MICRF229 datasheet(11/23 Pages) MICREL

11 / 23 page

Micrel, Inc.

MICRF229

April 15, 2015

Revision 1.0

There are eight selectable low-pass filter BW settings:

1625Hz, 3250Hz, 6500Hz, 11000Hz, 13000Hz, 19000Hz,

34000Hz and 46000Hz for 433.92MHz operation. The

low-pass filter BW is directly proportional to the crystal

reference

frequency,

and

hence

Operating

Frequency. Filter BW values can be easily calculated by

direct scaling. Equation 5 illustrates filter Demod BW

calculation:

BWOperating Freq = BW@433.92MHz ×













433.92

(MHz)

Freq

Operating

Eq. 5

It is very important to select a suitable low-pass filter BW

setting for the required data rate to minimize bit error

rate.

Use the sensitivity curves that show BER vs. bit

rates for different D[16:4:3] settings as a guide.

This low-pass filter

−3dB corner frequency bandwidth can

be configured by setting the registers as in Table 1 for

433.92MHz.

Table 1. Low-Pass Filter Bandwidth Selection @ 434MHz RF

Input

D[16]

D[4]

D[3]

Low-Pass

Filter BW

Maximum

Encoded Bit Rate

1625Hz

2.5KBps

3250Hz

5KBps

6500Hz

10KBps

13000Hz

20KBps

11000Hz

Do Not Use

19000Hz

34000Hz

46000Hz

Bit rate refers to the encoded bit rate. Encoded bit rate is

1/(shortest pulse duration) that appears at DO, as

illustrated in Figure 2.

Figure 2. Transmitted Bit Rate through the Air

Alternatively the default registers setting for D[16:4:3] is

011 at power up, without programming the setting of

these bits, the demodulation bandwidth can be selected

externally by SEL1 pin.

Table 2. Demod Bandwidth

− SEL1 External Input

SEL1

Bandwidth at 434MHz

3250Hz – D3,D4 must be 11

13000Hz – default internal pull up

Slicer and CTH

The signal prior to the slicer, labeled “Audio Signal” in the

Functional Diagram, is still baseband analog signal. The

data slicer converts the analog signal into ones and zeros

based on 50% of the slicing threshold voltage built up in

the CTH capacitor. After the slicer, the signal is

demodulated OOK digital data. When there is only

thermal noise at ANT pin, the voltage level on CTH pin is

about 650mV. This voltage starts to drop when there is

RF signal present. When the RF signal level is greater

than

−100dBm, the voltage is about 400mV.

The value of the capacitor from CTH pin to GND is not

critical to the sensitivity of MICRF229, although it should

be large enough to provide a stable slicing level for the

comparator. The value used in the evaluation board of

0.1μF is good for all bit rates from 500bps to 40kbps.

The data slice level can be set by programming D[6:5]

bits, which also has the effect on the sensitivity of the

receiver as indicated in the sensitivity graphs.

Table 3. Slice Level

− Serial Register Control

Mode

Slice level 60%

Slice level 30%

Slice level 40%

Slice level 50% - default

CTH Hold Mode

If the internal demodulated signal (DO in the Functional

Diagram) is at logic LOW for more than about 4msec, the

chip automatically enters CTH hold mode, which holds

the voltage on CTH pin constant even without RF input

signal. This is useful in a transmission gap, or “dead

time”, used in many encoding schemes. When the signal

reappears, CTH voltage does not need to re-settle,

improving the time to output with no pulse width

distortion, or time to good data (TTGD).

MICRF229 Datasheet(PDF) 11 Page - Micrel Semiconductor

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