MICRF501 datasheet(13/18 Pages) MICREL

13 / 18 page

March 2003

MICRF501

Micrel

Gyrator Filter

The main channel filter is a gyrator capacitor implementation

of a seven-pole elliptic lowpass filter. The elliptic filter mini-

mizes the total capacitance required for a given selectivity

and dynamic range. An external resistor can adjust the cut-

off frequency of the gyrator filter. The table below shows how

the cut-off frequency varies with bias resistor:

Bias Resistor (k

Ω)

Cut-Off Frequency (kHz)

6.8

8.2

The gyrator filter cut-off frequency should be chosen to be

approximately the same as the cut-off frequency of the

Sallen-Key filter.

Cut-Off Frequency Setting

The cut-off frequency must be high enough to pass the

received signal (frequency deviation + modulation). The

minimum cut-off frequency is given by:

C(min) = fDEV + Baudrate/2

For a frequency deviation of f

DEV = 30kHz and a baudrate of

20k baud, the minimum cut-off frequency is 40kHz. Bit setting

Fc1 = 1 and Fc0 = 0, which gives a cut-off of (60

±15) kHz,

would be the best choice. The gyrator filter bias resistor

should therefore be 7.5k

Ω or 8.2 kΩ, to set the gyrator filter

cut-off frequency to approximately 60kHz.

The crystal tolerance must also be taken into account when

selecting the receiver bandwidth. If the crystal has a tempera-

ture tolerance of say

±10ppm over the total temperature

range, the incoming RF signal and the LO signal can theoreti-

cally be 20ppm away from each other.

The frequency deviation must always be larger than the

maximum frequency drift for the demodulator to be able to

demodulate the signal. The minimum frequency deviation

DEVmin) is equal to the baudrate, according to the electrical

characteristic's. This means that the frequency deviation

has to be at least equal to the baudrate plus the maximum

frequency drift.

The frequency deviation may therefore vary from the mini-

mum frequency deviation to the minimum frequency devia-

tion plus two times the maximum frequency drift. The mini-

mum cut-off frequency when crystal tolerances are consid-

ered is therefore given by:

Cmin = ∆f × 2 fDEVmin + Baudrate/2

where

∆f is the maximum frequency drift between the LO

signal and the incoming RF signal due to crystal tolerances.

A frequency drift of 20ppm is 8680Hz at 434MHz. The

frequency deviation must be higher than 28.68kHz for a

baudrate of 20k baud. The frequency deviation may then vary

from 20kHz, when the RF signal is 20ppm lower than the LO

signal; to 37.36kHz when the RF signal is 20ppm higher than

the LO signal. The minimum cut-off frequency is therefore

47.36kHz.

Limiter

The limiter serves as a zero crossing detector, thus removing

amplitude variations in the IF signal, while retaining only the

phase variations. The limiter outputs are ideally suited to

measure the I-Q phase difference, since its outputs are

square waves with sharp edges.

Demodulator

The demodulator demodulates the I and Q channel outputs

and produces a digital data output. It detects the relative

phase difference between the I and the Q channel signals.

For every edge (positive and negative) of the I channel limiter

output, the amplitude of the Q channel limiter output is

sampled, and vice versa. The output of the demodulator is

available on the DATAIXO pin. The data output is therefore

updated 4 times per cycle of the IF signal. This also means

that the maximum jitter of the data output is 1/(4

×∆f) (valid

only for zero frequency offsets). 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 inputs and the output of the demodulator are filtered by

first order RC lowpass filters and then amplified by Schmitt

triggers to produce clean square waves.

It is recommended for low bitrates (<10kbps) that an addi-

tional capacitor is connected to Pin 18 (DataC) to decrease

the bandwidth of the Rx data signal filter. The bandwidth of

the filter must be adjusted for the bitrate. This functionality is

controlled by bit RxFilt.

Received Signal Strength Indicator (RSSI)

The RSSI provides a DC output voltage proportional to the

strength of the RF input signal. A graph of a typical RSSI

response is shown in Figure 9 (f

DEV = 30kHz, Gc = 1).

0.6

0.8

1.2

1.4

1.6

1.8

2.2

PIN (dBm)

Figure 9. Typical RSSI Characteristics

This graph shows a range of 0.7V to 2.05V over a RF input

range of 70dB.

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, it could tell the transmitter to

reduce the transmit power to reduce current consumption.

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