[time-nuts] GPS signal simulator

WarrenS warrensjmail-one at yahoo.com
Wed Dec 10 09:03:25 UTC 2008


    In my quest to keep improving my GPSDO, a simple test tool 
that I find indispensable is a GPS 1PPS (or 100Hz) signal simulator 
reference.  Others may also find this useful because it allows testing the 
PLL & DO in their GPSDO to Cs type accuracy using a standard OCXO.

    The advantages of using the 1PPS GPS signal simulator instead of the 
real thing for high accuracy test, is that it eliminates all the errors and unknowns 
that come before the 1PPS signal so that the performance and limitations of 
just the PLL and it disciplined oscillator can be quickly tested independently 
of the receiver and the GPS signal. 
    This also gives higher accuracies than are available from the GPS system, 
while using a relatively low accuracy oscillator for its reference.

    The simulator generates the 1PPS signal from a divided down 
standard 10MHz OCXO. 
 For advanced test a simple Phase modulated 'Noise' can be added to the 
1PPS to represent  the basic phase noise of the GPS receiver's 1PPS signal.
The phase & freq  difference between the reference oscillator and the 
tracked PLL oscillator can then be compared to any desired accuracy 
and resolution limited by the noise, but not the accuracy, of the oscillators. 


Now I am wondering:
Has anyone made or considered the tricks needed to simulate a GPS 
RF signal in a simple and cheap way? 
Something that could be used to send a 1PPS timing signal to a GPS receiver 
for similar basic high accuracy receiver testing from the antenna to the 1PPS? 
It would seem that this may be feasible when used with a receiver that can 
output its 1PPS when locked to a single satellite? ? ?

WarrenS




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