MICRF008

Micrel

MICRF008

10

July 2003

C

In order to calculate the right value for the C

data format needs to have a preamble that resembles the

data pattern, that is it has the same period and duty cycle.

(See

preamble, a large capacitor value should be used, such as

1

µF to 2.2µF. However, if the data pattern has a preamble,

the C

instead of a large capacitor. This will guarantee a stable and

reliable performance for the receiver. If the data pattern has

variable data rates, the C

for the lowest data rate and optimized in range tests. To find

the C

1. Find the data period or bit period, and the

reference oscillator frequency. The reference

oscillator frequency is the RF carrier frequency

divided by 129. The bit period is the elapsed

time from one high and one low of the data

pattern.

2. The C

C=

5 bit period REFOSC

144.55 10

F

TH

3

××

×

[]

where:

• REFOSC is the reference oscillator frequency in

MHz.

• Bit period is given in seconds and is the inverse

of the baud rate for Manchester encoding.

• The result obtained is in farads.

It follows the C

mentioned above in the Table 4. Again, as mentioned before,

the data pattern needs preamble.

Baud Rate (Hz)

1000

2400

4800

Frequency (MHz)

315

82nF

39nF

18nF

390

100nF

47nF

22nF

418

120nF

47nF

22nF

433.92

120nF

47nF

22nF

C

The function of the C

the AGC control voltage by using a sufficiently large capaci-

tor. It is suggested a value between 1

µF to 10µF depending

on data dead time, noise, and recovery time from strong to

low RF signals. Large capacitor values can be connected to

V

V

noise from AGC pin being thrown into the V

on this pin is current-based, with an attack current of 15

µA,

and a decay current of 1.5

µA. It is suggested the following

values for the C

further optimized during receiver range tests.

Baud Rate (Hz)

1000

4.7 to 10

2400

2.2 to 4.7

4800

1 to 2.2

Table 5. Recommended Values for C

AGC

Reference Oscillator Frequency

A Colpitts oscillator inside the chip generates the reference

oscillator frequency. It requires a resonator of some kind

connected to the REFOSC pin. Either a ceramic resonator or

a crystal can be used. A resonator is chosen due to its lower

cost and because the MICRF008BM is running in sweep

mode, which does not require the precision of a crystal.

Resonators found in the market normally have a precision of

0.5%. This precision is sufficient for the MICRF008BM. The

reference oscillator frequency can also be generated by an

external source through connector J1 and capacitor C1. The

maximum level should not exceed 0.5V

oscillator frequency is calculated by the following equation

and Table 6 shows the resonator frequency for the most

common used frequencies:

REFOSC =

fc

129

where:

• REFOSC is the reference oscillator frequency in

MHz.

• fc is the RF received carrier frequency of interest

in MHz.

Freq. (MHz)

REFOSC (MHz)

315.0

2.44

390.0

3.02

418.0

3.24

433.92

3.36

Table 6. Reference Oscillator Frequency

For a list of ceramic resonator manufactures, see

“Applica-

tion Hint 35.”

SEL0 Pin – Setting the Demodulator Bandwidth

The SEL0 pin sets the demodulator bandwidth. When the pin

is connected to ground the demodulator bandwidth is set to

its minimum. When the pin is left floating (internal pull-up) or

connected to V

maximum. The demodulator bandwidth is a function of the RF

frequency used. See Table 7 for the most common frequency

used versus the demodulator bandwidth.

RF Carrier Frequency (MHz)

SEL0

315

390

418

433.92

0

1.6kHz

2.0kHz

2.2kHz

2.4kHz

1

3.6kHz

4.0kHz

4.4kHz

4.8kHz

Table 7. Demodulator Bandwidth