Mocomtech CDM-QX operation manual

CDM-Qx Satellite Modem	Revision 5
Functional Description	MN/CDMQx.IOM

The RX IF signal is translated and filtered at an intermediate frequency (IF) using the coarse step synthesizer. This is mixed with a second synthesizer, resulting in the signal being IF sampled with a high-speed analog to digital converter (A to D). The sampled IF is then digitally split into an in-phase (I) and a quadrature (Q) component. An AGC circuit keeps the desired signal level constant over a broad range of input levels. The I and Q signals are then decimated to reduce the computation rate into the poly phase matched filter. Carrier and clock recovery is performed on the baseband I and Q signals after the matched filter. The resultant demodulated signal is fed, in soft decision form, to the selected FEC decoder (which can be Viterbi, TCM, Reed-Solomon, or Turbo if installed). After decoding, the recovered clock and data pass to the de-framer (if EDMAC framing is enabled) where the overhead information is removed. Following this, the data passes to the Plesiochronous/Doppler buffer, which has a programmable size, or alternatively bypasses the buffer. From here, the receive clock and data signals are routed to the terrestrial interface, and are passed to the externally connected DTE equipment.

Physically a modem chassis is comprised of three main card assemblies:

1.The first of these is the IF Backplane card, which includes the frequency reference; power splitters, power summers, the FSK link and the IF Loop back functions.

2.The second card is the Digital Back plane card. This routes all the control signals, the data path switching, the carrier-in-carrier signals and power for all modules.

3.The third is the M&C, which controls all functions in the unit.

Within the chassis are four slots, which allow any combination of modulators or demodulators to be installed. If configured as a single modem, two plug-in cards comprising a modulator and demodulator are required.

A Modulator card contains the transmit interface circuits, the framer, the encoder or encoders and the signal processing functions of modulation.

A Demodulator card performs all of the signal processing functions of carrier search, cancellation, demodulation, Forward Error Correction, the de-framer, plesiochronous/Doppler buffer and the receive interface circuits.

Terrestrial data interface cards can be on the modulator cards or demodulator cards. When a modulator and demodulator are grouped together, the data interface card can be used for full-duplex data interface. When 1 up to 4 ports of E1 (with D&I) are needed the Quad E1 Data Interface Module can be installed in slots 3 and 4.

Figure 3-1 shows a functional block diagram of the modem with either modulators and demodulators in all 4 slots and the figure also shows a modulator in slot 1 and a demodulator in slot 2 along with a Quad E1 Data Interface Module in slot 3 and 4.

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