Your GPSDO efficiency - Page 1
Forum section: electronics / projects

author: Koen
author_rank: Frequent Contributor
timestamp: January 18, 2016, 08:07:23 pm
content: Hello,I've noticed multiple users owned GPSDO and was wondering if some of you would like to share the efficiency of your devices by sharing average clock bias, clock drift, time accuracy and frequency accuracy then mentioning the GPS model, antenna type or model, antenna position and oscillator type or model. Here's a template :Code:GNSS receiver :Model : __,Constellations : __ (GPS/GLONASS/QZSS/BeiDou/...),Average satellites in use : __,GNSS antenna :Model : __,Type : __ (Patch/Whip/...),Position : __ (Inside/Outside/...),Oscillator:Model : __,Type : __ (Crystal/XO/TCXO/VCTCXO/OCXO/...),Frequency tolerance : __,Average results :Clock bias : __ nanoseconds,Clock drift : __ nanoseconds/second,Time accuracy : __ nanoseconds,Frequency accuracy : __ ppb.Which will look like this :GNSS receiver :Model : __,Constellations : __ (GPS/GLONASS/QZSS/BeiDou/...),Average satellites in use : __,GNSS antenna :Model : __,Type : __ (Patch/Whip/...),Position : __ (Inside/Outside/...),Oscillator:Model : __,Type : __ (Crystal/XO/TCXO/VCTCXO/OCXO/...),Frequency tolerance : __,Average results :Clock bias : __ nanoseconds,Clock drift : __ nanoseconds/second,Time accuracy : __ nanoseconds,Frequency accuracy : __ ppb.Thank you,Koen

author: edpalmer42
author_rank: Super Contributor
timestamp: January 18, 2016, 10:41:56 pm
content: It's an interesting idea, but I'm not sure if it makes sense. I don't like the way I phrased that, but I can't come up with anything better. I don't mean it to sound insulting, but it does.The core info in your survey is in the results section. The average clock drift is zero because that's what a GPSDO does. Same with the average time and frequency accuracy.If you've read some of the other message threads on the various GPSDOs that are on the used market, I'm sure you realize how slippery the question of GPSDO performance is. They all provide an absolute frequency and time reference. Yes, there are 'outliers' like the BG7TBL unit that has a very small frequency offset, but even that one is perfectly acceptable for the vast majority of users. But we know that somearebetter than others, so how can you quantify that?The long-term performance will be identical if they all track the same satellites. There may be differences between the different satellite systems (GPS, GLONASS, etc). I've never seen any info on that. The biggest differences will be seen in the short-term performance where the performance of the local oscillator will dominate. To measure that you can use measurements like standard deviation or Allan Deviation. You can't use standard deviation for long-term performance (the details of why are beyond me) so they invented Allan Deviation to handle both short and long term measurements.Here's some info on the short-term performance of some bare GPS receivers and GPSDOs that I've measured in the past.GPS Devices -- Performance of 1 PPS OutputDevice ............... Std Dev (ns).... Range (max-min)(ns) ... Device TypeNavsync CW12 ......... 4 - 5 .......... 20 - 25 ............... GPS Rcvr (1) see belowMotorola UT+ ......... 40 - 55 ........ 95 - 110 .............. GPS Rcvr (2)Rockwell Jupiter ..... 10 ............ 50 ................... GPS Rcvr (3)Motorola M12M ........ 10 - 15 ........ 40 - 60 ............... GPS RcvrTrimble Thunderbolt .. 0.4 - 0.5 ...... 2 - 4 ................. GPSDOHP Z3801A ............ 0.1 - 0.2 ...... < 1 ................... GPSDOHP Z3817A / CW12 ..... < 0.1 .......... < 1 ................... GPSDO (4)Jackson Labs GPSTCXO . 0.3 - 0.4 ...... 2 - 3 ................. GPSDOResults are based on multiple runs of ~ 1000 measurements each.Sawtooth correction has not been used for any of the GPS receivers. Where supported, it would reduce the numbers substantially.GPS Rcvr measurements made with HP 5372A. GPSDO measurements made with HP 5370B.All units were connected to the same antenna system.1. Sawtooth correction not supported.2. Most 'range' results were in this group, but there were a few at 20 - 30.3. Only one test.4. Requires external 1 PPS input. Equipped with E1938 oscillator.I see that I have to make some more measurements and update this table with info on a couple of recently-acquired units.Ed

author: Koen
author_rank: Frequent Contributor
timestamp: January 19, 2016, 07:43:54 am
content: Hello and thank you for your answer. Yes, I'd like to quantify several GPSDO efficiencies. I had no interest for it before reading the multiple active GPSDO topics and later realizing I had a good quality GNSS receivers and oscillators on my boards. Now I'd like to make sense of the numbers the receiver outputs and how real life, different components, different situations impact efficiency.