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 18.104.22.168 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 22.214.171.124 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.