GPS signal simulator
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
Hi Warren:
Here's some info on a GPS signal generator made by Stanford Telecom for
testing their GPS ICs:
http://www.prc68.com/I/5001A.html
I'm sure it could be greatly reduced in size and complexity be replacing all
the logic with a micro controller, but the RF part would remain.
There is a very strong correlation between the hardware and the ICD-200 GPS spec.
Have Fun,
Brooke Clarke
http://www.prc68.com
WarrenS wrote:
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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Brooke Clarke wrote:
Hi Warren:
Here's some info on a GPS signal generator made by Stanford Telecom for
testing their GPS ICs:
http://www.prc68.com/I/5001A.html
I'm sure it could be greatly reduced in size and complexity be replacing all
the logic with a micro controller, but the RF part would remain.
There is a very strong correlation between the hardware and the ICD-200 GPS spec.
Have Fun,
Brooke Clarke
http://www.prc68.com
Brooke
An FPGA would be far more practical than a microcontroller given the
1.023MHz chip rates for the civilian signals.
Bruce
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.
Warren,
There's been some discussion about a GPS simulator for the
purpose of playing with GPSDO algorithms a couple of times
on the list over the years.
But since you're making more progress than most have, here
is a nice pair of 400,000 (!) sample data sets for you to use:
http://www.leapsecond.com/pages/gpsdo-sim/
For playing with GPSDO, the character of the typical 1PPS is
very easy to model (it's basically a few ns of noise that nicely
averages down at 1 over tau out past a day) so going all the way
back to RF simulation of the GPS SV signal(s) might be overkill.
In any event, I found it much easier to just take real GPS 1PPS
data, and real OCXO data, and then simulate a GPSDO from
those two raw data sets. You can plot phase, frequency, ADEV
of both inputs and one output and see the GPSDO humps we
always talk about, etc.
/tvb