The fundamental frequency in the GPS system is 1.023 MHz, chosen so that a 10-bit maximum-length feedback shift register (MLFSR) clocked at that frequency would complete one cycle of 1023 states in precisely one millisecond. The civilian "C/A codes" or "Gold codes" are generated by combining two such MLFSRs. This fundamental frequency is often called "f0", pronounced "f-naught". It's just far enough away from 1.000 MHz to get you into trouble if you mistake one for the other.
The military "P/Y-code" signal is transmitted at precisely ten times that rate, or 10.23 MHz. Some military-focused documentation calls that frequency "f0" instead. The L1 frequency is 1540 * f0 = 1575.42 MHz. The L2 frequency is 1200 * f0, or 1227.6 MHz. Many civilian receivers use an IF of 4 * f0 = 4.092 MHz. This requires an LO frequency of 1536 * f0. I'm not familiar with the 4000S, but some Trimble receivers of similar vintage (late 80's, early 90's) used a LO at 768 * f0 with a sub-harmonic image-reject mixer to produce the 4* f0 IF frequency. > The L1 1575.42 MHz chain uses a 16.368 MHz VCXO locked to the 10 MHz > reference, running an LO of some integer multiple that results in a > reference around 38.4 MHz labeled "ECL 38.4 F0" on the main board, and > an unlabeled signal IF called "TTL LIMITER." Internal markings "768" and > "384" may indicate PLL IFs of 76.8 and 38.4 (76.8/2) MHz. > "38.4 F0" almost certainly means 38.4 * f0. This would produce a digital clock fast enough to sample P-code data. (Recall that the P-code wasn't encrypted to Y-code until about 1990, so this receiver was probably built to track P-code.) > > The L2 1227.6 MHz chain uses a 28.644 MHz VCXO locked to the 16.368 MHz > reference, and LO that results in another unknown IF that runs through a > similar TTL limiter. It appears that the LO is an integer multiple of > the 28.644 MHz, with a PLL IF possibly around 59.2 MHz, marked "592 FO." > Only the unknown signal IF from this section goes to the processing > boards - no PLL IF seems to go beyond these modules. The unknown signal > IF goes only to one of two apparently identical DSP boards, unlike the > others that all go to the main board. > If "592 F0" were used in a subharmonic mixer as described above, simulating an LO of 2 * 592 = 1184 * f0, that would produce an L2 IF signal at 16 * f0 = 16.368 MHz, which is a high enough IF frequency to carry the P/Y modulation (+/- 10 f0 = 10.23 MHz) without aliasing. (Not sure that's what's going on, but it's a plausible explanation for your observations.) > The L1 downconverter appears to use quadrature mixing, but I can't tell > what happens after that - the I-Q signals go into a bunch of baseband > circuitry. The L2 one also has a quadrature mixer, but only one output > goes into its baseband circuits - the other is just terminated. The L1 signal carried P-code and C/A code in quadrature. The L2 signal only carried P-code. Sounds like the L1 outputs are split between the P-code and C/A code processors. The L2, carrying no C/A code, only goes to the P-code processor. > As I > understand, the L2 is always encrypted, so useless for data, but its > carrier can be used to enhance overall accuracy - I recall studying that > a few years ago, but forgot the details. So, maybe the L2 portion is > only for carrier recovery of some sort. > As above, P-code (on L1 and L2) was not encrypted when this box was designed. > I'd appreciate any info or ideas on deciphering the rest of the way - > maybe the modules will be useful for something as a system, rather than > just parts. I'm especially interested in GPS carrier recovery techniques > for frequency only - not time. > The GPS signals are spread-spectrum, well below the thermal noise level, so you can't easily see the carriers without actually tracking the signals. Hope this helps. Cheers! --Stu _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
