); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Hi all,
I have been playing with a phase and frequency locked local oscillator in my 3816A GPS receiver. The simple setup uses the OCXO output as an external reference input to a HP 3325A and with a trivial mod to the VP GPS receiver the 3325A becomes its local oscillator. The setup improves the short term stability of the OCXO. Which I guess in hindsight is not very surprising. Logging data every 5 seconds using GPSCon the AD drops to under 0.5. I tested two different approaches. One setup used a fixed LO frequency of 19095750.191 Hz. To my surprise the prime number fraction chosen to give the time interval interpolator a workout results in an auto correlation peak at 1000 seconds when the EFC is viewed with the plotter program. The second approach used dithering. The LO center frequency was set to 19095750.2 Hz and continuously ramped up and down every 10 seconds +-0.035 Hz. This causes the pps pulse to dither slightly more than one sawtooth period over a 5 second interval (the GPS pps pulse edge is just a LO clock edge). Dithering however is a pain to adjust because timing the start of the ramp with respect to the pps output is trial and error and not at all repeatable with my setup. Both approaches result in a pps standard deviation of about 6.4ns. Further small improvements might be possible but would require adjusting the phase of the 3325A over the gpib interface to lock the sawtooth residual to some constant value. For the truly adventurous the next level requires some artificial intelligence like <http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/19/30540/01408297.pdf?arnumber=1408297>http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/19/30540/01408297.pdf?arnumber=1408297 unfortunately only the abstract is on the web. As I recall the authors report 2.5ns of uncertainty over a day when compared to a cesium reference and over 10 times improvement in the MDEV plot of the OCXO. A second rather simple modification to reduce temperature induced tuning I tried is to power the oven with a separate isolated 12V supply. The only common connections are the EFC pin the reference output voltage pin and whatever point you decide to attach in the oven as a signal ground (if you are worried about power supply sequencing you can always add a diode). I connected the signal ground to the inner oven ground pin which does not have any voltage drops caused by the outer oven heater current. The ideal signal ground point would be somewhere inside the inner oven where no current induced voltage drops would modulate the EFC tuning voltage. To get an idea of the magnitude of the improvement you can measure the voltage drop from the oven ground pin to the ground pin of the DAC voltage reference. I haven't combined the isolated oven supply and the phase locked LO in a single test yet. For the curious I have a 6 day GPSCon log file of the two LO setups compressed down to a half Meg let me know if interested and I will email it to you. Enjoy, Peter [EMAIL PROTECTED] _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
