MICRF506YML datasheet(21/42 Pages) MICREL

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Micrel

MICRF506BML/YML

Baudrate: The baud rate given is bit/sec

Switched Capacitor Filter

A6..A0

0001000

‘1’

ScClk_X2

‘0’

ScClk4

ScClk3

ScClk2

ScClk1

ScClk0

RSSI

A6..A0

0000001

Modulation1

Modulation0

‘0’

RSSI_en

LD_en

PF_FC1

PF_FC0

The main channel filter is a switched-capacitor

implementation of a six-pole elliptic low pass filter.

The elliptic filter minimized the total capacitance

required for a given selectivity and dynamic range.

The cut-off frequency of the switched-capacitor filter

is adjustable by changing the clock frequency.

RSSI

33kohm, 1nF, 125kbps, BW=200kHz, Vdd=2.5V

0.6

0.8

1.2

1.4

1.6

1.8

2.2

-125

-115

-105

-95

-85

-75

-65

-55

-45

-35

-25

Pin [dBm]

The clock frequency is designed to be 20 times the

cut-off frequency. The clock frequency is derived

from

the

reference

crystal

oscillator.

programmable 6-bit divider divides the frequency of

the crystal oscillator. To generate the correct non-

overlapping clock-phases needed by the filter this

frequency is then divided by 4. The cut-off frequency

of the filter is given by:

Figure 13. RSSI Voltage

fCUT

fXCO

⋅ ScClk

C10

1nF

33k

Pin 14

RSSI

fCUT: Filter cutoff frequency

fXCO: Crystal oscillator frequency

ScClk: Switched capacitor filter clock, bits

ScClk5-0

For instance, for a crystal frequency of 16MHz and if

the 6 bit divider divides the input frequency by 4 the

cut-off frequency of the SC filter is 16MHz/(40 x 4) =

100kHz. 1

st order RC low pass filters are connected

to the output of the SC filter-to-filter the clock

frequency.

Figure 14. RSSI Network

A Typical plot of the RSSI voltage as function of

input power is shown in Figure 13. The RSSI has a

dynamic range of about 50dB from about -110dBm

to -60dBm input power.

The lowest cutoff frequency in the pre- and the main

channel filter must be set so that the received signal

is passed with no attenuation, which is frequency

deviation plus modulation. If there are any frequency

offset between the transmitter and the receiver, this

must also be taken into consideration. A formula for

the receiver bandwidth can be summarized as

follows:

The RSSI can be used as a signal presence

indicator. When a RF signal is received, the RSSI

output increases. This could be used to wake up

circuitry that is normally in a sleep mode

configuration to conserve battery life.

Another application for which the RSSI could be

used is to determine if transmit power can be

reduced in a system. If the RSSI detects a strong

signal, if could tell the transmitter to reduce the

transmit power to reduce current consumption.

fBW

= + fOFFSET + fDEV + Baudrate / 2

where

fBW: Needed receiver bandwidth, fcut above

should not be smaller than fBW [Hz]

fOFFSET: Total frequency offset between

receiver and transmitter [Hz]

fDEV: Single-sided frequency deviation, see

chapter Modulator on how to calculate [Hz]

July 2006

M9999-092904

+1 408-944-0800

MICRF506YML Datasheet(PDF) 21 Page - Micrel Semiconductor

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