Evaluation Kit for CMX838 Family Radio Processor
22
EV8380 User Manual
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
2. (5.15MHz / 12.5kHz) = 412d = $019C
3. $019C unsigned = $FE64 signed
Frequency IF shift required between FRS channels 14 & 1
Number of FRS channels (12.5kHz wide) in the required shift
Frequency shift is negative, so load $FE64 (the first four bytes of
the signed version of $019C) into
SYNTHESIZER 1st OFFSET ($8D) register
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.
¤2001 MX-COM, Inc.
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480230.001
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