The CMX138A is a half-duplex audio scrambler and sub-audio signalling processor for analogue two-way radio applications. This makes it a suitable device for the leisure radio markets (FRS, MURS, PMR446 and GMRS).
This device provides a user programmable frequency inversion audio scrambler with compander and pre/de-emphasis – performing simultaneous processing of sub-audio and in-band signalling.
Other features include: an auxiliary ADC channel and an auxiliary DAC interface (with optional RAMDAC) to facilitate transmitter power ramping.
The device has flexible powersaving modes and is available in a 28-pin (E1) TSSOP package.
This enhanced pin-compatible IC supersedes the CMX138.
Devices are available through our distributors below:
Leisure Radio Systems:
Q. I have been looking for a CTCSS and Voice band scrambler IC and have found the CMX138A device, which will be ideal for my project.
However, I note that it has only one modulator output, this is a shame, as I would prefer to use it with a radio that allows Two Point Modulation.
What is the best method of interfacing the CMX138A to such a radio?
A. CML has a number of devices that are available with two modulator outputs, designed specifically for radios that operate two-point modulation, for the CMX7041 – 7041FI-1.x is one such example.
The CMX138A is optimised for low cost radios, which means that some compromises were required, this includes the use of a single modulator output pin. However, it is possible to use the device with such a radio by allowing for a method of external alignment in production.
The hyper-linked (Two-point Modulation Example) illustration demonstrates a simple method of interfacing and comes complete with a suggested method of adjustment. When designing a two-point balancing circuit, such as that shown, it is important to remember the 20k ohm output load limit of the CMX138A.
Note: Alignment of a radio is normally required when using two-point modulation as it is important to ensure that the deviation of VCO and reference are balanced across the frequency band of operation.
Even if the in-band and sub-audio signals are supplied independently to the VCO and reference, as is possible with the CMX7041FI-1.x family, alignment is still usually required to set the relative deviations of the two components.
There are application notes available on our website that discuss this phenomenon in much greater detail than we do in this FAQ.
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Guides to operation and interfacing
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