MICRF230 datasheet(11/20 Pages) MICREL

11 / 20 page

Micrel, Inc.

MICRF230

ASK/OOK Demodulator

The demodulator section is comprised of detector,

programmable

low

pass

filter,

slicer,

and

AGC

comparator.

Detector and Programmable Low-Pass Filter

The demodulation starts with the detector removing the

carrier from the IF signal. Post detection, the signal

becomes baseband information. The low-pass filter

further enhances the baseband signal.

There are four selectable low-pass filter BW settings:

1625Hz, 3250Hz, 6500Hz and 13000Hz for 433.92MHz

operation. The low-pass filter BW is directly proportional

to the crystal reference frequency, and RF 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)

Freqruency

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 SEL0:SEL1 settings as a guide.

This

low-pass

filter

with

−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

SEL1

SEL0

Low-Pass

Filter BW

Maximum

Encoded Bit Rate

1625Hz

2.5KBps

3250Hz

5KBps

6500Hz

10KBps

13000Hz

20KBps

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

1/(shortest pulse duration) that appears at DO:

Figure 3. Transmitted Bit Rate through the air

Slicer and CTH

The signal before the slicer, labeled “Audio Signal” in

Figure 1, is still a 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 capacitor value from the CTH pin to GND is not

critical to the sensitivity of MICRF230. However, it should

be large enough to provide a stable slicing level for the

comparator. The

0.1μF value used in the evaluation board

is good for all bit rates from 500bps to 20kbps.

CTH Hold Mode

If the internal demodulated signal (DO in Figure 1) is at

logic LOW for more than approximately 4ms, the chip

automatically enters CTH hold mode, which holds the

voltage on CTH pin constant even without a 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 resettle. This

improves the time to output with no pulse width distortion,

or time to good data (TTGD).

AGC Loop

The AGC comparator monitors the signal amplitude from

the output of the programmable low-pass filter. The AGC

loop in the chip regulates the signal from the output point

to be at a constant level when the input RF signal is

within the AGC loop dynamic range (about

−115dBm to

−40dBm).

When the chip first turns on, the fast charge feature

charges the AGC node up with 120µA typical current.

When the voltage on AGC increases, the gains of the

mixer and IF amplifier go up, increasing the amplitude of

the audio signal (as labeled in Figure 1), even with only

thermal noise at the LNA input. The fast-charge current is

disabled when the audio signal crosses the slicing

threshold, causing DO’ to go high, for the first time.

When an RF signal is applied, a fast-attack period

ensues when 600µA current discharges the AGC node to

reduce the gain to a proper level. Once the loop reaches

equilibrium, the fast attack current is disabled, leaving

only 15µA to discharge AGC or 1.5µA to charge AGC.

The fast attack current is enabled only when the RF

signal increases faster than the ability of the AGC loop to

track it.

The ability of the chip to track to a signal that decreased

in strength becomes much slower, since only 1.5

μA is

available to charge the AGC to increase the gain. When

designing a transmitter that communicates with the

April 15, 2015

Revision 2.0

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