H

M

8

4

.

6

z

H

M

2

9

.

3

3

4z

H

M

3

7

.

6

Table 3. Recommended Reference Oscillator Values

For Typical Transmit Frequencies (sweep-mode)