Message 5/6 From BOB LIESENFELD Aug 6, 95 10:51:14 pm -0100 Return-Path: Date: Sun, 06 Aug 1995 22:51:14 -0100 Subject: VFO STABILITY To: ROHRWERK@netcom.com Organization: HamLink RBBS Hi John, Good to see someone else working this problem. I have been stalking the stable HF VFO for some time now. My experiences pretty much paralels yours. In one 20 Meter VFO, I ended up puting the osc in a metal box along with a homebrew oven circuit. Just a transistor that would dissapate heat and was turned on/off by a thermistor driving a comparator IC. It ran at about 102 degrees F. I have found that stability is tuff (esp at 14 MHZ!) but that stability over temperature is *REALLY* the tuff nut to crack. I have had some luck in just sticking in some neg temp coff. caps in the tank somewhere. Have not done any fine scientific approach to this, just pulled out an NP0, and put in whatever neg coff. I happen to have handy. This has helped many times, as I too am a musician (play the flute) and obsessively chase drift! There was an article in QST a couple years back on a scientific approach to temp compensation, and altho there were a few numerical typos, it was a good intro to this technique. Hope this all generates a thread, as I'm sure there is much expertiese out there on this subject. 72 Bob WB0POQ Technology is *OUT* of control..... ---NoSnail v1.17 ******************************************************************* HAM>link< RBBS - Serving the Amateur Radio Community Since 1983 - 612/HAM-0000 v.34 Ham Radio Spoken Here!! - 612/HAM-1010 v.32b Reply to sender @ hamlink.mn.org ******************************************************************** Xref: netcom.com rec.radio.amateur.homebrew:9655 Path: netcom.com!noc.netcom.net!news.sprintlink.net!nuclear.microserve.net!pinet ree From: jackl@pinetree.microserve.com (WB3U) Newsgroups: rec.radio.amateur.homebrew Subject: Re: VFO Stability Date: Sun, 06 Aug 95 23:30:47 GMT Organization: Microserve Information Systems (800)-380-INET Lines: 41 Distribution: world Message-ID: <403k0p$ab@nuclear.microserve.net> References: NNTP-Posting-Host: pinetree.microserve.com X-Newsreader: News Xpress Version 1.0 Beta #4 rohrwerk@netcom.com (John Seboldt) wrote: > >Better yet, a close-wound wire coil on a 1/2 inch ceramic form (no slug) >did even better in the above circuit. One run I left it sitting all >night, and it stayed with a half-step at about 200 Hz audible pitch -- a >drift of about 10 Hz! Spot checks during the day showÙd it staying >within about this range. Other runs varied, but were significantly better >than other configurations. And all this without a box, though in a room >without windows with fairly steady temperature (when the central air isn't >on). This agrees closely with my own experiences building VFOs. One of the most stable I ever produced was built around the older W7ZOI high-stability (JFET) Hartley circuit. I used an ARC-5 transmitter covering the 80M band as a test bed and simply wired the FET, diode and one or two small fixed ceramic caps onto the tube pins of the original oscillator. The caps I added were *not* NPO. For anyone not familiar with these transmitters, their VFOs used a very large ball-bearing tuning cap, an air trimmer, and a large ceramic coil form (about 1-1/2") with heavy gauge wire and a tunable iron slug. All of the tuned components were enclosed in a formed aluminum shield on the top side of the chassis. Anyway, after installing the circuit in the original chassis, I found that the original dial markings were almost exactly correct, and that only a slight adjustment of the trimmer and slug were required for perfect alignment. Most of the cold start drift occurred in the first 15 minutes and was measured at better than 100 Hz over the first hour, improving to better than 10 Hz per hour thereafter. After confirming this with cold starts on several consecutive days, I let the oscillator run overnight. The <= 10 Hz wanderings recorded during previous runs were both positive and negative, and the final measurement after more than 24 hours was still within 10 Hz of the frequency achieved after one hour of warmup. FWIW, the counter used for these tests had a 10 MHz ovenized oscillator and was adjusted to WWV (as necessary) before each measurement. There is no question in my mind that ceramic forms and large gauge wire are a significant aid to good VFO design. At the very least, starting with these components can save many hours of cut and try in attempting to build stable VFOs. 73, Jack WB3U Message 6/8 From Stephen Parry Aug 7, 95 07:52:36 am PDT Return-Path: Date: Mon, 7 Aug 95 07:52:36 PDT To: rohrwerk@netcom.com Subject: Re: VFO Stability Glass tubing is good, you can score it with a glass cutter and it will break cleanly. Scientific supply stores carry it. You can probably also score the ceramic and break it. I've had bad experience with regular pvc water pipe, it appears to be pretty lossy. You can get acrylic rod from a plastics store which would be a better choice. Stephen Message 7/8 From Paul Harden Aug 7, 95 09:11:19 am -0600 Return-Path: Date: Mon, 7 Aug 1995 09:11:19 -0600 To: rohrwerk@netcom.com Subject: Re: VFO Stability John K0JD, I read your VFO observations with interest ... as they are pretty much the same as mine. No matter what book you pick up by the "experts" on VFO's or ARRL ... right off the bat you are inculcated with this belief that toroids are the only way to go and the old air-core inductors and transformers are way too unstable for VFO use. I no longer use toroids in my HB stuff hardly at all, except perhaps in the BP filters after the PA transistor (for the current/voltage aspects). It just makes sense that an iron core takes a while to heat up and stabilize, all the while introducing stability errors into the circuit. Air core/ceramic types have been used for years, even by the military, for stable designs. I have the Toko designers data book here, and the charts in it showing various effects vs. temperature are far more stable than we've been led to believe. Keep up the good work and experimentation. Paul NA5N ----NATIONAL RADIO ASTRONOMY OBSERVATORY ----- Socorro, New Mexico ----- | VLA - Very Large Array Observatory - Worlds largest radio telescope | |VLBA - Very Long Baseline Array - even larger | --------(pharden@zia.aoc.nrao.edu) --- (73 de NA5N) -------------------- Message 8/8 From Harry Chase Aug 7, 95 11:31:15 am EST Return-Path: Date: Mon, 07 Aug 95 11:31:15 EST To: rohrwerk@netcom.com Subject: VFO Stability If you are *really* picky about the coils for stability, you might consider the fancy ones that are made by JFD or Johansen -- these are sputtered silver onto a glass cylinder (pyrex, or some other kind of good temp-stable glass) and they come with all kinds of test data; Q is claimed at upwards of 250, depending on the value. They are very expensive, and they arent made in values (that I've seen) which would be convenientbelow 40M or so, (gemnerally intended for low-VHF ranges) but they could be your answer when freq. is really critical. BTW, I sympathize with your complaints; I have a devil of a time keeping even a phase-locked LO on 10 GHz stable with anything less than a good ovenized crystal reference. When mountain-topping with the 10GHz setup, I first turn on a beacon I built which is on my tower at home, and controlled by a Vectron 10 MHz 3rd. overtone, double-oven type oscillator I got by luck at a flea mkt.; and it sits inside another box to keep drafts off it, and runs constantly. Only with this do I depend on the freq setting on the portable rig... Harry WA1VVH Message 2/3 From Paul Harden Aug 7, 95 02:54:12 pm -0600 Return-Path: Date: Mon, 7 Aug 1995 14:54:12 -0600 To: rohrwerk@netcom.com Subject: Re: VFO Stability Gads, 17Hz per hour drift is pretty darn good. You oughta be proud for attaining that. Most VFO's in the QRP kits and rigs I've played with are easily 200 Hz/hr 1st hour and slightly <100 Hz/hr thereafter. Of course with a little effort and know how you can improve on that fairly easy, but <20 Hz/hr is pretty good for your basic L-C circuit (i.e., not a phased lock loop or something). Paul NA5N Message 3/3 From N5EM@aol.com Aug 7, 95 04:55:29 pm -0400 Return-Path: Date: Mon, 7 Aug 1995 16:55:29 -0400 To: rohrwerk@netcom.com Cc: qrp-l@lehigh.edu Subject: Re: VFO Stability >But I'm coming to the conclusion from my experiments and some concurring >messages, that air core coils are quite superior. John, Of that there is no doubt. Air cores are superior to anything with core with regards to temp. stability and, generally Q because to be self-supporting, they are wound with larger wire. Problem is, most projects now-a-days don't have room in them. Consider the VFO coil you are working on. How big would it have to be to work on 40 meters and how much space would you have to leave around it to avoid unwanted coupling problems. We would probably be much better off, in general, if we used larger cases and air solenoid inductors because we wouldn't be as tempted to cram everything together into a Sucrets box (oops, Altoids box). Consider the W1AW problem with the QRP+. By simply re-routing a coaxial cable, the problem is cleared up or greatly reduced. There's a lot of stuff in that little package. Now, don't misunderstand. I love the Plus. I'm glad that Index got it into that little package. It just highlights the problems of unwanted and unexpected interaction when you get everything so close together. Begining builders take note. If you are building from scratch without the benefit of a kitted design that SHOULD assure reproducibility, don't pick a case that is too small. Give yourself some room to work and give your electronics some breathing room (both thermally and electromagnetically). You'll probably end up with more hair in the long run. 72 Ed Manuel, N5EM n5em@aol.com Date: Mon, 7 Aug 1995 10:42:51 -0700 (PDT) From: Steven Wilson To: ges@praxis.ppco.com Cc: Multiple recipients of list Subject: [2320] Re: VFO Stability Message-ID: BATHTUB CAULK will eat up copper and other metal type things. If it has a vinger smell it will attack metal...... I know from experience having sealed a PC Board in it. Looks nice, ran very good for about 90 days. The copper just disappeared from the board. de stan ak0b Date: Mon, 7 Aug 1995 14:40:16 -0400 (EDT) From: JEVERHART@cayman.vf.mmc.com To: qrp-l@Lehigh.EDU Subject: [2322] Re: VFO Stability Message-ID: <950807144016.2023d163@carib.vf.mmc.com> randyw@crl.com writes: > BATHTUB CAULK will eat up copper and other metal type things. If it > has a vinger smell it will attack metal...... I know from experience > having sealed a PC Board in it. Looks nice, ran very good for about > 90 days. The copper just disappeared from the board. de stan ak0b Boy does he ever speak the truth! I have some related experience. Back in the days when Iran was a valued ally (and very good customer for military and other hardware), I designed some equipment for a company that had Iran as a customer. Since the stuff was going to be exposed to the elements (weather, but no like Dayton, so don't grouse!), we "potted" our assemblies with a foam material. (Gee that's not a bad run-on sentence but it's not world class like NILs's :-)). Anyhoo, the foam potting material intentionally expanded as it set to ensure that it filled all of the nooks and crannies to keep out nasty stuff like moisture and desert sand. Being a conscientious engineer (how many of them do you know that could deliver that word without a dictionary or spell-checker?), I carefully sealed relays and potentiometers so that the potting foam could not get inside. After several months of outdoor exposure the electronics stopped working! It seems that the pots I had worked so carefully to seal and protect, developed a severe case of corrosion. Chemical analysis showed that the corroded material inside the pots was copper actetate. AHA! It seems that ordinary RTV and bathtub caulk (and most ordinary silicone sealing caulks) generate acetic acid during the curing process - that is what makes the familiar vinegar smell. Better yet, the curing process requires the presence of water vapor in the air. In the presence of a corrosive sensitive material like copper, this is a sure recipe for disaster! My fix was very simple - I replaced the damaged components and sealed the new ones with a specially formulated RTV, specifically RTV 3140. The latter is not corrosive because it does not use acetic acid in its chemical processes. But RTV 3140 is not commonly available. The bottom line is that I do NOT recommend ordinary RTV anywhere that will expose a corrosive metal to moist conditions. Use something more benign. BTW, if you read thru all of this you definitely do not have Attention Deficit Disorder but perhaps altogether too much free time on your hands (watches on your hands instead of vice versa?) I must be reading too much of Nils' stuff lately. It's starting to make sense. 72/73, Joe E., N2CX ------------------------------ Date: Mon, 7 Aug 95 14:18:42 EDT From: (MSgt Cameron C.R. Bailey) To: Cc: QRP-L@LEHIGH.EDU Subject: [2323] Re: VFO Stability Message-ID: Yes you are correct. We used to use Bathtub Caulk sealants to seal repacement antennas on aircraft. The Air Force does not approve it for that use. There are approved sealants known as potting compounds. The ones that comply with MIL specifications are known commonly as RTV (Room Temperature Vulcanizing). They are in most cases a two part mixture. Some of the ones used on aircraft can withstand temparature extremes. Be careful, some potting compounds are known as reverting potting compounds. They will revert back to liquid after 2 years to reseal connectors. They can be messy. There are some encapsulants out there for sealing cable splice cases. Talk to your telephone/catv friends. I don't know if you can buy small quantities of it. I prefer to use the insulating varnish (GLYPT) on coil forms. There are other special coatings you can use made just for coils called Q-dope. If you are not concerned with heat, you may consider using sealing wax. It is easy to remelt. Don't let your circuit in the hot sun though. :^) Chuck would have problems. 72 de kt3a --------------------------------------------- Date: Mon, 7 Aug 95 15:36:55 EDT From: Clark Fishman (FSAC) To: qrp-l@lehigh.edu Subject: [2326] VFO Stability Message-ID: <9508071536.aa01095@FSAC3.PICA.ARMY.MIL> Try the T68-7 core for your next VFO..the 7 material has a lower terperature coefficient then the 6 material. Keep on oscillating wa2unn Date: Mon, 7 Aug 1995 17:50:47 +0600 From: ges@praxis.ppco.com (ges@240pob) To: randyw@crl.com Cc: qrp-l@lehigh.edu Subject: [2335] Re: VFO Stability Message-ID: <9508072250.AA13563@praxis.ppco.com> Thanks to Steven Wilson for the info about silicone rubber corroding copper, etc. What a surprise - I will have to find a better way to stablize the vfo in the xmtr I am putting together. It is a vfo/2N3053/IRF510 job and I am just transferring the parts to a real pcb. Steven no doubt saved me a lot of disappointment down the road - proving once again the value of the qrp list. BTW, a tube of RTV has a warning right on the tube that it will corrode copper and brass! ---------------------------------------------------------- Glen E. Stockton K5UP Bartlesville, OK ges@ppco.com ---------------------------------------------------------- Date: Mon, 7 Aug 1995 16:54:07 -0700 From: ji3m@scubed.com (James R. Duffey) To: qrp-l@lehigh.edu Subject: [2336] Noncorrosive RTV, Beeswax, and VFO Stability Message-ID: JEVERHART (and others) writes about sealing electronics in Room Temperature Vulcanizing (RTV) silicone sealant: >Better yet, the curing process requires the presence of water vapor in the >air. >In the presence of a corrosive sensitive material like copper, this is a sure >recipe for disaster! My fix was very simple - I replaced the damaged >components and sealed the new ones with a specially formulated RTV, >specifically RTV 3140. The latter is not corrosive because it does not use >acetic acid in its chemical processes. But RTV 3140 is not commonly available. It is, but you need to buy a whole case. However a similar sealant is available as aquarium chaulk or sealant. It should be available at stores that sell tropical fish supplies. The ones I have used do not require release of acetic acid to cure, but they too requires absorption of moisture (usually from the air) to cure, so if you use it allow it to cure fully before sealing it to the atmosphere. This could take a while in dry --More-- climates or during the winter when the air in houses is very dry. I know from experience that moisture is necesary for a cure, having watched someone try to seal a dry box with it, and having RTV that never cured, at least for months and months! If all you are concerned about is preventing VFO microphonics use beeswax which is available in most hobby shops. Melt a bit with a double boiler. If you don't know what a double boiler is, or you can't use the one that is normally used for cooking, put the wax in a can ,then put the can in boiling water. When it melts pour it onto your VFO so that it covers all the components. Usually a quarter inch or so is sufficient. Allow to set. This will prevent most of the movement of components with respect to each other and prevent microphonics. The wax can be removed with gentle heating for rework. Duffey KK6MC/5 James R. Duffey ((505) 764-3143) Maxwell Laboratories, S-Cubed Division 2501 Yale Blvd SE Suite 300 Albuqueruqe, NM 87106-4125 ji3m@scubed.com Message 7/9 From Steven Wilson Aug 7, 95 08:35:02 pm -0700 Return-Path: Date: Mon, 7 Aug 1995 20:35:02 -0700 (PDT) To: John Seboldt Subject: Re: VFO Stability air core is better.. but -7 is a nice material to use. they are about the same price as a -6 material. Dan hd it before he went on his "walkabout". Message 8/9 From Mark J. Dulcey Aug 8, 95 00:47:06 am -0400 Return-Path: Date: Tue, 8 Aug 1995 00:47:06 -0400 (EDT) To: John Seboldt cc: Multiple recipients of list Subject: Re: VFO Stability On Mon, 7 Aug 1995, John Seboldt wrote: > Meanwhile, the basic consensus is that ANY core material is a > compromise for VFO use. An air core coil is by far the most stable > inductor to use. The traditional ceramic form is most recommended, with > glass tubing and acrylic rod also getting some nods. Toroids have become > popular for compactness, of course, but unless that -7 material is really > good, air core is still the ultimate if you make room for it. This is true for rigs intended for use while standing still. Air-core inductors, especially the large ones needed for HF use, might present a problem with mechanical stability in mobile and hand-held gear, though, resulting in a drift cure that is worse than the disease of core instability. The best bet there, of course, is a frequency synthesizer, but a well-built ceramic or even powdered-metal toroidal core is likely to be a better bet than air-core. Message 9/9 From John F. Woods Aug 8, 95 07:56:16 am -0400 Return-Path: To: rohrwerk@netcom.com cc: Multiple recipients of list Subject: Re: VFO Stability Date: Tue, 08 Aug 1995 07:56:16 -0400 > Meanwhile, the basic consensus is that ANY core material is a > compromise for VFO use. An air core coil is by far the most stable > inductor to use. The traditional ceramic form is most recommended, with > glass tubing and acrylic rod also getting some nods. True, but it turns out that air core is also something of a compromise: the disadvantages are the large physical size (obviously), and the coupling to other circuits. (Enclosing an air core inductor in a metal can lowers its Q because of the coupling to the can.) Toroids are self-shielding and have little (if not quite no) coupling to nearby wires. I believe the -7 material has a TC of +30ppm/*C, as opposed to -6 which is +35ppm/*C (a small difference, but better than nothing). I happen to have a bunch of T44-7 cores, from when Amidon accidently shipped me T44-7 instead of T50-7, and then sent, as replacements, another batch of T44-7... Message 4/5 From N1OOQ Tom R. @ MRO1 08-Aug-1995 1001Aug 8, 95 10:22:11 am EDT Return-Path: Date: Tue, 8 Aug 95 10:22:11 EDT To: rohrwerk@netcom.com Cc: qrp-l@lehigh.edu Subject: re: VFO stability > The simple Hartley circuit (such as W7EL's Optimized QRP Xcvr) never > satisfied me -- it never seemed to settle down, with a slow drift downward > that never stopped. This is after boiling the toroid (which helped), Yup, I had the same experience with a Hartley. Didn't try boiling the toroid. > Then I built up the series-tuned Colpitts used by W1FB, a breadboard > version without a box. At 7 MHz, a toroid inductor (about 6 uH) again > proved unsatisfactory, the drift being about the same. Using a piece of > close spaced airwound coil stock, stability improved a lot, with many 1 I tried series-tuned "Clapp" configuration for the 40m superhet I'm building. I tried two coils: one wound on a T68-6 toroid, the other an air core coil of about 1 inch diameter, hand wound. Results were worse than the same circuit with a parallel-tuned tank. I was attempting to get lower drift, as mentioned by W7ZOI and W1FB in "Solid State Design". I finally gave up on series-tuned, and went with parallel-tuned Colpitts, which is a great deal better than the Hartley mentioned above. I don't really know why I got these results for the Clapp ckt... and now you've confirmed Heyward and DeMaw's findings. > Better yet, a close-wound wire coil on a 1/2 inch ceramic form (no slug) > did even better in the above circuit. One run I left it sitting all > night, and it stayed with a half-step at about 200 Hz audible pitch -- a > drift of about 10 Hz! Spot checks during the day showed it staying This I have to try. I've been keeping an eye out for a supply of ceramic-form coils or just the forms at the flea markets... nothing so far. -Tom R. N1OOQ randolph@est.enet.dec.com --More-- Message 5/5 From Murray Kelly Aug 9, 95 03:45:48 am +1000 Return-Path: Date: Wed, 9 Aug 1995 03:45:48 +1000 (EST) To: rohrwerk@netcom.com Subject: Re: VFO Stability X-Newsreader: NN version 6.5.0 CURRENT #83 I have found much of the same stuff. A sIELER (damn editor) circuit qv. is very stable . I even wrote bit of basic code to help. From my experience the heat is the worst enemy. As you describe. A box filled with styrofaom 'chips' would speed up heating and reduce drift. After that the power disipated by the oscillaotor is next most important. Reduce it 'till it stops! Then go back a bit. My ear is not acute as your own altho I played first violin for many years. Hi. The 850TS bugs me with its drift after the 930. Good luck. Murray Kelly. vk4aok. Message 3/3 From David D. Meacham Aug 8, 95 01:11:18 pm -0700 Return-Path: Date: Tue, 8 Aug 1995 13:11:18 -0700 (PDT) To: John Seboldt cc: Multiple recipients of list Subject: Re: VFO Stability John, The -7 material IS a lot better than the others, but it turns out that it is very sensitive to stray magnetic fields, too! Anything magnetic near the rig can cause VFO frequency shift. There was even a report of shift when mobile and driving under a steel bridge! 72, Dave, W6EMD --------------------------------------- On Mon, 7 Aug 1995, John Seboldt wrote: > Clark Fishman recommended -7 core material > to help stability in VFOs. I have heard of this, but the last Amidon > flyer I got with an order (about 3 months ago) had nothing to say about > them, though it was listed on the enclosed price list. Something to try. > > Meanwhile, the basic consensus is that ANY core material is a > compromise for VFO use. An air core coil is by far the most stable > inductor to use. The traditional ceramic form is most recommended, with > glass tubing and acrylic rod also getting some nods. Toroids have become > popular for compactness, of course, but unless that -7 material is really > good, air core is still the ultimate if you make room for it. > > : John Seboldt rohrwerk@netcom.com / CW: It don't mean a thing > : Amateur radio K0JD... / if it ain't got that swing! > : Church of the Annunciation, / Di dah, di dah, di dah, di dah... > : Minneapolis / (sorry, Duke!) > > > Date: Tue, 08 Aug 1995 08:44:04 -0600 From: LVE1@inel.gov (Larry East) To: qrp-l@lehigh.edu Subject: [2357] RE: RTV Message-ID: <9508081444.AA18459@garnet.inel.gov> Gee... I thought the WHOLE WORLD knew by now that most RTV caulking compounds can cause damange to copper! Oh well... there ARE some RTVs that don't generate acetic acid in the process of curing that are readily available -- the kind that are used for automotive (and other) gasket sealing. Check your local automotive parts supply; the "good stuff" should say on the label someplace that it is non-corrosive and/or acid free. I have used the stuff to seal antenna traps (among other things) with no sign of corrosion after 10+ years. 72, Larry W1HUE/7 LVE1@inel.gov --More-- Message 14/17 From N1OOQ Tom R. @ MRO1 14-Aug-1995 1Aug 14, 95 10:44:29 am EDT Return-Path: Date: Mon, 14 Aug 95 10:44:29 EDT To: rohrwerk@netcom.com Subject: re: VFO stability One more comment on boxes/inductors: When I tried the series-tuned Colpitts, I made a box out of copperclad to house the whole thing. I was originally going to use a hand-wound air-core coil as the L, so I made it big enough to not interact greatly with that, i.e. the box size was about 3x the largest dimension of the coil. I held the coil in the center of the box with some bits of polystyrene plastic... Anyway, all this was for naught. The thing drifted like crazy, pathetically bad. I think interaction with the box might have been a big factor. I replaced the coil with a toroid L of similar inductance to see if the so-called self-shielding property of the toroid would cut down the box effects... The answer was, yes, but it still drifted pretty badly. It was then that I tried parallel-tuned, and most of the drift went away immediately! Beats me... W7ZOI and W1FB found exactly the opposite... The only thing I could figure at the time was that I was getting much higher Q out of the parallel-tuned, because I put a very small value NP0 cap in series between the tank and the gate of the FET. You can't really do that with series-tuned, because it would mess up the freq tuning, so maybe the FET loads it down too much... de N1OOQ Xref: netcom.com rec.radio.amateur.homebrew:9757 Path: netcom.com!netcomsv!uu3news.netcom.com!netcomsv!uucp3.netcom.com!orlando.power.net!news.leonardo.net!la-news1.digilink.net!newsfeed.internetmci.com!psgrain!news.tek.com!emerald.cse.tek.com!royle From: royle@emerald.cse.tek.com (Roy W Lewallen) Newsgroups: rec.radio.amateur.homebrew Subject: Re: VFO stability Date: 11 Aug 1995 01:11:11 GMT Organization: Tektronix, Inc., Beaverton, OR. Lines: 47 Distribution: world Message-ID: <40eanf$mb7@tekadm1.cse.tek.com> NNTP-Posting-Host: emerald.cse.tek.com This has been an interesting thread, and the comments seem to generally agree with what I've believed for some time: 1. In a properly-designed oscillator with NPO capacitors, the inductor will generally be the dominant drift source. Neither the capacitors nor the active device will contribute significant drift compared to it. This may not be true if the inductor is made very, very, good but was true for all the cases I tried. 2. The temperature coefficient of polystyrene capacitors is opposite that of most inductors. Sometimes you can get lucky and the capacitors cancel the inductor drift (particularly if the inductor has poor drift character- istics). I prefer to first reduce the drift as much as possible by using low temperature coefficient parts, then compensate what's left. This leads to a more repeatable design. The less inherent drift and the less compensation, the better. 3. I suspect but haven't confirmed that inductor drift decreases pretty much as Q increases. Therefore, a good-sized air-core inductor would reasonably drift less than one wound on a small toroid core. (Of course, powdered iron cores also have a temperature coefficient so they further degrade the drift characteristics.) I've done little experimenting with larger air-core inductors but those of you who have, have confirmed this. One thing I'd watch out for when using air-core inductors is what the field is coupling into. There's a large field surrounding a solenoid. If there's metal nearby, it can lower the Q which may increase drift. Also, nearby material like an epoxy-glass PC board may degrade performance, since this material absorbs atmospheric moisture and has a dielectric constant which is influenced by temperature. It would be prudent to evaluate an oscillator in its final physical form to make sure it works as well as it may have at the breadboard stage. One other thing to watch for is poor-quality NPO capacitors. Apparently some of the imported ones really don't have low temperature characteristics. An oscillator and can of spray freeze should help you to get an idea of what you've got. Keep the comments coming! 73, Roy Lewallen, W7EL w7el@teleport.com [After August 16, any of my addresses which include ".tek" will be invalid. Send email to w7el@teleport.com.] Xref: netcom.com rec.radio.amateur.homebrew:9799 Path: netcom.com!ix.netcom.com!howland.reston.ans.net!tank.news.pipex.net!pipex!dispatch.news.demon.net!demon!pinetree From: jackl@pinetree.microserve.com (WB3U) Newsgroups: rec.radio.amateur.homebrew Subject: Re: VFO stability Date: Sat, 12 Aug 95 19:29:33 GMT Lines: 30 Message-ID: <808256566.23046@pinetree.microserve.com> References: <40eanf$mb7@tekadm1.cse.tek.com> NNTP-Posting-Host: pinetree.microserve.com X-NNTP-Posting-Host: pinetree.microserve.com X-Newsreader: News Xpress Version 1.0 Beta #4 royle@emerald.cse.tek.com (Roy W Lewallen) wrote: >This has been an interesting thread, and the comments seem to >generally agree with what I've believed for some time: >3. I suspect but haven't confirmed that inductor drift decreases pretty much as Q increases. Therefore, a good-sized air-core inductor would reasonably drift less I've noticed that too. I wonder if the explanation could be that a larger conductor incurs less IR loss, and is also better able to dissipate the minute amount of heat that's generated from this source? Much excellent information has been published on this subject, but I haven't seen any in quite some time. I think many of the tried and true principles that used to be applied have been gradually displaced by the desire to build ever-smaller components. The advent of synthesizers has also had an effect on the popularity of this material. Some of the best information on VFO stability was published in the 60's, a time when VFO builders were still faced with the effects of tube heat and changing operating characteristics. The same methods used then still hold true, and solid-state gear can take even better advantage of them. The trick is finding the info. 73, Jack Xref: netcom.com rec.radio.amateur.homebrew:9805 Path: netcom.com!ix.netcom.com!howland.reston.ans.net!news.sprintlink.net!psgrain!nntp.teleport.com!usenet From: w7el@teleport.com (Roy Lewallen) Newsgroups: rec.radio.amateur.homebrew Subject: Re: VFO stability Date: 13 Aug 1995 04:28:12 GMT Organization: ELNEC/EZNEC Software Lines: 34 Message-ID: <40jv0s$2mq@maureen.teleport.com> References: <808256566.23046@pinetree.microserve.com> NNTP-Posting-Host: ip-pdx7-34.teleport.com X-Newsreader: AIR News 3.X (SPRY, Inc.) > jackl@pinetree.microserve.com (WB3U) writes: > royle@emerald.cse.tek.com (Roy W Lewallen) wrote: > >This has been an interesting thread, and the comments seem to > >generally agree with what I've believed for some time: > > >3. I suspect but haven't confirmed that inductor drift decreases > pretty much as Q increases. Therefore, a good-sized air-core inductor > would reasonably drift less > I've noticed that too. I wonder if the explanation could be that a > larger conductor incurs less IR loss, and is also better able to > dissipate the minute amount of heat that's generated from this source? > . . . This stands to reason. Everything else being equal, a higher-Q inductor will generate less heat than a lower-Q one. But all else is seldom equal. The effect of heating is to change the physical dimensions of the coil, which in turn changes the inductance. I would imagine there could be quite a difference in inductance change as a function of temperature depending on the shape of the inductor and the expansion characteristics of the form, if any. Also, as you mention, the amount of temperature rise which results from the heat depends on the heat sinking/dissipating ability of the coil. Hmmm. . . wonder if a shape could be found such that the inductance is constant if all dimensions change in an equal ratio. I'm sure somebody thought of this a generation or two ago. . . Roy Lewallen, W7EL w7el@teleport.com