What does the GPS "Freq Error" mean?

David Allday

I want to brag about the accuracy/stability of my new FlexRadio 6700, but I have no idea of what does the GPS Freq Error mean? How does this relate to the frequency of my 6700. Is -1.250000e-12 good? Is -9.510000e-12 better?

Tim K8XS

Well David, everything is relative. The value given (ie -1.25e-12) is the difference between a perfect 10MHz reference and the 10 MHz reference generated in your GPS disciplined oscillator unit. The values are given in scientific notation. The -12 at the end means that you move the decimal place 12 places to the left and fill with zeros in front of the leading number before you place the decimal place. For example if it said -1.1e-0 then you would move zero places to the left which would give you -1.1Hz error. if it was -1.1e-1 then iyou would move the decimal point one place to the left and get -.11Hz. if it was -1.1e-12 then you would move the decimal 12 places to the left by adding 11 zeros. That would be .0000000000011Hz error. That is pretty dang small. The minus sign means you were either above or below the absolute 10 MHz frequency. Plus means one direction and the - means the other direction. So -1.25e-12 would be a smaller error that -9.91e-12. Either way they are both pretty small errors. A really good typical crystal oscillator with compensation might be +/- 5.0 e-6Hz error at 10MHz, or +/- 1.000005Hz error. Big difference. Hope this helps. (Also remember those errors are average error over some period of time.)

Tim K8XS

last example, erase the 1 that would be +/- .000005Hz error.

Tim K8XS

Sorry, too late at night. The compensate crystal osc would have an error of .5 ppm or 5 parts in 10 e6 or 5 Hz error.

Steve-N5AC

The error value that is supplied does not presuppose any particular frequency of operation. A 1E-6 value (plus or minus) indicates that your error can be up to one part per million or 1ppm. A crystal oscillator alone would typically have, as Tim says, something like 0.5ppm (5E-7) to 5ppm (5E-6) stability. An ovenized oscillator (OCXO) might be 0.5ppm down to 0.01ppm (1E-8), also written as 10ppb or parts per billion. On a bad day, you might see 100 parts per TRILLION out of this GPSDO or 1E-10. Now what does this mean? If you grab your scientific calculator and punch in the value from the GPSDO and multiply it by your frequency in Hz, it will tell you how far off you could be in Hz at that frequency. So let's say David is operating on 10m at 28.2MHz, taking the two different numbers provided at different times provided from his GPSDO, we would multiply and get: 28.2 E 6 x 1.25 E -12 = .000035Hz 28.2 E 6 x 9.51 E -12 = .000268Hz (The E6 is one million for MHz to Hz conversion) So the first number is better -- it's a smaller error. But heck, the second number says that on 10m you are only off by 268 MICROHERTZ ... That's pretty good so from a frequency precision perspective you're in great shape either way. The really great thing is that with a GPSDO, you never drift over time. You will always be on frequency, even 10 years from now, with no adjustment necessary. Steve

Tim K8XS

I stand corrected. The error is part per unit and must be multiplied by the frequency of interest as Steve pointed out. Sorry.

Patrick

Steve can you answer these questions for me.

 I come late to this discussion, but my question has to with time and not frequency.  I have 2 questions.  1) The displayed time in the SmartSDR window comes from the GPSDO or the computer?  2) If it comes from the GPSDO do you compensate for the IP protocol latency between the computer through the network to the radio and back.  I have noticed differences in time between the computer time and the SmartSDR time in the order of .15 to .25 seconds.  In the computer I run Dimension 4 time sync app and it is generally within .004 and .008 seconds off. 

Thanks,

Pat   WR1Z

Steve-N5AC

If the GPS is installed, we use the time from the GPS.  No effort is made to ensure that it rolls over to the next second at visually the precise time -- there is some propagation error going from the radio to the PC, etc.  The main purposes of the GPS are as a frequency reference and to precisely time stamp the data for later use so we didn't make a huge effort to ensure that it visually rolled over at the correct time.  The time on the display is more for informational purposes if that makes any sense.

Patrick

Steve thanks for the info. Your answer has helped my understanding.  I do think however that the GPSDO should however make available timing to the computer.  The better the timing the more precise the communications can be between the computer and the radio using digital protocols running from the computer.  Right now I time using NTP using the Dimension SW and it is very precise and keeps the computer well synced for the digital apps. I now this sounds like I am making a problem were there is not one, but more for maybe for future improvement in the precision of the radio when doing digital modes.  Thanks for you time.

Pat   WR1Z