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Wednesday, April 26, 2023
Retro QRP Rigs of the 1960's, 70's, and 80's -- Video by Mike WU2D
Thursday, April 13, 2023
The Franklin Oscillator: A Super-Stable VFO. Why No Attention? Why So Little Use?
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).
Tuesday, April 11, 2023
Arnie Coro: Jaguey Rig Designed in 1982, More info on the Rig
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.
Friday, December 2, 2022
But why? Why Can't I Listen to DSB (or AM) on my Direct Conversion Receiver?
I've said this before: I just seems so unfair. We just should be able to listen to DSB signals with our beautifully simple homebrew Direct Conversion receivers. I mean, building a DSB transmitter is a natural follow-on to DC receiver construction. And we are using AM shortwave broadcast stations (Radio Marti --I'm looking at you) to test our DC receivers for AM breakthrough. But when we tune these stations in, they sound, well, awful. So unfair! Why? Unfortunately it has to do with laws. Laws of physics and mathematics. Blame Fourier, not me.
Over the years there has been a lot of handwaving about this problem. From Doug DeMaw, for example:
In his "W1FB's Design Notebook," Doug wrote (p 171): "It is important to be aware that two DSSC (DSB) transmitters and two DC receivers in a single communication channel are unsatisfactory. Either one is suitable, however, when used with a station that is equipped for SSB transmissions or reception. The lack of compatibility between two DSSC (DSB) transmitters and two DC receivers results from the transmitter producing both USB and LSB energy while the DC receiver responds to or copies both sidebands at the same time."
That's correct, but for me, that explanation didn't really explain the situation. I mean we listen to AM signals all the time. They produce two sidebands, and our receivers respond to both sidebands, and the results are entirely satisfactory, right? Why can't we do this with our Direct Conversion receivers? I struggled with this question before: https://soldersmoke.blogspot.com/2015/07/peter-parker-reviews-dsb-kit-and.html You can see in that post that I was not quite sure I had the answer completely correct.
It took some discussion with a fellow Vienna Wireless Society member, and some Googling and Noodling for me to figure it out. But I think I've got it:
Imagine a station transmitting a DSB signal at 7100 kHz with a 1 kHz tone at the AF input. There will be signals at 7101 kHz and at 7099 kHz. Assume the carrier is completely suppressed.
We come along with our DC RX and try to tune in the signal.
Remember that they heart of the DC RX is a product detector, a mixer with the VFO (or PTO) running as close as we can get it to the suppressed carrier frequency (which we can't hear).
Lets assume that we can somehow get our VFO or PTO exactly on 7100 kHz. The incoming signals will mix with the VFO/PTO signal. We are looking for audio, so we will focus on the difference results and ignore the sum results of the mixing.
The difference between 7101 and 7000 is 1 kHz. Great! And the difference between 7099 and 7000 is 1 kHz also. Great again, right? We are getting the desired 1 kHz signal out of our product detector, right? So what's the problem?
Here it is: SIDEBAND INVERSION. Factoring in this part of the problem helps us see the cause of the distortion that plagues DSB-DC communication more clearly.
Remember the Hallas Rule: Whenever you subtract the modulated signal FROM the unmodulated signal, the sidebands invert. So, in this case, we are subtracting that 7099 "lower sideband" signal FROM the 7100 VFO/PTO signal. So it will invert. It will become an upper sideband signal at 1 kHz. We will have two identical 1 kHz signals at the output. Perfect right? Not so fast. Not so PERFECT really.
The perfect outcome described above assumes that our VFO/PTO signal is EXACTLY on 7100 kHz. And exactly in phase with the suppressed carrier of the transmitter. But if it is even SLIGHTLY off, you will end up with two different output frequencies, signals that will move in and out of alignment, causing a wobbling kind of rapid fade-in, fade-out distortion. You can HEAR this happening in this video by Peter Parker VK3YE, starting at 6:28:
And you can see it in this LTSpice simulation.
On paper, using simple mixer arithmetic, you can tell that it will be there. With the VFO/PTO just 1 Hz (that's ONE cycle per second) off, you will end up with outputs at 1.001 kHz and at .999 kHz. Yuck. That won't sound good. These two different frequencies will be moving in and out of alignment -- you will hear them kind of thumping against each other. And that is with a mere deviation of 1 Hz in the VFO/PTO frequency! We are scornful when the SDR guys claim to be able to detect us being "40 Hz off." And before you start wondering if it would be possible to get EXACTLY on frequency and in phase, take a look at the frequency readout on my PTO.
Now consider what would happen if the incoming signal were SSB, lets say just a tone at 7101 kHz. We'd put our VFO at around 7100 kHz and we'd hear the signal just fine. If we were off a bit we'd hear it a bit higher or lower in tone but there would be no second audio frequency coming in to cause distortion. You can hear this in the VK3YE video: When Peter switches to SINGLE Sideband receiver, the DSB signals sound fine. Because he is receiving only one of the sidebands.
The same thing happens when we try to tune in an AM station using a Direct Conversion receiver: Radio Marti sounds awful on my DC RX, but SSB stations sound great.
My Drake 2-B allows another opportunity to explore the problem. I can set the bandwidth at 3.6 kHz on the 2-B, and set the passband so that I will be getting BOTH the upper and the lower sidebands of an AM signal. With the Product Detector and the BFO on, even with the carrier at zero beat AM sounds terrible. It sounds distorted. But -- with the Product Detector and BFO still on -- if I set the 2-B's passband to only allow ONE of the sidebands through, I can zero beat the carrier by ear, and the audio sounds fine.
There are solutions to this problem: If you REALLY want to listen to DSB with a DC receiver, build yourself a synchronous detector that gets the your receivers VFO EXACTLY on frequency and in phase with the transmitter's oscillator. But the synchronizing circuitry will be far more complex than the rest of the DC receiver.
For AM, you could just use a different kind of detector. That will be the subject of an upcoming blog post.
Please let me know if you think I've gotten any of this wrong. I'm not an expert -- I'm just a ham trying to understand the circuitry.
Wednesday, November 2, 2022
Understanding a Very Simple Two-Diode Mixer
What do you guys think? Do I have this right? How would you characterize this mixer: Is it multiplying by 1 and 0? Or is it multiplying by 1 and -1?
Friday, September 16, 2022
Fixing Up An Old Homebrew Rig -- Barebones Superhet and VXO 6 Watter
Tuesday, July 19, 2022
Putting a Real LC VFO in My Ceramic-Resonator, Direct Conversion 40 Meter Receiver. LC JOVO! (Video)
The VFO circuit comes largely from W1FB's Design Notebook page 36. I followed most of the conventional tribal wisdom on VFOs: NP0 caps, often many of them in parallel. Air core coil (in my case wound on a cardboard coat hanger tube).
Monday, February 7, 2022
SolderSmoke Podcast #235 NE-602, Azores Rig, Spur Problems, SSB Rigs, Peashooter, HB Filters, MAILBAG
SolderSmoke Podcast #235 is available for download:
One contact on uBITX. More SW listening.
Repaired my Chrome Book in Santo Domingo!
Christmas Present for All: James Web Space Telescope launch
Tuesday, December 7, 2021
Junk Box Sideband from the Azores (2004 QST Article)
Tuesday, November 30, 2021
Putting a Barebones Superhet on 17 Meters with an NE602 Converter (Video)
Saturday, November 6, 2021
M0NTV's "Crystal Filters for the Fearful" (video)
Tuesday, July 27, 2021
QST Recognized Error on Sideband Inversion, But Continued to Make the Same Mistake
Monday, July 26, 2021
QST Repeatedly Got Sideband Inversion Wrong
It kind of pains me to do this. These articles are from a long time ago, and the author is an esteemed Silent Key, but the myth about the origins of the USB/LSB convention is still out there, and as a homebrewer of SSB gear I feel obligated to point out these examples of the error that that myth is based on.
Last Friday, Pete WB9FLW and I were talking about homebrewing SSB rigs. I recommended a series of QST articles by Doug DeMaw. "Beginner's Bench: The Principles and Building of SSB Gear" started in QST in September 1985. There were at least five parts -- it continued until January 1986. (Links to the series appear below.) I hadn't looked at these articles in years, but when I did, a big mistake jumped right out at me: In the first installment, on page 19, Doug makes the same mistake that he made in his Design Notebook:
"Now comes the conversion section of our SSB generator. We must move (heterodyne) the 9-MHz SSB signal to 3.75-4.0 MHz. Our balanced mixer works just as it does in a receiver. That is, we inject the mixer with two frequencies (9 MHz and 5 MHz) to produce a sum or a difference output frequency (9 - 5 = 4 MHz, or 9 +5 = 14 MHz) If we are to generate 75 meter SSB energy, we must chose the difference frequency. We could build an 20-meter SSB transmitter by selecting the sum of the mixer frequencies. The RF amplifiers and filter (FL2) that follow would then have to be designed for 14-MHz operation. In fact, many early two-band homemade SSB transmitters were built for for 75 and 20 meters in order to use this convenient frequency arrangement. The use of upper sideband on 20 meters and lower sideband on 75 meters may be the result of this frequency arrangement (the sidebands become inverted when switching from the difference to the sum frequency.) "
Those last two sentences are incorrect. They repeat the "Myth," or the "Urban Legend" about the origins of the LSB/USB convention. Contrary to what many hams now believe, with 9 MHz filter and a 5.2 MHz BFO it takes more than just switching from sum frequency to difference frequency to invert one of the sidebands.
There are two conditions needed for sideband inversion to take place:
1) You have to be taking the difference product (DeMaw got that right)
2) The unmodulated (VFO or LO) signal must be larger than the modulated signal. (DeMaw and the ARRL obviously missed that part. Repeatedly.)
This is another way of stating the simple, accurate and useful Hallas Rule: Sideband inversion only occurs when you are subtracting the signal with modulation FROM the signal without modulation.
For DeMaw's claim to be correct, one of the SSB signals going into the balanced mixer would have to invert, and the other would have to not invert. Let's see if that happens: He has the sideband signal being generated at 9 MHz and the VFO running around 5 MHz.
9 - 5 = 4 But we are not subtracting the modulated signal FROM the unmodulated signal. SO NO INVERSION
9 + 5 = 14 We are not subtracting at all. SO NO INVERSION.