MICRF219A datasheet(10/23 Pages) MICREL

10 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com

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

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 operating curves that show BER vs. bit

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

This low-pass filter

−3dB corner, or the demodulation

BW, is set at 13000Hz @ 433.92MHz as default

(assuming both SEL0 and SEL1 pins are connected to

VDD). The low-pass filter can be set by changing register

bits D[4:3]. Table 2 demonstrates the scaling for

315MHz RF frequency:

D[4]

D[3]

Low-Pass

Filter BW

Maximum

Encoded Bit Rate

1625Hz

2.5kbps

3250Hz

5kbps

6500Hz

10kbps

13000Hz

20kbps

or (408) 944-0800

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

D[4]

D[3]

Low-Pass

Filter BW

Maximum

Encoded Bit Rate

1170Hz

1.8kbps

2350Hz

3.6kbps

4700Hz

7.2kbps

9400Hz

14.4kbps

Table 2. Low-Pass Filter Selection @ 315MHz RF input

Slicer and CTH

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

Figure 1, 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 MICRF219A, 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

20kbps.

CTH Hold Mode

If the internal demodulated signal (DO’ in Figure 1) 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

“deadtime”, 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).

AGC Loop and CAGC

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 at this 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 CAGC node up with 120µA typical current.

When the voltage on CAGC 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 CAGC

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 CAGC or 1.5µA to

charge CAGC. The fast attack current is enabled only

when the RF signal increases faster than the ability of

the AGC loop to track it.

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