Lee KD4RE of the Vienna Wireless Society has been talking about the Franklin oscillator. He has been telling us that it is very stable, and capable of stable operation up through the ten meter band. Lee wants to build an direct conversion receiver for all of the HF bands with one of these circuits.
I was skeptical. First, I'd never heard of this circuit. I'd grown up in ham radio on a steady diet of Hartley and Colpitts and Pierce. Vackar or Clapp were about as exotic as I got. And second, I'd come to accept that it is just not possible to build a good, stable, simple, analog VFO for frequencies above around 10 MHz. For example, in his Design Notebook, Doug DeMaw wrote, "VFOs that operate on fundamental frequencies above, say, 10 MHz are generally impractical for use in communications circuits that have receivers with narrow filters." DeMaw was known for resorting to variable crystal oscillators.
But then this month Mike Murphy WU2D put out two videos about his use of the Franklin oscillator circuit in a direct conversion receiver at 21 MHz. The VFO was shockingly stable. I began to believe Lee. I fired up my soldering iron and built one.
Lee was right, it is in fact remarkably stable, even at higher frequencies. My build (see picture above) was a bit slap-dash and could be improved a bit, but even in these circumstances here is what I got. This was with a stable 6 Volt Supply and with only a cardboard box covering the circuit:
Local time Frequency
0543 19.1114 MHz (cold start)
0636 19.1116
0804 19.1117
1034 19.1118
1144 19.1117
I started digging around for references to the Franklin. There was nothing about it in Solid State Design for the Radio Amateur, nor in Experimental Methods of RF Design. Pat Hawker G3VA (SK) did discuss it in his Technical Topics column in RADCOM, February 1990. Pat gave a great bio on Charles S. Franklin (born in 1879 and a colleague of Guillermo Marconi). But tellingly, Pat writes that, "Despite its many advantages, the Franklin oscillator remains virtually unknown to the bulk of American amateurs."
It wasn't always unknown. In the 1940s, we see articles about the Franklin oscillator circuit. There is a good one in the January 1940 issue of "Radio." The author W6CEM notes that the circuit "is probably familiar to only a few amateurs." It shows up in the "How's DX" column (above). And the 1958/1959 issue of Don Stoner's New Sideband Handbook we see a lengthy description of the Franklin oscillator. Stoner wrote: "The author's favorite oscillator is the 'old time' Franklin, and it is believed to be the most stable of them all! This rock-solid device can put a quartz crystal to shame! Because it represents the ultimate in stability, it is the ideal VFO for sideband applications." And we see a PTO-tuned Franklin oscillator in the July 1964 QST. And it is in the fifth edition of the RSGB Handbook (1976).
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The Franklin circuit was not/is not "known" to many radio amateurs. My old pal W2LYH was instrumental in teaching me about it. Check out his detailed station building ideas in the QST archives~!~ Don K4ZA
ReplyDeleteGreat read and thanks for the thought provocation!
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ReplyDeleteThe PTO in the Collins 51J-3, R-388, and 75A-4 receivers use a Franklin oscillator in the same configuration as in the Jan. 1940 Radio article, although the feedback is to a tap on the PTO inductor. I suspect other Collins PTOs with two tubes are similar. - John Atwood, WA1ICI
ReplyDeleteMost oscillator configurations use a single active device. 2 active devices connected in cascade offers more power gain and better isolation from the resonant circuit compared to 1 active device (if done properly!). You may use either a series or parallel resonant circuit for the tank. Substituting 2 JFETs and a slightly different topology, we have built temperature stable VFOs that operate in the 150 MHz region. However, for portable outdoor use without an ovenized container, the temperature drift gets quite noticable. No better nor worse than any lumped element single device topology aka Colpits et al.
ReplyDeleteUltimately, the best oscillator for any 1 operator is 1 the 1 that you enjoy success with. We all want a VFO with a high Q tank that is temperature stable in the intended operating environment. VFO stability stories are the stuff of legend amd hype. Almost no articles show frequency measurements in a 1 Hertz resolution plotted over many hours at full mesh, 1/2 mesh, and minimal mesh of the tuning capacitor (or similar for the PTO). Then for the Y axis change the room temperature at least 10 degrees F (usually done in a temp controlled oven). Anything less is just adding to the folklore.
For us, at HF to VHF, the bipolar transistor Vackar is king and offers temperature stability and low thermal noise and low flicker noise when properly designed and built
Simon and Petre Gavanchy
Vackar relies on temperature compensation components. The franklin is not (at least by design) Going for temperature compensation and measurements in the 1Hz scale? Good luck! The Franklin is good "enough" for our HF ham purposes. We do not mind correcting the tine every 10-15 minutes, but we mind if this has to be done every minute. So this is "good enough". The only disadvantage of the franklin is the little bit increased phase noise due to the 2 devices, but on HF this is way less important.
DeleteI've known about the Franklin since the 90s. 73 had a test circuit for measuring inductance. Like the Lambda Diode, put an LC on it and it will oscillate (within the transistor limits)
ReplyDeleteIf you want rare try the Zachary (invented by Mrs. Zachary) and the slightly less rare beasty from the 1950s, quoted below.is from an article about a 72 Mc/s oscillator for 144 Mc/s with less than 500c/s at 144. 12AT7.
...the basic circuit of an oscillator which is reputed to have a high degree of stability. It is a twin triode push-pull circuit, very reminiscent of the multivibrator, and glories in the title Kalitron. It comprises very few components and is one of those delightful circuits which function not only in "breadboard" form, but repeats the performance when reengineered for final use!
I suspect it's the same reason why people don't use the Butler oscillator circuit for crystal oscillators. Two tubes or two transistors are used instead of the single device in other circuits.
ReplyDeleteChuck, WB9KZY
I have built Franklin VFO's and they provide sufficient stability such that the thermistor/varactor compensation I previously needed for a JFET Clapp oscillator was not necessary. So I just went with NPO chip caps, Air-core Inductor (staked), and the light coupling. To avoid spurious oscillation at LF, don't use a higher value cap (at C3 on WU2D's and C6 on mine) than you need to. You don't want your Franklin to turn into an LF Multivibrator! Use NPO here too. The key is to keep the loop gain adequate at HF where you want it, and lower at LF, where you don't.
ReplyDeleteI use these as free-running VFO's for 80/40 and 30 Meters, operating at 1/2 of the TX frequency, to prevent pulling.
Note the use of a separate die-cast VFO box below the die-cast PA Box. It's all about the thermal/mechanical stability. Otherwise, the normal good practices, use Single-Sided PCB, LM317 type +6.4Volt IC regulator,etc
I also replaced the JFET Clapp with a Franklin in my HB receivers. Same results.
Franklin VCO's work great at VHF+ in synthesizers. That's for a synthesized 2M FM rig near completion.
GL es 73!
P.S. Just a note: My Website is in dire need of updating. The information on the MOuSeFET Transmitters IS actually up to date. But the HBR-3 receiver, and other items are in need of updates. Too many projects, not enuf time!
ReplyDeleteOne of the reasons some "innovators" get a lot of attention (aside from their own often-prodigious efforts at self-promotion) is that they're pretty rare. Human nature isn't innovative, it's *imitative*, perhaps part of our primate heritage ("monkey see, monkey do"). To a large extent, this means we move in well-established ruts, and we follow gurus who deepen the ruts, though they might know of better paths. To switch metaphors, the Franklin oscillator may have appeared on the edges of the radar screens of DeMaw et al, but their almost-exclusive attention was on the flurry of activity at the center.
ReplyDeleteThe inclination toward and the habit of imitation is especially pronounced in pastime pursuits like amateur radio. Most of us have limited time and funds, and innovation is often expensive in terms of both. The reliance on "popcorn" components and tried-and-true topologies and designs is understandable, if a bit obtuse. In the last quarter century, how many SA602-2N3904-LM386 direct-conversion rigs been "designed," built, or offered for commercial sale as kits? There are better components readily available, and sometimes that's brought to the community's attention, but the ruts are deep and time is short.
To a certain--perhaps large--extent, the future of amateur radio will depend on the exploration of alternatives. That doesn't mean the lemming (another imitative creature)-like abandonment of entire technologies for the latest whiz-bang. It certainly doesn't mean we have to jump on the DDS, DSP, or SDR juggernauts. Innovation doesn't just (or even essentially) mean "new." *Better* is also innovative, and if a Franklin oscillator is better than a Colpitts or Hartley one in terms of what is most-wanted--frequency agility *and* stability--then I for one am going to give it a try. I won't be doing so, though, with 2N3904s, J310s, or any of the other usual suspects. There are lower-noise and higher-gain alternatives.
In fact, I was the one that "forced" Mike Murphy WU2D to do a series of videos on the Franklin VFO. He refers my call (sv3ora) in the first video.
ReplyDeleteHe did not build it as I did, (http://qrp.gr/minivfo/) but it was enough to show the features.
I have an ALC amplifier in the chain and taking the output from the LC, so it is of very low distortion Without loading the LC.
Mine works to 36MHz with a SINGLE UNTAPPED inductor and only capacitors switching.
73
Kostas sv3ora
I'm going to be building this VFO. It's come just at the right time for me, as I'm going to try building a direct conversion receiver for the first time. While I've done quite a bit of reading and watching videos, there's still quite a bit I don't quite grasp. I will be using Bill's school DCR as my guide. I do like the idea of the permeability tuned oscillator though. From what I've seen it allows very fine tuning. Maybe I could use that in the Franklin.
ReplyDeleteTrevor
Trevor: I understand that the Collins PTOs were basically tube-type Franklin oscillators with permeability tuning. But if you have never built a DC receiver before, it may be best to start with a proven design. You can experiment with Franklins and Franklin/PTOs later. 73 Bill
ReplyDeleteKostas SV3ORA ,
ReplyDeleteFor years I was aware of the Franklin (and other) oscillator circuits, but it was your webpage that provided the impetus, getting me to evaluate it more closely. I set up a laptop, connected to a Homebrew Frequency Counter via an Arduino UnoR3, and let it take the frequency data, while I go do some other stuff.
The Franklin did the best of the VFO's. Only a XTAL Osc did better. Each run was from a cold start to 150 minutes with data logged each minute, then plotted on the laptop. These were for 80/40 and 30 Meter transmitters, VFO at 1/2 of the Tx frequency, so VFO=1.8/3.55/5.05MHz approx. Since then, I did this for receivers and a 2M FM (Synthesized) HB Rig.
Kudos for providing the much-needed impetus!
73!
WN2A
The Temperature Coefficents of the other components (other than the "Tank") are often overlooked. One that should be considered is the TCL of any RFC's. These inductors are usually chosen for high-impedance, not so much for TC. Normally, a high TC with a high shunt impedance
ReplyDeleteisn't a concern, or is it?? These RFC's are often wound with a high perm ferrite, with very high TCL's.
If that part is coupled in some way to the tank, it could degrade VFO temp stability. That could explain some problems I have seen with both JFET Clapp and Vackars. -- Maybe some TC analysis is in order?