Hello Philip, I agree with Bruce about the digital stuff and semiconductor temperature sensors, etc. From your commentary I think you should do some reading before proceeding. Here are some suggestions;
The first is a series of Application notes from Agilent (old hp test div) called AN-200. A total of 5 App notes comprise the AN-200 series. If you go to the following Web page and enter AN-200 at the top of the page in the search box, you will get a return of all the AN-200 booklets in PDF that can be downloaded. The BIG one is AN-200-2, but it would be to your advantage to collect all of them. You need to paste in the entire link below if your browser doesnt see the whole thing when clicking on it. [1]http://www.home.agilent.com/agilent/facet.jspx?t=80030.k.1&cc=US&lc= eng&sm=g&pageMode=TM Next is the AN-52 series, also at the above site. The original, produced in the 1960's is AN-52. Later, in the 1970's, they rewrote and split this App note into two titled AN-52-1 & AN-52-2. There is also AN-52-4, but that does not cover your interests at the moment. I would suggest downloading ALL of them, including the original AN-52. An-52 does have historical perspective and a few things not included in the later rewrites. That should keep you busy for a while. A lot of stuff on the WEB, some good and some not so good, just take it with a grain of salt ! NIST (the old NBS) has several things worth reading, however, most of that deals with the measurement process or is a rigorous mathematical analysis of one thing or another. In my experience, an inexpensive metal can crystal and a decent oscillator circuit will hang in there under 10ppm in a regular room with a stable ambient temperature. HP used such a crystal in their 60KHz receiver because it was controlled in a loop from the 60KHz, thus approaching the accuracy and stability of the transmitted signal. Second it does not take much to get parts in 10^-7 range. Temperature compensated crystal oscillators easily handle that level. With care, a crystal oscillator in a well designed circuit can reach parts in 10^-8 with a bit-bang oven control. HP did that in the late 1950's. From that point the difficulty is logarithmic. Bill....WB6BNQ Philip Pemberton wrote: Hi folks, I've been following the mailing list for a few weeks using Pipermail (the web-based archive) and I figured now was a good time to jump in (so to speak). I'm working on a GPS-disciplined oscillator, based on a Trimble SVeeSix GPS receiver, and a homebrew OCXO. I've got a pair of 10MHz 50-degree-C oven crystals, and have a pretty good idea how to handle the temperature regulation. What I'm planning to do is mount the crystal on a copper plate with two power transistors, using heatsink compound between the copper and transistors/crystal case, and fit a temperature sensor to the top side of the crystal case. I'm planning to use a copper bracket to hold the sensor onto the crystal, and in turn mount the crystal to the copper base. As far as temperature regulation goes, I'm going to use a PIC microcontroller (one of the 8-pin chips with an A/D converter) to monitor the temperature of the crystal, and use a PID loop to control the two power transistors to maintain a temperature of 50C +/- 2 Celsius (the accuracy spec of the temperature sensor). I also have other higher-accuracy sensors (Dallas DS18S20 and DS18B20) that I can calibrate with; these are accurate to around half a degree Celsius with a resolution of 0.5C. The whole thing is going to be mounted in a metal box lined with 1/2in thick polystyrene, with all external connections made via Molex KK connectors and standard hookup wire. If there's any advantage to doing so, I might use RG174 cable for the oscillator output, but otherwise I'll stick to the KKs and maybe twist the OUT/GND wires together. What I'm stuck on is the oscillator itself. The crystals are standard parallel-resonant parts, with a load capacitance of 30 picofarads. I've got a few varicap diodes (varactors) that I'm planning to use to allow external trimming of the frequency, on top of what the ~20pf "coarse" preset will allow. So on one side of the crystal I'll have a 33pf capacitor, and on the other a 20pf load capacitor, the varicap and a low-value DC-blocking capacitor for said varicap. The standard oscillator circuit for TTL seems to be a pair of 74HC04 inverters and a few passives, or a transistor version that outputs a sine-wave. Are there any particular types of oscillator that are more suitable for high-accuracy timing? What I'd like to do is use this oscillator to calibrate frequency counters and check the calibration on oscilloscopes and similar. Being able to lock function generators (a mix of custom DDS sine generators based on Analog Devices DDS chips and FPGA-based complex-signal DDSes) against the oscillator would be very useful as well. Should I be going for a 1V sine output and then convert this to TTL in the generators (which are easy to retrofit with adapter boards) or output TTL from the reference and leave it at that? What design parameters should I be optimising for, and how? Given that a standard crystal is good to roughly 100ppm, and most commercial OCXOs are specified to be within 1x10^-9 or better, I'm aiming for around 1ppm to start with. Is even this realistic for a homebrew device? There seems to be quite a bit of difference between just building a 4MHz oscillator to run a PIC MCU to building an accurate frequency reference source... As far as parts are concerned, I'm planning to use either a BB153 or BB148 varicap, a Microchip TC1047AVNBTR temperature sensor, a National Semiconductor LM4040CIM3-4.1 voltage reference for the PIC's A/D, two BD139 power transistors and a PIC12F683 microcontroller. Thanks, -- Phil. [EMAIL PROTECTED] [2]http://www.philpem.me.uk/ _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to [3]https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. References 1. http://www.home.agilent.com/agilent/facet.jspx?t=80030.k.1&cc=US&lc=eng&sm=g&pageMode=TM 2. http://www.philpem.me.uk/ 3. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.