The Franklin Oscillator: A Super-Stable VFO. Why No Attention? Why So Little Use?

My Franklin VFO

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.  

WU2D's Franklin Oscillator

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."  

QST "How's DX" August 1947

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). 

Here is the January 1940 "Radio" with the Franklin oscillator article on page 41 by W6CEM: 

https://worldradiohistory.com/Archive-Radio/40s/Radio-1940-01.pdf

Here is the July 1964 QST article: 

http://jlandrigan.com/files/PTO%20VFO/PDF%20PTO%20VFO%20Horn%20QST%20July%201964.pdf

There was an article about the Franklin oscillator in 73 magazine by W4LJC in February 1999: 

https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1999/73-magazine-02-february-1999.pdf   

The author notes that: 

Much more recently (2022), Mike WN2A, modified his Mousefet transmitters (seen in QRP Classics in 1992) to include the use of the Franklin VFO circuit.  Mike's documentation is really excellent.  Kostas SV3ORA has a Franklin oscillator in his Super VFO circuit.  Hans G0UPL has one on his site. 

Look, there may be reasons why the Franklin oscillator has been ignored.  But the circuit sure seems to present a lot of advantages.  Stable operation beyond the 10 MHz barrier is the big one.  Simplicity is another.  If there are problems and shortcomings, let's hear about them. But it seems as if the Franklin oscillator may provide the opportunity for us to build stable VFOs beyond 10 MHz without resort to complicated PLL stabilization techniques, and without opting to go with an Si5351 or other complex digital devices.   

So let me ask:  Why hasn't the Franklin oscillator been given more attention, and why haven't we seen more use of this circuit by hams or even by manufacturers?  

Farhan's sBITX -- Taking Orders! (Video)

It is really looking great.  Congratulations to Farhan for bringing yet another amazing rig to the amateur radio community.   

Order yours here: 

https://www.hfsignals.com/

Arnie Coro: Jaguey Rig Designed in 1982, More info on the Rig

Jaguey, Matanzas, Cuba

Dxers Unlimited's mid week edition for 23-24 October 2007

By Arnie Coro
Radio Amateur CO2KK

...

My own personal experience with the original JAGUEY direct conversion 

transceiver, designed way back in 1982, is that when used with a well 
designed front end input circuit, those receivers provide amazing 
sensitivity, with signals as low as 1 microvolt easily detected but, 
they do have one drawback, their selectivity or ability to separated 
between stations is very poor. The direct conversion radio receivers are 
used for picking up CW Morse Code Signals , Digital Modes and Single 
Side Band, but they are not good for receiving AM signals, and can't 

pick up FM modulated signals at all...

The original JAGUEY 82 Cuban designed single band amateur transceiver, was tested against a sophisticated and really expensive factory built 

transceiver. The tests showed that our design was at least as sensitive 
as the very expensive professional equipment, registering a measured 
sensitivity of less than one microvolt per meter, producing perfect CW 
Morse Code copy of such a signal. Adding well engineered audio filtering 
to a direct conversion receiver can turn it into a really wonderful 
radio by all standards amigos. 

Radio is a fun hobby, and believe me amigos, there is nothing more 
magical than listening to a radio receiver you have just finished 
building !!!

-----

Peter Parker VK3YE Found a nice description of the Jaguey by Cuban radio Amateur Jose Angel Amador from the BITX40 Facebook Group: 

A translation.  This was apparently in response to someone who thought they'd found a Jaguey schematic: 

"That's not an original Jaguey, that was a simple, single band, unswitched, 5 watt, DSB, kit for beginners with no gear and needing something to put on the license.

Carbon microphone direct to balanced modulator, two stages with 20 dB gain, W1FB/W1CER style feedback, and final with 2 x 2N2102 class B.
The receiver was more like that of the schematic, with a TAA263, easy to get from the FRC in 1978, and headphones. No need for an RF stage: the mixer was overloaded at night with European broadcasts above 7150.
The VFO is also inspired by Solid State Design for the Amateur Radio, a Colpitts with 2SC372 and a low gain feedback buffer with two 2SC372s.
Binocular ferrites were taken from Soviet TV baluns. The conditions of Cuba 1978.
Today I would make an SSB rig with polyphase networks, mixer with 4066,  and VFO Si5351.
The big complication of BitX is the crystal filter, they either get it made, or stick to a recipe, but few have what is needed to measure and tinker with crystal filters.

Arnie Coro CO2KK (SK) Provides More Info on the Islander DSB rig

Peter Parker VK3YE found this message from Arnie CO2KK in the Wayback Machine.  We continue to look for more information on the Jaguey solid state DSB rig. 

Arnie wrote: 

Several years ago an amateur in central Cuba was approached by some of his young friends to help them build their first rig. CO7PR, Pedro, a telecommunications technician for the phone company, had a vast experience both with vacuum tubes and solid state circuits, plus that special gift of designing and building with whatever is available. After a few days, Pedro came out with the "Islander" prototype, a low parts count, easy to build single band transceiver!

Here is the circuit description of that little radio that has provided many cuban young and old, newcomers to our hobby, with their first rig... and the challenge to improve it.

RECEIVER:

It is a direct conversion, YES, a DC receiver made with vacuum tubes. The very thought of having those tube filaments fed from the AC power supply and at the same time having 80 or 90 dB of amplification made me shudder when I first talked to Pedro on the very popular here 40 meter band! You are LOCO Pedro, I told my good friend... CRAZY, those poor kids are going to hear 50 percent 120 hZ hum when they tune across the 7 megahertz band.

Yes Arnie, you are right, it has a little background hum, but by using a small loudspeaker and small coupling capacitors... it's tolerable! The receiver shares, in its original version, the same antenna input as the transmitter output stage, a PI network, but we soon learned to add a separate LC tuned circuit first and latter a bandpass double tuned input filter...PLUS a signal attenuator... a very primitive but effective attenuator... just a 10 k potentiometer!

For an RF amplifier stage the Islander uses a russian pentode, which is the equivalent of the popular TV IF amplifiers of the 50's... looks like a 6CB6, for those of you that fixed TV sets 40 years ago or so.

The 6 "little spider" five, as everyone knows that tube here,has a lot of gain, and it can be kept rather stable by a judicious choice of screen and cathode resistor values.. Noisy pentagrid converter follows!

The 6A2P... a russian 6BE6, was the first tube type used in the Islanders, later some people tried the ECH81 triode-hexode and found it works better.

The circuit of the 6A2P-6BE6 is quite straighforward... a... you guessed right... PRODUCT DETECTOR... fed from the vacuum tube VFO... and providing its audio output to the two stage audio amplifier.

Audio amplifier is made with a triode-pentode tube of which plenty are locally available from defunct TV's... the ECL82 and the 6F4P and 6F5P of east european and russian manufacture respectively provide a lot of gain.

So... that's your receiver.. quite straightforward, works on 160, 80 and 40 meters by just changing the input filter and the VFO injection, it does NOT provide very good selectivity at all, but during the daytime, when the 40 meter band is used for local and regional contacts, it puts those new hams ON THE AIR!

VFO... the big problem amigos!

CO7PR worked very hard to try to make a stable vacuum tube VFO... and he almost made it..

YES, ISLANDERS drift, some not too much, others are not so good, depending on who built the rig, and how close they followed Pedro's advice at first, and Arnie's CO2KK later (as yours truly became quite involved in the project, as soon as I found that it was THE way of getting all those guys ON THE AIR!)

VFO is made with ONE of the 6 "little spider" 5 pentodes... By the way, I am sure you will like to know why the tube is locally known like that... the ZHE letter of the Cyrillic alphabet is something difficult to pronounce to a cuban - or any other non slavic for the matter - and it resembles like a little spider on the tube's carton and... that's why it is not a 6 "ZHE" 5 but a 6 "little spider" five!!!

The VFO cleverly works at one half the operating frequency... and then it DOUBLES frequency at the plate circuit... output is via a link to the pentagrid or hexode mixer depending on which type you use.

BUT... the VFO also has a second output to the transmitting chain.. Well that's the receiver... OH YES... the VFO is fed from a VR tube, a gaseous discharge voltage regulator similar to a VR-150 or VR-105... CO7PR advises to use the VR105, but when building the Islander, special in the countryside, that's a very hard part to find, as old TV sets don't use VR tubes! So people use whichever VR they can find. ZENERS? They are only available locally for 6 to 24 volts, so they can't be used with this rig.

ISLANDER DSB AND CW transmitter circuit:

From the VFO plate circuit, you pick up 7 mHz energy (usually you must wait at least half an hour for that said 7 mHz energy to be stable enough in frequency) and feed two diodes (ex-video detectors from russian TV type D20) acting as what I like to call BALANCED AND UNBALANCED modulator!

When used for DSB, it is certainly a DSB generator... but when you want to work CW, it must be UNBALANCED.something easy to achieve with just a resistor from the +12 volts line and a switch!

The balanced modulator receives its audio from a carbon microphone capsule salvaged from an old telephone, and conveniently connected to same +12 volts with some additional filtering via biggest possible electrolytic + small ceramic dogbone from TV set IF amplifier as RF bypass... no dogbone capacitor there... strange howls on Islander audio as RF leaks into balanced modulator you know.

So dogbone ceramic capacitor is a must! No, disk ceramics are not locally available, so people must use the next best choice... dogbone ceramics in the 100 pf to 5000 pf range, usually rated at 300 volts or so... (that 300 volt rating we learned the hard way, but more about that later.)

The original version of CO7PR's Islander ran with the carbon microphone, no MIC LEVEL control option, as he really wanted to keep things simple... later versions have audio preamps of various designs, and some even have a sort of primitive compressor.. From the balanced modulator the DSB (plus a little carrier leak that is always there) drives the rig's one or two transistor low level RF amplifier, which is made using whatever NPN silicon transistor is available, usually KT315's salvaged from TV's too. the KT315 is sort of a russian version of the 2N2222, so you understand why we use it here!

RF voltage reaches then the grid of an ex-video output amplifier vacuum tube, and there you are... about 2 to 5 watts of either DSB or CW on 40 meters and a new cuban amateur ON THE AIR!

Before I forget... keying... a little chirpy always because of so many interactions between simple circuits, sometimes not too well shielded, first time builder etc.

BUT... ISLANDER is ON THE AIR providing that young kid from the local junior high school or that doctor that always wanted to be a ham, or maybe the fresh out of school electronic technician, with the fascination of their first ever rig. YES, they drift, and some drift badly, when the frequency determining capacitors in the VFO are not too good... (most of the time), as I said they are a little chirpy. and the receiver's selectivity makes working 40 meters at night almost impossible (although some wizards do make nightime contacts at the low end of 7 mHz) BUT. YES, they are ON THE AIR.

Today there are a few Islanders still on the air, and some are even still built brand new (with many of CO7PR's and CO2KK's mods), but the trend is for all solid state rigs centered around CO5GV's and CO2JA's prototype the "JAGUEY," a design that draws a lot from Wes Hayward's Solid State Design for the Radio Amateur, and as of late, with lots of ideas coming from SPRAT, the G-QRP club magazine and QRPp from NORCAL, the Norther California QRP club!!!

In a future posting I will describe the "Jaguey," too.

-------------

More info on the Cuban DSB and AM rigs can be found here:

https://soldersmoke.blogspot.com/2023/04/more-info-on-cuban-jaguey-solid-state.html 

https://soldersmoke.blogspot.com/2023/03/homebrew-am-from-cuba.html

https://soldersmoke.blogspot.com/2023/03/more-cuban-homebrew-from-80s-and-90s.html

https://soldersmoke.blogspot.com/2023/03/schematics-for-cuban-islander-double.html

https://soldersmoke.blogspot.com/2023/03/the-islander-homebrew-dsb-rig-from-cuba.html

https://soldersmoke.blogspot.com/2023/03/needed-more-info-on-cuban-islander-or.html

Europa -- "Attempt No Landing There."

As we get ready to send two probes to the Galilean moons of Jupiter, this sci fi movie made its way into my feed.   It is pretty good, and the ending will appeal to all true radio amateurs. 

https://www.syfy.com/syfy-wire/searching-for-life-on-europa-the-science-behind-europa-report

https://www.esa.int/Science_Exploration/Space_Science/Juice

More Info on the Cuban Jaguey Solid State DSB Transceiver

ZL2BMI Transceiver Layout (not full size here!) 

Continuing our search for information the Cuban "Jaguey" DSB rig, Trevor Woods pointed me to Dick Pascoe's QRP column in the (below) July 1998 issue of Ham Radio Today.  I think the first SPRAT article about Eric Sears' ZL2BMI DSB rig was in SPRAT 83 in the summer of 1995.  This fits well with the sequence described below by Arnie Coro CO2KK. 

I am still looking for a schematic and pictures of the Jaguey rig: If you can help in this, please let me know.  

Speaker Made from Potato Chip Bag: Tim Hunkin's Solenoid and Electromagnet Video

The potato chip ("crisp") speaker is very cool (I have it cued up here), but the rest of Tim Hunkin's video is also wonderful and worth watching.  (Note:  Posh bags work better as speakers.) 

Thinking of our use of signal relays, I kept wanting Tim to tell viewers to put a diode and a capacitor across the relay terminals to prevent back EMF from frying circuits.  But I guess this is not much of a problem with the arcade games that Tim builds.  

As always, Tim's scrounging and use of discarded parts is really admirable.   

I noticed in the credits that he is dedicating these videos to the memory of his colleague Rex. 

Thanks to Chuck WB9KZY for bringing this video to our attention.  And thanks Tim. 

The IRF510: The Car Turn-Signal Blinkers Used As RF Amplifiers

 

I have many of these MOSFETS in my rigs.  Yesterday I came across Paul Harden NA5N's excellent Handiman's Guide series.  It had this really wonderful paragraph about the history of this part.  I-R is the International Rectifier company.  

https://hyse.org/pdf/www.aoc.nrao.edu/~pharden/hobby/_ClassDEF1.pdf

https://hyse.org/pdf/www.aoc.nrao.edu/~pharden/hobby/Hobby.shtml

Helge LA6NCA Builds a Double Sideband Thermatron Transmitter

Helge is an amazing homebrewer. Check out the shack.  Note the R-390 and the Tek 'scope.  Watch how Helge designs his rig.  Watch him check the 3-D printed coil and the variable caps for resonance.  Most of all, watch his happiness when the new transmitter works.  I just wish he would have showed us some OM complaining that he was on the "wrong" sideband. FB Helge!  Thanks. 

W2UO's Michigan Mighty Mite -- Made a Contact!

Hi Bill,

I found SolderSmoke about six months ago, and it's quickly become one of my favorite podcasts.

I've been a ham since I was 11, but never tried my hand at homebrewing anything.  I've always heard how hard it would be, and how a project like a SSB transmitter is just too far out of reach.

You and Pete are inspirational, so I set out to build a simple starter project, the venerable Michigan Mighty Mite.  However not just any mighty mite, a usable one, not just a proof of concept.  One intended to sit on a desk and look good doing it.  Complete with built in low pass filter, tx/rx switching, and an internal dummy load.

I don't know if I accomplished all that, but I did make a contact on it this afternoon.  Next logical step I suppose will be to build a DC receiver to sit next to it.

Please find pictures attached, I've learned a lot about what not to do with project, so criticism is welcome.

73!
-Jim W2UO

My response: 

Wow Jim, that is really wonderful.  Congratulations on the build.   I've built many of them, but I don't think I ever made a contact with a MMM.  FB.  

It looks great to me!   Indeed, you should do a Direct Conversion receiver next.    Maybe do a receiver for 40, then do a version of the MMM for that same band.  Then you could 

make a completely homebrew QSO.   I did this recently on 40:  https://soldersmoke.blogspot.com/2023/02/first-qso-with-high-school-receiver-100.html    It was a real hoot!  

Our friend Dean also built a MMM as his first project:  https://soldersmoke.blogspot.com/2019/12/dean-kk4das-puts-michigan-mighty-mite.html

Please keep us posted on your progress. 

Again, congratulations!   73  Bill N2CQR

Technology and Methods from Wes Hayward W7ZOI

Great stuff.  Lots of wonderful articles filled with wisdom to ponder. 

 http://w7zoi.net/oldtech/ponder.html

Thanks to Tony G4WIF for spotting this gem and alerting us.  And thanks to Wes for all of this.  

Nice Ham Radio Documentary from Montana

Thanks to Thomas K4SWL for alerting us to this.  The 25 minute film is well-done, using modern film techniques and editing. It is a bit appliance oriented (but so is ham radio, unfortunately).  The EME stuff is interesting.  And I think the film captures the friendly, fun spirit of ham radio.