EMRFD Message Archive 12561

Message	Date	From	Subject
12561	2016-03-31 20:41:16	Rod KM6SN	vfo stability
Hi Nick and others, Some tricks to vfo stability 1. a type 6 core is good 2. wind the inductor under tension with the biggest wire you can manage 3. boil it in water for a minute or two 4. when it cools and dries, coat it with the XYLs "Harder than Nails" stuff 5. use polystyrene caps wherever possible in the frequency determining area Tthe attached ltspice file is an oscillator which is designed for immunity from active device parameters, from the "Oscillator Design Handbook", a collection from RFdesign magazine, page 76 (out of print). I will upload copy of that article to emrfd FOR EDUCATIONAL PURPOSES (it is copyrighted). It is 4 jpegs about 6mb each. Using the above techniques, and ugly construction, and a high quality variable cap, a 10.9 MHz vfo holds within 1Hz for hours. For other frequencies: C4 XC= 10k C1 XC=300 ohms C6 XC=10 ohms L1 XC=300 ohms Be advised, that oscillator does NOT produce a sine wave- rather it appoximates a square wave with about a 70% high duty cycle. This oscillator topology can be used at 2x the desired frequency, divided by two with a 74hc74, and then LPFed into a beautiful rock-steady sine wave. (or, don't use an LPF, and drive your mixer with a square wave-- be careful on signal levels.) That oscillator circuit is amazingly stable up into VHF. In summary, type 6 is good pay special atention to the inductor use polystyrene caps select an oscillator circuit that is inherently immune to active device parameter changes (Seiler, Vakar, or the one mentioned above). Remember, homebrewing is about learning, in addition to the joy of using HB gear. Rod KM6SN [Non-text portions of this message have been removed]
12565	2016-04-01 00:26:25	Dana Myers	Re: vfo stability
12567	2016-04-01 09:05:29	markwdunning	Re: vfo stability
Rod Actually my LT spice simulation shows that it can produce a very good sine wave if you take an output off the junction of C2/C1 and L1 and lightly couple it to a J310 source follower. I am tempted to build a version for 160M but I have so many other projects in the way. Thanks for sharing this circuit. 73 Mark VK6WV
12569	2016-04-01 09:56:52	Alberto I2PHD	Re: vfo stability
12571	2016-04-01 11:13:02	Rod KM6SN	Re: vfo stability
Mark, I agree about the waveform. The purpose of this oscillator is stability; I wonder if the light coupling to the J310 would risk that stability. I should test that some day..... Thanks, Rod
12572	2016-04-01 13:59:42	Dana Myers	Re: vfo stability
12573	2016-04-01 14:31:26	Rod KM6SN	Re: vfo stability
Alberto, Yes, that circuit has some similarities to a Vackar. Here is a very good article on a Vackar: http://www.qsl.net/va3iul/Very_Low_Phase_Noise_VFO/Very_Low_Phase_Noise_VFO.htm The circuit I posted probably is more stable, but I cannot attest to the phase noise. I suspect the posted circuit is more stable because: 1. The capacitance swamping the base-emitter is much larger, and 2. the coupling to the collector is much lighter, and those both reduce the vulnerability to changes in the active device parameters. Have fun with both, and in bocca al lupo! Rod