Using six modules, one can easily assemble a variable data rate L-band modulator.
The user forwards data to be transmitted over the LAN to a TCP-IP socket. The TCP-IP protocol ensures proper flow control. The receiver also provides another TCP-IP socket for monitoring and control purposes.
The data to be transmitted is encoded with both Reed-Solomon and convolutional error correction codes. A unique word is inserted for frame synchronization purposes. Interleaving is used to improve the concatenated codes performance.
The resulting data stream is QPSK modulated. The modulator digitally implements an accurate raised cosine square root filter in order to shape the transmit spectrum. The modulation data rate is variable up to 20 Mbit/s.
The signal is then converted to analog baseband and low-pass filtered to reject spurious spectral lines and out-of-band signals.
The last module synthesizes a low-phase noise carrier. High frequency stability can be obtained by locking the carrier frequency to an external ultra-stable 10 MHz reference clock. The carrier frequency is user-selectable over the range of 950 MHz to 1.450 GHz.
In order to keep the RF modulated signal at a fixed power level, an accurate RF power measurement is performed in the last stage. The user can control the transmitted power accurately over at least 20 dBs.
Several test modes are included in this assembly, including transmitting a known 2047-bit pseudo-random sequence.
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