I'm currently using GnuRadio/GRC/USRP to implement a proof-of-concept transmitter to demonstrate the feasibility of using open-source SW and low-cost HW to build a transceiver that is compatible with NASA's Telemetry Data Relay Satellite System (TDRSS).
TDRSS uses Direct Sequence Spread Spectrum (DSSS), and has extremely tight specs on the chip clock stability. In the mode that I'm demonstrating, the chip clock runs at 3077799 Hz and the jitter is supposed to be 0.01 Hz or less. When I attach my transmitter to the TDRSS test set for validation, the test set's receiver keeps skirting on the edge of synchronizing with the PN code, but never correlates long enough for the receiver to move from "acquisition" to "tracking". The carrier frequency, PN sequence, chip rate, and signal level have all been verified to be correct. The prevailing opinion in the lab is that the chip clock must be jittering too much. They further claim that the jitter is inescapable because I have so few samples per chip (I'm running at 8e6 samples/sec) and there isn't an integer relation between the chip rate and the sample rate. So, after that long-winded introduction, I have two questions for the list: 1) Is 0.01 Hz jitter at 3.077799 MHz achievable with GnuRadio? I'm running 3.1.1 on a MacBook Pro with a Core2 Duo processor running at 2.3 GHz. 2) Is the lab technician's explanation of the source of the jitter correct? Somehow, it doesn't seem mathematically reasonable. 8 Msamples/sec is about 2.6 times the Nyquist rate for a 3.077799 MHz signal. There should be more than enough sample data to accurately represent the chipping sequence. @(^.^)@ Ed _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
