MICRF505BML datasheet(18/27 Pages) MICREL

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MICRF505

Micrel

M9999-051304

May 13, 2004

Sallen-Key Filters

A6..A0

0000001

Modulation1

Modulation0

"0"

RSSI_en

LD_en

PF_FC1

PF_FC0

Each channel includes a pre-amplifier and a prefilter, which

is a three-pole Sallen-Key lowpass filter. It protects the

following switched-capacitor filter from strong adjacent chan-

nel signals, and it also works as an anti-aliasing filter. The

preamplifier has a gain of 22-23dB. The maximum output

voltage swing is about 1.4Vpp for a 2.25V power supply. In

addition, the IF amplifier also performs offset cancellation.

Gain varies by less than 0.5dB over a 2.0 - 2.5V variation in

power supply. The third order Sallen-Key low pass filter is

programmable to four different cut-off frequencies according

to the table below:

Switched Capacitor Filter

A6..A0

0000000

‘1’

ScClk_X2

ScClk5

ScClk4

ScClk3

ScClk2

ScClk1

ScClk0

PF_FC1

PF_FC0

Cut-off Freq. (kHz)

100

150

230

340

The main channel filter is a switched-capacitor implementa-

tion of a six-pole elliptic low pass filter. The elliptic filter

minimizes the total capacitance required for a given selectiv-

ity and dynamic range. The cut-off frequency of the switched-

capacitor filter is adjustable by changing the clock frequency.

The clock frequency is designed to be 20 times the cut-off

frequency. The clock frequency is derived from the reference

crystal oscillator. A programmable 6-bit divider divides the

frequency of the crystal oscillator. To generate the correct

non-overlapping clock-phases needed by the filter this fre-

quency is then divided by 4. The cut-off frequency of the filter

is given by:

80 ScClk

cut

XCO

◊

when ScClk_X2=0 and

40 ScClk

cut

XCO

◊

when ScClk_X2=1 where:

fcut: Filter cutoff frequency

fxco: Crystal oscillator frequency

ScCLK: Switched capacitor filter clock, bits ScClk5-0

For instance, for a crystal frequency of 32MHz and if the 6 bit

divider divides the input frequency by 5 the cut-off frequency

of the SC filter is 32MHz/(80

¥ 5) = 80kHz

1st order RC low pass filters are connected to the output of the

SC filter to filter the clock frequency.

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, that 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;

Baudrate / 2

OFFSET

DEV

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)

Baudrate: The baud rate given in bit/sec

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