This page contains small MPG video movies(~1.4MB each) of the GPS100SC receiver in action. Specifically sliding correlation from time and frequency perspectives as well as a spectrum of the frequency pull in process. A brief description is given for each clip, Click on the MPG video links to see the short clips. For all the videos below using the GPS100SCS simulator is clocked from the receivers reference oscillator at 10.23MHz.
The ca_codes_slidelock.MPG video shows the dynamics of C/A code scan / lock process using a GPS100SC receiver and GPS100SCS signal simulator. The top o'scope trace shows a reference C/A code from the GPS100SCS simulator. The 2nd trace down is the C/A code from a GPS100SC receiver. Below that trace is the C/A epoch of the same GPS100SC C/A code generator. Not shown is the C/A epoch signal that is being used as a trigger signal to the o'scope. With simulator C/A epoch signal as trigger a solid trace of the simulator results on the o'scope. Without such an arrangement it is not possible to obtain a stable trace for simulator C/A epoch. As the movie plays the the receivers C/A code slides from right to left across o'scope display. The receivers C/A epoch is of course is in lock with it as it slides. When the two codes are lined up the C/A code control circuit switches from scan to track which stops sliding process. Now C/A code lock is achieved by dynamically controlling receiver C/A code Advance and Retard.
The ca_epoch_lock_vid.MPG shows the code slide and lock process in more detail. Here just the reference C/A code epoch from simulator and the receivers C/A epoch are displayed on the o'scope. The top trace is receivers code epoch and the bottom the simulators code epoch. Again the receiver slides it C/A code ( represented by epoch) with respect to the simulators C/A code looking for code alignment. When the epochs are lined up the C/A codes are aligned. In the video we see this occur and the receiver stops sliding its C/A code and enters C/A track mode. If you look closely you can see some details of the lock process. First it overshoots a bit and then settles down. Once loop is settled residual jitter can be seen. This is due primarily to the 1/20 chip resolution of the receivers C/A code sliding system.
The freqacq_sv06oct2304.MPG shows the frequency pull in process for from GPS SV 06 on Oct 23, 2004 using a GPS100SC receiver. The video shows the HP8566A spectrum analyzer with the correlated 10.7MHz IF signal test point as its input. The 8566A display span is 2KHz using the 100Hz RBW. At this time the receiver has achieved C/A code lock and is in the process of getting Doppler lock. At first the signal is not centered in the 10.7IF filter which is roughly the center of the frequency span. The receiver uses the frequency counter to measure the received signal frequency error and adjusts the VCXO (LO2) frequency so as to center the received signal in the 10.7Mhz IF filter which has a BW~ 1KHz . Its a slow process and does not quite finish before the video clip ends. If it went a but further the movement from left to right would stop, as the track mode would be entered. At that time the the signal shifts slightly, slowly back and forth about the IF center frequency as the receiver dynamically tracks the signal Doppler. Of interest in video is what appears to be a momentary small hump in the center of the display, a temporary cross correlation with another SV signal??
The freqdomain_slidecorr.MPG shows a spectral display of the correlation process. A HP8566A spectrum analyzer is fed the 10.7MHZ IF test point signal from a GPS100SC receiver. The receiver is being fed by a GPS100SCS simulator signal with no data modulation on it , just C/A code modulation. The receiver is in the process of sliding its C/A code, looking for correlation. This movie clip is really the frequency domain view of the same process as shown in first video from above. At first there is just a noise hump as there is no correlation, that is codes are not aligned yet. The noise hump is roughly 1KHz wide reflecting the 10.7MHz crystal filter BW. As the clip progresses we see the signal suddenly appear. Note that with no data modulation it is a clean CW response. In this test setup the Doppler tracker was disabled and the VCXO control voltage held constant at its mid point value.
The manual_code_movement.MPG shows how the C/A code from GPS100SCS signal simulator can be manually moved so as to investigate receiver response. The top trace shows the C/A code epoch from the simulator. The bottom trace is the C/A epoch from the receiver. The receiver Code Scan and Lock circuits are disabled , leaving its code fixed. The scope is triggered from receivers epoch. The video show the simulator epoch moving to the left at first just past correlation and the moving back towards correlation point. This is all done with single pushes of the code shift button on the simulator. You can see the 1/20 chip resolution of the movement. Such static control of the C/A code is very useful for investigating the correlation circuits used in a GPS receiver.