Design Resources

CMX838 and 10kHz Channel Spacing FAQ

 Q. I am interested in obtaining 10kHz channel spacing with the CMX838, but I see that 10kHz is not a supported channel spacing. Is there any way that I can achieve this channel spacing?

A. It is possible, but many considerations will need to be made to ensure everything works as desired. At a top level, the plan involves:

– program the CMX838 for 20kHz channel spacing.

– program the CMX838 to use a particular reference oscillator value (e.g. 24MHz).

– actually use a reference oscillator of one-half the programmed value (e.g. 12MHz).

– this would result in 10kHz spacing instead of 20kHz spacing.

– use a recommended crystal for the baseband clock.

The key to making this approach work is to use a reference oscillator whose value, when doubled, is also on the list of supported reference oscillators (i.e. Section of CMX838 data sheet, bits 5-2). This will allow for programming one supported frequency while using another.

The fact that the “real” oscillator is half of what the CMX838 thinks it is should cause the real channel spacing to be half of what the CMX838 thinks it is, namely, 10kHz.

NOTE: if a particular RF channel center frequency is programmed into $8B (“Synthesizer Channel Select” register), and if the actual RF reference oscillator is half of its programmed value, the RF channel center frequency produced by the CMX838 will be half of the programmed value. Consequently, this approach will require generic RF channel programming per Section of the CMX838 data sheet.

Another caveat with this approach is the baseband clock generation:

– if the CMX838 is generating the baseband clock from the RF reference oscillator…

– and if the CMX838 is programmed for a RF reference oscillator of 20.95MHz…

– but the CMX838 is actually supplied with a RF reference oscillator of 10.475MHz…

…all of the CTCSS tones will be half of their programmed value, and the internal SCF clocks will be half of their intended value too.

Therefore, if this approach is used to obtain 10kHz channel spacing, a dedicated baseband clock signal (from the list of supported frequencies in the data sheet) must be used for the baseband clock generation.

CMX838 – Tone Cloning FAQ

Q. What is “Tone Cloning”?

A. Tone CloningTMCMX838 is a process of automating the configuration of Family Radio Service (FRS) radios, especially when using radios from different vendors. Tone Cloning requires the use of a fast CTCSS processor such as the CMX838.

An FRS radio that supports Tone Cloning can copy (clone) the CTCSS tone(s) used by another FRS radio. This greatly simplifies radio configuration and reduces the process to two steps:

1. Configure a “first” radio with a desired CTCSS tone.

2. Use the Tone Cloning feature of the other radios so that they clone (copy) the hidden code in the “first” radio”s transmission.

Since different FRS radio vendors may describe their CTCSS tones differently (e.g. “67.0Hz” or code “A” or code “F” for the same CTCSS tone frequency), customers may not understand how to program different vendors’ radios for proper operation. An FRS radio with Tone Cloning can clone tones from any CTCSS-based FRS radio, thereby allowing radios from different manufacturers to operate together. In this way, FRS radios with the tone cloning feature can be nearly universally compatible with other CTCSS-based radio models and so save the cost of having to purchase the same FRS radio model for each member of a group.

To support maximum Tone Cloning an FRS radio must be able to quickly identify ANY valid received CTCSS tone, without prior knowledge of what that tone will be. (This includes: identify the received CTCSS tone; configure both tone encoder and decoder to match the identified tone; and restore the pre-cloning configuration for the 6 remaining decoder tones.) The CMX838 uses an innovative tone decoder technology to non-predictively decode any standard CTCSS tone in less time than other decoder approaches take to decode a single, predicted tone. Other common tone decoder techniques could require 10 seconds or more to perform the same function and would be unsatisfactory to FRS radio users.

Group Calling FAQ

Q. What is “Group Calling”?

A. Group calling is the ability of an FRS radio to simultaneously transmit a message to multiple radios. To support group calling, an FRS radio must be able to quickly transmit any one of multiple CTCSS tones (one CTCSS tone for each group supported). Support for group calling also requires an FRS radio to quickly recognize or “decode” one of several received CTCSS tones because it must open squelch (turn on the loudspeaker) upon receiving transmissions from any and all configured group’s members. Decoder speed is very important because no received speech is heard at the receiving radio until the decoder responds.

Rapid “one-of-many” tone decoding is an especially challenging requirement in tone decoding technology. The CMX838 uses an innovative tone decoder technology to decode any standard CTCSS tone in less time than other decoder approaches take to decode a single, predicted tone. This makes the CMX838 uniquely suited to deliver and support a practical (fast) group calling FRS radio feature.

C-BUS Guides

Guides to operation and interfacing


Package: E1

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