Just go to http://soldersmoke.com. On that archive page, just click on the blue hyperlinks and your audio player should play that episode.
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Obviously there is a lot here that it applicable to ham radio. Feynman admonished us to try to deeply understand what we are doing. Do we risk cargo culting when we make use of gear that we really don't know anything about? Or when we use a chip that we don't really understand? (I'm looking at you, Si5351.)
I guess we can't really understand some of this stuff as deeply as Feynman would like -- can anyone describe the signal flow in a CPU chip? I don't think so. And Feynman would be the first to admit that no one really understands quantum mechanics. Still, as the author notes, we should be cognizant of the gaps in our understanding. For there dragons lie. Or opportunities to learn. The comments on the Hack-A-Day post are mostly pretty good.
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.
This LTSpice model just shows two diode ring mixers. The transmitter is on the top, the receiver is on the bottom. The transmitter has RF at 7100 kHz at L1 and audio at 1 kHz at R1. The receiver has the VFO at 7100.001 L7, DSB from the transmitter at L12 with audio appearing at R4. It is instructive to watch the output as you move the VFO frequency. If you move the VFO freq away from the transmit carrier osc frequency you will see the distortion. Here is the netlist for the 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.
Above is the screenshot of the LTSpice model of the 40 meter Direct Conversion receiver that Dean KK4DAS and I have been working on. I will post a larger scale version of the picture below. Click on the images for a better view. Comments welcome. Please let us know if you find any errors or mistakes. Realize that we wanted to keep this all simple, discrete, and entirely analog.
Here (I hope!) is the net list for the LTSpice model:
First, one of the surprising things about the LTSpice model: IT IS ALIVE! I never had a VFO or PTO actually turn on for me in LTSpice. This one did! So I just connected the PTO to the Mixer and the receiver works in LTSpice. I just put an RF signal at the receiver input, and you can see the resulting AF across the 8 ohm resistor at the audio amp output. I was even able to calculate the precise frequency of the PTO: 7078 kHz. As in the real world, in an effort to stabilize the frequency, I changed the capacitors to NP0 in LTSpice. Very cool. Dean joked that all we need is a way to get RF in and audio out and we will have made an SDR receiver.
About the receiver:
-- Four stages that will be built by students Manhattan-style on four copper clad boards: Bandpass filter, diode ring mixer, Permeability Tuned Oscillator (PTO), AF Amplifier.
-- The bandpass filter is a simple dual-tuned circuit device based on the info on the QRP Labs site. (Thanks Hans!) We out a 10k pot as an RF gain control between the antenna and the filter.
-- The mixer is a standard diode ring. We included a diplexer at the output using a circuit from the famous W7EL Optimized transceiver. (Thanks Roy!)
-- The Permeability Tuned Oscillator is a very simple and very stable Colpitts design developed by Farhan VU2ESE. We added a simple FET buffer using the circuit in Farhan's Daylight Again rig. (Thanks Farhan!)
-- The AF amp is a very simple three transistor amplifier based loosely on designs from Forrest Mims and from the Herring Aid 5 receiver. Both these designs use just two stages -- we added a third and put an AF gain pot between the first and the second stages. There is an impedance mismatch between the diode ring and the AF amp, but we found that most of the proposed solutions were more trouble than they were worth, so we left it as is.
--Thanks to Wes W7ZOI for his November 1968 QST article on the solid-state DC receiver. Wes's article inspired our efforts.
Dean and I have both built these receivers. They work very well. Dean has even decoded FT-8 with his. We used Radio Marti at 7355 kHz to test for AM breakthrough -- with the diode ring, the diplexer, and the RF gain control we were able to bring the AM breakthrough down to acceptable levels. You can see many videos of my receiver in action over on my YouTube channel: (355) SolderSmoke - YouTube
Here is a larger image of the schematic (click for a full view):
And here is a nicer schematic done by our friend Walter KA4KXX:
The above video popped up on the BBC channel a few days ago. Three cheers for the Beeb for doing this, but I'd like to point out that we have been building Trivial Electric Motors for at least 16 years. We were inspired by Alan Yates VK2ZAY W7ZAY.
Below is a video from 2006:
And there are several links (and a video) about Alan and the Trivial Electric Motor here:
The improved resolution could be useful -- we may now be able to see the sidebands coming out of a mixer that is producing AF out (as in a DC receiver).
The bigger screen is nice.
Looks like Dean and I will not have to modify our TinySAs for audio out. We will just upgrade to Ultra so we can listen in style to Vatican Radio and Radio Marti.
I recently commented that I was building a discrete LM386 for a Direct Conversion receiver. I think I was exaggerating my project. In fact I just built a very ordinary push-pull amplifier using a 2N3904 and a 2N3906 as a complementary pair AF amplifier. This is part of an LM386, but there is much more inside that little chip. Dave went much further. His schematic is above.
Please find enclosed the LT spice circuit for the small audio amp that I developed. I have included numerous notes on the schematic as to component function, suggested values to tweak, etc.
My intention was to come up with a relatively low parts-count design while adhering to the classic three-stage topology [diff pair / voltage amp / voltage follower] that has been used as the basis for so many audio power amplifiers for decades. I'm sure some of the parts could be eliminated at the risk of possible stability issues, but a lot of that also depends on proper layout, length of speaker leads, speaker load impedance, etc.
In any case I thought it would be great if someone wanted to build up this design to see if it works in real life or whether any serious mods are needed to get it to behave (I have no illusion that the spice models are entirely accurate, nor is my analysis thorough).
It should be capable of at least 500mW into an 8 ohm load at 9VDC supply, and over a watt at 12VDC. It should handle a 4 ohm load although at reduced voltage swing on the output, and with increased output transistor heat dissipation requirements.
The output stage is a complimentary compound ("sziklai") pair which should, in theory, maximize voltage swing from a low supply voltage (as compared to the more traditional complimentary Darlington configuration.
The bias current is set by a fixed pair of resistors and could be quite different from the simulated value, so I would be careful on initial power-up to monitor the quiescent current draw. I would guess it need not be more than a few mA or so, and definitely less than 40mA or so. This can be made adjustable if desired.
Regards,
Dave
-----------------
So, does anyone out there want to give this a try? You could be helping to save generations of homebrewers from the indignity of using an integrated circuit!
November 18, 2022 1244 UTC. I was using a TinySA spectrum analyzer to look at noise levels on the 40 meter ham radio band. I also wanted to take a look slightly above the band (in frequency) to see Radio Marti at 7355 kHz. As I was doing this I remembered that Vatican Radio was on the air at 7305 kHz from 1230 UTC to 1245 UTC. So was just going to catch the last moments of that day's transmissions. Sure enough, I caught it, and watched it disappear from the TinySA screen. See the video above.
Radio Marti continued on. In the morning we can hear the rooster recordings from that station. We are using it to test how well our homebrew Direct Conversion receivers avoid AM detection. In the video I mistakenly said these two transmitters were on the air with 250 megawatts. The correct power is 250 kilowatts. Both transmit from Greenville NC. I think the signal from Vatican Radio is stronger here because they are using a different antenna pattern -- Radio Marti is aimed at Cuba.
This reminds me of a cool project I have not yet done: modifying the TinySA to allow the user to listen to the station: https://soldersmoke.blogspot.com/2021/10/how-to-listen-with-your-tinysa.html I notice that Dean KK4DAS (my colleague in DC receiver design) was the only commenter on the blog post describing the TinySA mod. TRGHS. We need to to do this.
Here are the reports showing when Vatican Radio and Radio Marti were on the air on November 18, 2022:
In April 1966, Lewis Fitch W4VRV of Columbus, Ohio built a Permeability Tuned Oscillator. It is remarkably similar to the devices we are building today.
Lewis opened his article with this:
Clearly, this guy was one of us!
His article is filled with good practical advice on VFO construction, with a special focus on PTOs. I was intrigued by the way his PTO mechanism allowed for the use of a reduction drive. This would help us avoid the indignity of attaching a digital frequency counter to such a quintessentially analog device.
Thanks to Michael (VE2BVW ?) for suggesting that I dig up some old 73 Magazine articles on PTOs. A quick search revealed that there weren't many. If anyone out there knows of good PTO articles in the ham or EE literature, please let me know.
This is really an amazing project. Way back in 1967 (that's 55 years ago) John Aggers W5ETT of Ponca City, Oklahoma decided to homebrew a solid state version of our beloved Drake 2-B receiver. Triple conversion. No crystal filters. Twenty two discrete transistors and no ICs. Tuned circuits at 50 kHz to provide most of the selectivity. And he did it. Just look at the picture above. It even LOOKS like a Drake 2-B.
I sent this to our friend Dale Parfitt, W4OP who more recently built a receiver like this. He too was amazed by this project.
The article by John Aggers is very clear and provides a lot of good information on how he designed and built this receiver using the technology of 1967 and junk box parts. I was struck by the lack of diode ring mixers. And I was somewhat taken aback by his use of plug-in socketed transistors. The AF amplifier is our still-familiar transformer-less push-pull complementary pair design. John did a wonderful job on the mechanical tuning and slide rule mechanism.
Three cheers for John Aggers W5ETT. This article is a reminder of the great benefit to the hobby of writing up a project and putting out there in the world. Here we are, more than half a century later, reading John's article and learning from it. FB OM.
I sometimes hear hams claim that our efforts to build simple direct conversion receivers are "very relevant" to modern technology and are "directly applicable" to today's communications techniques. These hams will say that direct conversion receivers are at the heart of modern rigs.
That's a nice thought, and it might have been true in the past, but I don't think it is true anymore.
I think the future is what you see written on the black box (!) that encloses the receiver in the above video: "DIRECT SAMPLING RECEIVER." In the recent past we did have two direct conversion receivers in the front end of SDR receiving systems. These receivers produced I and Q signals that were fed into the computer (often via the sound card). That was nice.
But the writing has been on the wall for a long time. There is no longer a need for all that direct conversion and I and Q. Just put a fast Analog-to-Digital converter chip at the front end, convert the entire HF spectrum to a digital stream, and send that stream to your computer. Or to another part of your "rig." As in the ubiquitous 7300.
I don't mean to be a Luddite here. That big waterfall is very nice. The receiver sounds great. But I am a homebrewer and I prefer to build my own gear. Ordering this black box on my phone, having it delivered by Bezos to my front step, and then updating the driver, is not what I consider homebrew radio.
A couple of things I spotted: The Si5351 chip in the box -- at least one part was recognizable. And the completely vertical skirts on all the SSB signals -- lots of 7300s out there.
Hey, to each his own, YMMV, whatever floats your boat. Just don't kid yourself into thinking that our beloved DC receivers are still somehow being used in these modern black boxes.
"SolderSmoke -- Global Adventures in Wireless Electronics" is now available as an e-book for Amazon's Kindle.
Here's the site:
http://www.amazon.com/dp/B004V9FIVW
Bill's OTHER Book (Warning: Not About Radio)
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