MICRF620 datasheet(15/21 Pages) MICREL

15 / 21 page

Micrel, Inc.

MICRF620/MICRF620Z

July 2006

M9999-120205

Front End

A6..A0

0000000

LNA_by

PA2

PA1

PA0

Sync_en

Mode1 Mode0

’1’

A low noise amplifier in RF receivers is used to boost the

incoming signal prior to the frequency conversion process.

This is important in order to prevent mixer noise from

dominating the overall front-end noise performance. The

LNA is a two-stage amplifier and has a nominal gain of

approximately 23dB at 434MHz. The front end has a gain

of about 33dB to 35dB. The gain varies by 1-1.5dB over a

2.0V to 2.5V variation in power supply.

The LNA can be bypassed by setting bit LNA_by to ‘1’.

This can be useful for very strong input signal levels. The

front-end gain with the LNA bypassed is about 9-10dB.

The mixers have a gain of about 10dB at 434MHz. The

input impedance is shown in Figure 9.

Figure 9. Input Impedance

Sallen-Key Filters

A6..A0

0000001

‘1’

‘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

channel 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

lowpass filter is programmable to four different cut-off

frequencies according to the table below:

PF_FC1

PF_FC0

Cut-off Freq. (kHz)

100

150

230

340

Switched Capacitor Filter

A6..A0

0001000

‘1’

ScClk5

ScClk4

ScClk3

ScClk2

ScClk1

ScClk0

The

main

channel

filter

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.

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. The cut-off

frequency of the filter is given by:

ScClk

XCO

CUT

⋅

fCUT: Filter cutoff frequency

fXCO: Crystal oscillator frequency

ScCLK: Switched capacitor filter clock, bits ScClk5-0

st order RC lowpass 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

DEV

OFFSET

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

Baudrate: The baud rate given is bit/sec

In battery operated applications that do not need very high

selectivity, the main channel filter can be bypassed by

SC_by=1. This will reduce the Rx current consumption

with ~2mA.

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