Micrel, Inc.

MICRF008

August 2008

11

M9999-080108

the data format needs to have a preamble that

resembles the data pattern, that is it has the same

period and duty cycle. (See “Application Hint 42”) If the

data pattern has no preamble, a large capacitor value

should be used, such as 1μF to 2.2μF. However, if the

calculated and should be used instead of a large

capacitor. This will guarantee a stable and reliable

performance for the receiver. If the data pattern has

calculated for the lowest data rate and optimized in

procedure below:

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.

[]

F

10

144.55

REFOSC

period

bit

5

C

3

TH

×

×

×

=

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.

frequencies mentioned above in the Table 4. Again, as

mentioned before, the data pattern needs preamble.

Baud Rate (Hz)

Frequency

(MHz)

1000

2400

4800

315

82nF

39nF

18nF

390

100nF

47nF

22nF

418

120nF

47nF

22nF

433.93

120nF

47nF

22nF

ripple on the AGC control voltage by using a sufficiently

large capacitor. It is suggested a value between 1μF to

10μF depending on data dead time, noise, and recovery

time from strong to low RF signals. Large capacitor

times faster. The drawback is the ripple noise from AGC

is current-based, with an attack current of 15μA, and a

decay current of 1.5μA. It is suggested the following

be 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

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

MICRF008YM 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 MICRF008YM. The

reference oscillator frequency can also be generated by

an external source through connector J1 and capacitor

reference oscillator frequency is calculated by the

following equation and Table 6 shows the resonator

frequency for the most common used frequencies:

129

fc

REFOSC

=

where:

•

REFOSC is the reference oscillator frequency in

MHz.

•

fc is the RF received carrier frequency of interest

in MHz.

Frequency (MHz)

REFOSC (MHz)

315

2.44

390

3.02

418

3.24

433.92

3.36

Table 6. Reference Oscillator Frequency

For a list of ceramic resonator manufactures, see

“Application Hint 35.”