MICRF505L datasheet(21/43 Pages) MICREL

21 / 43 page

Micrel

MICRF505BML/YML

August 2006

M9999-092904

+1 408-944-0800

Receiver

The receiver is a zero intermediate frequency (IF)

type in order to make channel filtering possible with

low-power integrated low-pass filters. The receiver

consists of a low noise amplifier (LNA) that drives a

quadrature mixer pair. The mixer outputs feed two

identical signal channels in phase quadrature. Each

channel include a pre-amplifier, a third order Sallen-

Key RC lowpass filter from strong adjacent channel

signals and finally a limiter. The main channel filter is

a switched-capacitor implementation of a six-pole

elliptic lowpass filte. The elliptic filter minimizes the

total capacitance required for a given selectivity and

dynamic range. The cut-off frequency of the Sallen-

Key RC filter can be programmed to four different

frequencies: 100kHz, 150kHz, 230kHz and 340kHz.

The demodulator demodulates the I and Q channel

outputs and produces a digital data output. If detects

the relative phase of the I and Q channel signal. If

the I channel signal lags the Q channel, the FSK

tone frequency lies above the LO frequency (data

‘1’). If the I channel leads the Q channel, the FSK

tone lies below the LO frequency (data ‘0’). The

output of the receiver is available on the DataIXO

pin. A RSSI circuit (receive signal strength indicator)

indicates the received signal level.

Front End

A6..A0

0000000

LNA_by

PA2

PA1

PA0

Sync_en

Mode1 Mode0

Load_en

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 900MHz. 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 going of about

10dB at 915MHz. The differential outputs of the

mixers can be made available at pins IchOut and

QchOut. The output impedance of each mixer is

about 8kΩ.

The input impedance is close to 50Ω as shown in

Figure 12, giving an input reflection of about -20dB.

The receiver does not require any matching network

to optimize the gain. However, a matching network is

recommended for harmonic suppression in Tx and

for improved selectivity in Rx.

Figure 12. LNA Input Impedance

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 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

MICRF505L Datasheet(PDF) 21 Page - Micrel Semiconductor

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