MCS3142-ISLASHST datasheet(23/37 Pages) MICROCHIP

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DS40001747A-page 23

MCS3142

6.4

Center Frequency and Frequency

Deviation

The RF transmitter is capable of generating many of

the popular RF frequencies that are permitted within

the radio regulations of the country the finished product

will be sold. The RF frequency configuration is

performed by selecting the frequency band, the

reference crystal frequency and the frequency value to

be stored in the Encoder Frequency Configuration

deviation is set in the Transmitter Configuration

Unlike other configuration options, the two encoders of

the MCS3142 device share the same frequency

configuration, which is shown in Table 3-18. Frequency

deviation is individually configurable. See Table 3-7

and Table 3-15 for the Classic KEELOQ and Ultimate

KEELOQ encoders, respectively.

6.4.1

BAND SELECTION

The Band bit in the Transmitter Configuration register

configures the RF transmitter for a range of frequencies

for a given crystal frequency, as shown in Table 6-2. The

Transmitter Configuration registers are shown in Table 3-

7 for the Classic KEELOQ encoder and Table 3-15 for the

Ultimate KEELOQ encoder.

Although each encoder has its own band selection

configuration, the requirements of proper antenna

tuning and the inability to configure the fundamental

frequency per encoder will likely require that this setting

be identical for both encoders.

The reference crystal frequency tolerance and

frequency stability over the operating temperature

range depend on the system frequency budget.

Typically, the receiver crystal frequency tolerance,

stability and receiver bandwidth will have the greatest

influence. For OOK modulation, the transmitted RF

signal should remain inside the receiver bandwidth,

otherwise signal degradation will occur. For FSK

modulation, fRF should remain inside the receiver

bandwidth and within 0.5 fDEV.

As a general practice, do not choose an RF transmit

signal with an integer or near integer multiple of fXTAL.

This will result in higher noise and spurious emissions.

TABLE 6-2:

FREQUENCY CALCULATION(1)

Reference Oscillator

(fREF)

Band

Frequency Range

(fRF)

fRF Equation

fDEV Equation

22 MHz

310-450 MHz

24 MHz

312-450 MHz

26 MHz

338-450 MHz

860-928 MHz

Note 1:

212992 < DF < 344064 and 10 kHz

 fDEV  200kHz.

14 fRF

fREF

-----------

14fDEV

fREF

-----------

13 fRF

fREF

-----------

13fDEV

fREF

-----------

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