PacComm Modem Technology
PacComm wireless communication products use several different modulation techniques to best serve a variety of applications.
Selection of the appropriate product and modulation method is based upon a comparison of product cost, the type of radio to be used, the RF link data rate required, and ease of installation.
(Products designated -1200 and -2400)
AFSK is a simple, reliable, and low cost method for transmitting data through unmodified voice-grade radios. 1200 baud signals are easily handled by almost all radios through connections to the microphone and speaker connectors.
2400 baud AFSK often presents too broad an audio signal for standard voice radios, but works well with telemetry grade (data) radios.
(Products designated -24M and -MSK)
MSK modulation allows higher data rates to be used on conventional voice radios via microphone and speaker connections.
The MSK modems are usable are usable at 1200 and 2400 baud (selectable) and use tone pairs at 1200/1800Hz for 1200 Baud and 1200/2400Hz for 2400 Baud. Transitions between tones occurs at zero crossing points reducing transients and bandwidth spreading. The modems are full-duplex for maximum data throughput.
The MSK modems are not compatible with 1200/2400 baud AFSK because of the different tone pairs and clock synchronization methods used. It can be mixed with AFSK on the same channel, but data connection between MSK and AFSK units will not occur.
The MSK modems will operate at baud rates higher than 2400 BPS when used with direct connection to the Varactor and Discriminator circuits or when used with telemetry (data) radios.
(Products designated -4800, -9600, -19200, and GMSK)
Direct Frequency Modulation is used where higher data rates are required. A key feature of the DFM modem design is digital generation of the transmit audio waveform. Precise shaping limits the signal bandwidth. Deviation of +/- 3KHz gives an RF spectrum 20KHz wide (-60dB).
Some PacComm modems use a large scale integrated circuit modem which produces a waveform very similar to DFM. This modulation method is termed Gaussian Minimum Shift Keying (GMSK) by the IC manufacturer.
Using a DFM modem entails modification to a voice radio so that direct connection can be made to both the modulation varactor and receiver discriminator. These connection points are normally brought out to the data connector of a telemetry radio module.
(Products designated -PAD)
The direct digital output is at TTL levels (no modem) for connection to a digital radio.
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