Evaluation Kit for CMX838 Family Radio Processor

22

EV8380 User Manual

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¤2001 MX-COM, Inc.

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Doc. # 20480230.001

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5.4

RF Programming Examples

5.4.1

How to Change an FRS RF Channel

To change the RF channel (for FRS, GMRS, or PMR446), write a byte to the SYNTHESIZER CHANNEL

SELECT ($8B) register that contains the desired channel number. If the device is in Tx mode, the new

channel carrier frequency will automatically be synthesized by the CMX838 RF synthesizer.

5.4.2

How to Set the IF Offset For A FRS RX Channel

In the instances where it is desired to transmit on one channel but receive on another, the SYNTHESIZER

1ST IF OFFSET ($8D) register allows for a “high side” or “low side” IF offset to be programmed into the

CMX838.

For example, suppose the CMX838-based FRS radio is transmitting on FRS channel 14 (467.7125MHz), but

the radio user wishes to receive on FRS channel 1 (462.5625MHz). To accomplish this synthesizer

manipulation, a value can be loaded into the SYNTHESIZER 1ST IF OFFSET ($8D) register that will

automatically produce the desired offset when the CMX838 is switched from transmit mode to receive mode.

For this example, the following calculations allow the determination of the appropriate setting for the

SYNTHESIZER 1ST IF OFFSET ($8D) register:

1.

467.7125MHz – 462.5625MHz = 5.15MHz

Frequency IF shift required between FRS channels 14 & 1

2.

(5.15MHz / 12.5kHz) = 412d = $019C

Number of FRS channels (12.5kHz wide) in the required shift

3.

$019C unsigned = $FE64 signed

Frequency shift is negative, so load $FE64 (the first four bytes of

the signed version of $019C) into

SYNTHESIZER 1

If the IF shift were a “high side” shift of 21.4MHz:

(21.4MHz / 12.5kHz) = 1712d = $06B0 unsigned

This means that for a “high side” IF shift of 21.4MHz, a value of $06B0 should be loaded into the

SYNTHESIZER 1ST IF OFFSET ($8D) register.

Additionally, if the PMR446 service (6.25kHz channels) is being used and a “low side” IF shift of 45MHz is

required:

(45MHz / 6.25kHz) = 7200d = $1C20 unsigned = $E3E0 signed

So, a value of $E3E0 should be loaded into the SYNTHESIZER 1ST IF OFFSET ($8D) register to accomplish

a “low side” IF shift of 45MHz with the PMR446 radio service.

5.5

Modulation Section Programming Example

5.5.1

How to Manipulate the Tx MOD 1 & 2 Outputs

The TX MOD 1 & 2 outputs were designed with flexibility in mind. These output modulation drivers can

independently be programmed to present either (1) Vbias, (2) audio, (3) inverted audio, (4) subaudio tone, (5)

inverted subaudio tone, (6) audio + subaudio tone, (7) inverted (audio + subaudio tone).

The Tx MOD 1 & 2 outputs can be configured by writing two data bytes (containing the desired configuration

settings) to the Tx MOD 1 & 2 CONTROL ($88) register.