Venus838LPx-T preliminary testing
I got the first breakout boards back today. There’s an error on them I had to workaround, but having done that I was able to observe it getting first-fix (with good reception, TTFF is remarkably good, FWIW), then performing the 2000 point survey and generating quantization error messages.
I’ve collected an hour’s worth of quantization error data (as reported by the unit) and attached it here, for those who wish to analyze it. It’s very, very jittery, but confined to a ±6 ns corridor. I was able to see what appeared to possibly be very, very brief hanging-bridge-like anomalies, but overall it looks just really noisy to me.
After the survey, the GGA position reported has not changed, which is as I’d expect. I don’t know if the unit periodically validates the position solution to insure it hasn’t been moved.
I don’t have matching PPS jitter data to go along with this. It’ll be interesting to attempt to correlate the two to get a picture of post-correction PPS stability, if I can.
I’m going to make a breakout board for these and list them on Tindie next week, for those who wish to play with them.
On Aug 5, 2016, at 8:13 PM, Nick Sayer via time-nuts time-nuts@febo.com wrote:
I’m going to make a breakout board for these and list them on Tindie next week, for those who wish to play with them.
As promised, the breakout boards are now available.
https://www.tindie.com/products/nsayer/skytraq-venus838lpx-t-timing-gps-module-breakout/
Wow... fast turn-around...
I might grab one of these for my NTP project for my home network...
Reckon they'd integrate with the Rasberry-Pi and other similar?
Clay Autery, KY5G
MONTAC Enterprises
(318) 518-1389
On 8/8/2016 5:13 PM, Nick Sayer via time-nuts wrote:
On Aug 5, 2016, at 8:13 PM, Nick Sayer via time-nuts time-nuts@febo.com wrote:
I’m going to make a breakout board for these and list them on Tindie next week, for those who wish to play with them.
As promised, the breakout boards are now available.
https://www.tindie.com/products/nsayer/skytraq-venus838lpx-t-timing-gps-module-breakout/
On Aug 8, 2016, at 7:26 PM, Clay Autery cautery@montac.com wrote:
Wow... fast turn-around...
I might grab one of these for my NTP project for my home network...
Reckon they’d integrate with the Rasberry-Pi and other similar?
Absolutely. In fact, the prototype has replaced the PA6H on my Pi Zero public NTP server.
The Pi doesn’t have any use for the sawtooth correction, of course, but one thing I’ve noticed from the substitution is that the serial timestamp message latency seems much more regular than with the PA6H. I’ve been able to set the fudge value to 235 ms and actually get a “*” instead of an “x” from that line of ntpq.
I had an opportunity to examine the P1PS2 pin’s output from the Venus838LPx-T. By default it’s a 10 MHz output nominally phase locked to GPS time. It didn’t take more than a second of looking at it on the scope to discover that it’s jittery as hell. My guess is that they’re synchronizing its leading edge once a second (or once in a while?) and letting it free-run from there and then correcting it with a sledgehammer at the next opportunity.
Certainly it’s not usable as a frequency standard on its own. However, for those who had been using James Miller’s GPSDO design with the 10 kHz output from a Jupiter, there might be something here. I wonder if simply adding a cleanup oscillator to the default output might yield results that were good enough. If not, you likely can configure the output to be 10 kHz instead of 10 MHz and then it would be a drop-in replacement. I’ve not (yet) investigated configuring this module for non-default behavior.
I think I’ll stick with the design I’ve got, but these modules might represent an alternative to the Jupiter.
Hi
This problem is not unique to the P1PS2. It is the way all of these devices
generate an arbitrary output. They drop pulses from a series to “correct”
the frequency. A frequency counter with a long enough gate time will read
them as being right. A spectrum analyzer will show a very different picture.
Bob
On Aug 15, 2016, at 12:07 PM, Nick Sayer via time-nuts time-nuts@febo.com wrote:
I had an opportunity to examine the P1PS2 pin’s output from the Venus838LPx-T. By default it’s a 10 MHz output nominally phase locked to GPS time. It didn’t take more than a second of looking at it on the scope to discover that it’s jittery as hell. My guess is that they’re synchronizing its leading edge once a second (or once in a while?) and letting it free-run from there and then correcting it with a sledgehammer at the next opportunity.
Certainly it’s not usable as a frequency standard on its own. However, for those who had been using James Miller’s GPSDO design with the 10 kHz output from a Jupiter, there might be something here. I wonder if simply adding a cleanup oscillator to the default output might yield results that were good enough. If not, you likely can configure the output to be 10 kHz instead of 10 MHz and then it would be a drop-in replacement. I’ve not (yet) investigated configuring this module for non-default behavior.
I think I’ll stick with the design I’ve got, but these modules might represent an alternative to the Jupiter.
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