Description
PSM-500/500L/500LT SCPC Satellite Modem
PSM-500/500L/500LT - Rev. 0.91
Page 1-8
1.2.1 Modulator
The PSK/QAM modulator in the modem employs a unique digital modulation scheme requiring no
heterodyne operations (mixing and filtering to an IF) to arrive at the transmit RF frequency. The
desired carrier frequency is synthesized and directly modulated with the baseband signal. The
baseband signal is itself digitally derived and generated using a digital to analog (D/A) converter.
The digital signal processing of the transmit signal includes the equivalent of a 144 tap FIR filter
function.
As previously shown in Figure 1-2, synchronous transmit data and clock signals are accepted by the
modulator, then processed by the V.35/Intelsat scrambler and differential encoder. The modulator
can be set by the processor to operate at a number of data rates between 1.2 kbps (BPSK, rate 1/2)
and 20 Mbps (8PSK +, M520 feature set). Refer to the specification in Appendix A for exact rate
capabilities. The data is then encoded for Forward Error Correction (FEC) at rate 1/2, 3/4, 5/6 or 7/8
resulting in an encoded signal at between 2.4 and 14,760 ksps (kilo symbols per second). The
Viterbi convolutional encoder can be programmed for rate 1/2, 3/4, 5/6 or 7/8 and is set for a
constraint factor (K) of 7 for use by a (receiving end) Viterbi convolutional decoder with the same
rate and K factor. A Reed-Solomon FEC is available for concatenated operation with the Viterbi
Codec and two types of “Turbo Codes” Codecs are also available to replace the Viterbi Codec. A
special case is 8PSK, which only operates in a Trellis Code Modulation (TCM) mode at rate 2/3,
unless a non-Viterbi FEC is added such as TPC or
FlexLDPC
.
The FEC is followed by an optional differential encoder. The differential encoder output is then sent
to the transmit baseband signal processor whose main function is to convert the data stream into
analog baseband I and Q channels for modulating the carrier. The actual conversion process is
accomplished in a lookup table, latch and D/A converter. The lookup table represents a digitally
preprocessed function required to produce the proper RF signal output when mixed with the desired
carrier frequency. A low-pass filter is applied to the D/A output to reduce the level of sampling
components.
Transmit Local Oscillator generation is accomplished in two parts. A PLL step synthesizer is used to
generate a basic LO in the 52 to 92 (or 104 to 184 or 952 to 1752) MHz range with 500 kHz step
size. A Direct Digital Synthesizer (DDS), consisting of an NCO and D/A conversion, is used to
generate an approximate 2 MHz signal with fine step size of approximately 1 Hz and a range of
±
1.25 MHz. When the DDS is subtracted from the step synthesizer output in a second PLL, the
available LO can be tuned in 1 Hz steps over the full range of 50 to 90 MHz (100 to 180 MHz if built
for that version).
The processed baseband signal is then mixed with the transmit synthesizer's LO carrier signal to
generate an output modulated carrier in the 50 to 90 MHz range (or 950 to 1750 MHz in the L-Band
modem). A classic IQ modulator with two mixers is used and the LO is fed into the second mixer
shifted 90 degrees from the first. The modulated baseband signal can take two forms at this point
depending on whether BPSK or QPSK modulation is used. In BPSK mode, the baseband signal fed
to the two mixers is identical. In QPSK mode, the two signals represent the baseband I and Q
channels of the baseband.
The resultant RF signal is then low pass filtered and amplified to produce a signal at approximately
over 5 dBm into 75
Ω
. An output attenuator controlled by the onboard processor is used to set the
modulator output level over a range of +5 to –35 dBm. The actual attenuator is a set of pin diodes
whose voltage is derived from the processor via a 12-bit D/A converter. The processor also holds a
calibration table of DAC input vs. RF output level/frequency in non-volatile memory.
No physical adjustments are present in the modulator. All necessary adjustments are electronically
performed during calibration and are intended to last the life of the unit without requiring resetting.
The modulator is capable of operating in two different modes: Continuous mode for SCPC use and
“Burst” mode for use at a VSAT location. When set to VSAT operating mode, the transmit signal is
turned off and on according to the status of the data interface control lines and framing information in
the data stream as described in the “Operation” Chapter of this document. The burst mode allows
multiple station modulators to link up consecutively with a single master station “burst demodulator”.
