Just go to http://soldersmoke.com. On that archive page, just click on the blue hyperlinks and your audio player should play that episode.
http://soldersmoke.com
This 1971 training film is pretty good. I like how they break the RF circuitry into just four components, then describe the AM receiver stage by stage. The way they handle diode (envelope) detection is exactly right. But their description of how mixing moves the incoming signal from the broadcast band to the IF is overly simple, and sort of just repeats the hetrodyne story from music. Real mixing is, of course, more complicated than that, but too complicated for a 15 minute film.
In this video, Nick M0NTV takes on a hot topic in ham radio homebrewing: The matching of diodes in diode ring mixers. How should the matching be done and -- more controversially -- is this matching necessary?
I won't spoil it for you by giving the answer. Watch Nick's video to find out if it matters. (But a hint appears below.)
I think it is great that Nick has taken the trouble to look carefully at this issue, and has found info that will be of great use to homebrewers. And I really liked Nick's response to the fellow who suggested just going out and buying a commercial diode ring: Nick replied that he homebrews because he likes to, and because he wants to know how these circuits work. FB Nick.
I was also pleased that Nick gave some much warranted recognition to Pete Juliano for his idea regarding the placement of a trim pot on a diode ring. This idea made it into the Experimental Methods in RF Design book (under Pete's old call: W6JFR). Page 6.56.
Wonderful progress from Rick N3FJZ. Having completed the PTO, Rick went on to build the diode ring mixer and diplexer. (See video above.) He then connected these to boards to a band pass filter and an AF amp from previous project. And wow, the resulting receiver sounds really great! I think it sounds better than the ones Dean and I have built.
-- It will be interesting to see how Rick's receiver sounds with the very simple AF amplifier that we are using.
-- We need to get Rick a coil form for the PTO variable inductor! And getting him the PTO form will allow him to dispense with the varactor circuit (which, I must say, is pretty cool).
-- In the PTO, for the coupling capacitor (C15) , I now have a 260 pF NP0 cap. But your .1 uF ceramic disc seems to be doing just fin. So maybe we don't need the NP0.
-- I'm glad that Rick grounded the brass screw. Without that, the hand capacitance effect is bad.
-- I think results will be much the same with the dual tuned circuit bandpass filter that we use. But it will be interesting to see if Rick's triple tuned circuit helps with AM breakthrough (I think I heard Radio Marti when Rick tuned to the top of the band).
-- I haven't had a hum problem and I don't think Dean has either. Dean is running his on two 9V batteries. Maybe that would take care of the hum. Our PTOs are completely Al Fresco!
When "direct conversion" receivers came along in 1968 (the concept existed before, just not the name), it was to build simple receivers. They took over from regens (which of course for the purpose of CW and SSB, were "direct conversion"), and kind of bumped simple superheterodyne receivers out of the magazines.
And they were easy to build, so long as the meaning of the dots were standard, but good performance was elusive. Endless articles about better mixers or more front end selectivity, and still the same basic results The Heathkit HW-7 comes along, and endless mods to that, but still no perfection.
Slowly the move was back to simple superhets, especially with some of the early seventies ICs intended for radio, and then ladder filters came along (actually they came early at least by 1974 from the UK and/or France, but while they got mention in North America early-ish, it took some years before the KVG filters were pushed aside and ladder filters got the spotlight).
And then wham, in the mid-eighties someone caught on. The problem with direct conversion receivers wasn't the mixer (well not once it was a balanced mixer) or lack of front-end selectivity, it was the matter of properly terminating the mixer. The problems that had been there all along were gone. And direct conversion receivers started their climb to being complicated receivers.
I guess it was that receiver by Gary Breed in QST circa 1986 with diode balanced mixers and termination that changed things. A new concept, but not really, I remember an article in QST in 1974 where a DBM diode mixer for VHF was properly terminated, and yet the concept went no further until a decade later.
Actually, I think there is a tiny bit about mixer termination in "Solid State Design for the Radio Amateur" but it never went so far as to say "this is what we need".
Or perhaps that tiny transceiver by Roy Llewellyn in QST was the first, I cant' remember. It certainly used a diode mixer with termination for the receiver.
And that set the stage for Rick Campbell's various receivers, all counting on termination of the mixer.
The ideas can often be there, but not applied because technology doesn't allow it yet, or just not looking that far beyond this month's construction article.
Most of us grew up with the above diagram of how a receiver detects (demodulates) an AM signal. Here is how they say it works:
-- Because of the way the sidebands and the carrier in the transmitted signal interact, we end up with a signal whose "envelope" matches the frequency of modulation. And we just need one side of the envelope.
-- We used a simple diode to rectify the incoming signal.
-- A simple filter gets rid of the RF.
-- We pass the resulting signal through a capacitor and we get audio, which we listen to.
REASONS FOR SCEPTICISM
But recently, a member of my local radio club has questioned this explanation of AM detection. He maintained that "envelope detection" is not real, and that was actually happening was "square law" mixing. I guess there are reasons for skepticism about the envelope detection explanation: The envelope detection explanation does seem very (perhaps overly) simple. This does sound a bit like the kind of "dumbed down" explanation that is sometimes used to explain complex topics (like mixing). Envelope detection does seem consistent with the incorrect insistence from early AMers that "sidebands don't exist." (Of course, they do exist.) All the other detectors we use are really just mixers. We mix a local oscillator the incoming signal to produce audio. Envelope detection (as described in the diagram above) seems oddly different.
Denial of envelope detection can even be found in the ARRL handbook: On page 15.9 of the 2002 edition we find this: "That a diode demodulates an AM signal by allowing its carrier to multiply with its sidebands may jar those long accustomed to seeing diode detection ascribed merely to 'rectification.' But a diode is certainly non-linear. It passes current only in one direction and its output is (within limits) proportional to the square of its input voltage. These non-linearities allow it to multiply."
ISN'T THIS REALLY JUST MIXING, WITH THE CARRIER AS THE LO?
It is, I think, tempting to say -- as the ARRL and my fellow club member do -- that what really happens is that the AM signal's carrier becomes the substitute for the VFO signal in other mixers. Using the non-linearity of the square law portion of the diode's characteristic curve, the sidebands mix with the carrier and -- voila! -- get audio. In this view there is no need for the rectification-based explanation provided above.
But I don't think this "diode as a mixer, not a rectifier" explanation works:
In all of the mixers we work with, the LO (or VFO or PTO) does one of two things:
-- In non-switching mixers it moves the amplifier up and down along the non-linear characteristic curve of the device. This means the operating point of the device is changing as the LO moves through its cycle. A much weaker RF signal then moves through the device, facing a shifting operating point whose shift is set by the LO. This produces the complex repeating periodic wave that contains the sum and difference frequencies.
-- In a switching mixer, the device that passes the RF is turned on and off. This is extreme non-linearity. But here is the key: The device is being turned on and off AT THE FREQUENCY OF THE LO. The LO is turning it on and off. The RF is being chopped up at the rate of the LO. This is what produces the complex repeating wave that contains the sum and difference frequencies.
Neither of these things happen in the diode we are discussing. If you try to look at the diode as a non-switching mixer, well, the operating point would be set not by the carrier serving as the LO but by the envelope consisting of the carrier and the sidebands. And if you try to look at is as a switching mixer you see that the switching is being controlled not by the LO but by the envelope formed by the carrier and the sidebands.
Also, this "diode as a mixer" explanation would require the diode to be non-linear. That is the key requirement for mixing. I suppose you could make a good case for the non-linearity of solid state diodes, but the old vacuum tube diodes were quite linear. The rectifying diode mixer model goes back to vacuum tube days. The "diode as rectifier" model worked then. With tubes operating on the linear portion of the curve, the diodes were not -- could not -- have been working as mixers. We have just substituted solid state diodes for the tubes. The increased non-linearity of the solid state diodes does introduce more distortion, but the "detection by rectification" explanation remains valid.
Even in the "square law" region (see diagram below) an AM signal would not really be mixed in the same way as signals are mixed in a product detector. Even in the square law region, the diode would be responding to the envelope. Indeed, the Amateur Radio Encyclopedia defines "Square Law Detector" as "a form of envelope detector." And even in the square law region, the incoming signal would be rectified. It would be moving above and below zero, and only one side of this waveform would be making it through the diode. Indeed the crystal radio experts discuss "rectification in the square law region" (http://www.crystal-radio.eu/endiodes.htm ) So even in the square law region, this diode is a rectifying envelope detector.
Here is what I think is the best proof that the "envelope detection" explanation is real: In this video, we see someone build an envelope detector in a simulator. Watch as he then traces the signals as they move through the diode, the RC filter, and the coupling capacitor. He goes through it cycle-by-cycle. You can clearly see how the rectification of the AM leads to envelope detection.
The rectifying envelope detection model goes way back in radio history, back to when authors did not shy away from complex technical explanations. Terman knew how mixers worked, and his 1943 "Radio Engineers Handbook" went to 1019 pages. Terman presented it as a rectification-based detection of the envelope. I think envelope detection is real, and that Dr. Terman was right.
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Some links that might help:
Analog Devices has a very good, rigorous site showing how envelope detectors work:
The crystal radio guys have a good take on square law detection (note, they just see it as rectification, but on a lower, more parabolic portion of the curve): http://www.crystal-radio.eu/endiodes.htm
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.
Jack Welch AI4SV has been an important member of the SolderSmoke community for many years. I remember fondly our Straight Key Night CW contact in which he told me that my HT-37 had "presence" even on CW. His thoughtful (!) piece on time crystals was also quite memorable. Jack has finally settled down (a bit) after a string of foreign assignments. He has landed happily in France, in a villa, on a vineyard, surrounded by wild boar and hunters. FB OM.
Hi Bill & Pete,
I've packed up the shack and moved from Cyprus to France, so no more 5B4APL. To obtain a French callsign, you have to submit proof that you've lived in France for three months, so I'm F/AI4SV until December and then we'll see.
I'm not sure how long we will be here, but probably a few years at least. Since we know next to nothing about French real estate, we are renting for the first couple years -- a château on the outskirts of Bordeaux. Before you think that I've come down with delusions of grandeur, I should point out that in that area, château means an old, stone house that is hard to heat in the winter -- and particularly difficult to run wiring around. Antennas and grounding are going to be particularly challenging. The selling point for the house was not so much my hobby as its location in wine country. In fact, there is a Sauterne my house's name on it (although I have nothing to do with production of the wine, that's in professional hands).
Back in the early days of Soldersmoke, Bill used to occasionally mention the dreaded Italian wild boar, the cinghiale. I didn't think that would ever be terribly relevant to me, but it is. A couple days after arriving in the Bordeaux suburbs, a sanglier (French cousin of the cinghiale) strolled across a road as I came around a bend. We almost had a month-long supply of bacon, but I managed to steer around him.
Since it will be a while before all our belongings arrive and even longer to set up a proper station, I have focused on operating QRP in the field and activating SOTA summits. That has gone well, but I aborted my most recent attempt when I ran into a bunch of orange-clad rifle-toting hunters who were combing the mountain in search of sanglier. Apparently it's a big thing here. I decided to survive to activate the peak on another day.
Finally, I have attached a journal article, which at first glance doesn't seem to have a lot to do with radio, but kind of does. It turns out that both the human ear and violins have non-linear characteristics that cause them to function as audio frequency mixers. Looking through the article, you'll find some familiar looking formulas about mixing products, harmonics and resonance. If Bill wants to get away from ICs, perhaps his next rig could include a 17th Century Italian violin as a mixing stage.
Cheers & 73,
Jack F/AI4SV
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Hello Jack:
Great to hear from you. Wow, France! You are rivaling my string of nice-to-go assignments. FB OM. Have fun.
Yes, the Cingales. Hunting season was always a bit of an uneasy time. We used to dress the kids up in reflective vests. One time we found a very drunk Italian hunter wandering around with a shotgun (that was kind of scary). We would know when hunting season started by the sound of gunfire in the morning. Kind of reminded me of other places!
In retirement I have gotten back into VWS. We are having a lot of fun. Just yesterday 30 students at the Thomas Jefferson High School got their Technician licenses. They will soon build Direct Conversion receivers.
As for mixing, what you sent reminded me of my early confusion on this subject. In the SS book I describe the "Terzo Suono" -- it is really just an additive heterodyne. I confused it
with a true mixing product. But it was an educational confusion.
Please keep in touch and let us know how things are going at the Villa!
Here is another update on Direct Conversion receiver construction. In Northern Virginia we get very strong signals from the Radio Marti transmitter in Greenville NC. During the morning hours it is just above the 40 meter band at 7335 kHz. In the evening it is a bit higher in frequency at 7435 KHz. (in the video above I mistakenly give the morning frequency, when in fact they were on the higher evening frequency). In either case, Radio Marti has been a big source of unwanted AM breakthrough in our simple DC receivers. It now serves as something of a test of our bandpass filters and mixers.
The following morning, I tested the mixer with Radio Marti (in fact) on 7335 kHz. By adjusting the VFO signal input to the minimum value needed to turn on the diodes, I was able to bring Radio Marti AM breakthrough to minimal levels. But I could still hear it (weakly) in the background. Putting a very simple diplexer at the audio output of the mixer (just a .1uF capacitor in series with a 47 ohm resistor to ground) helped a lot.
I could also hear break through from Spanish-language broadcasts from Vatican Radio on 7305 kHz (using the 250 kW transmitter in Greenville NC) from 11:30-11:45. Perhaps most surprisingly, I was also getting AM breakthrough from 40 meter FT8!
Here is a short video showing the simple two-diode mixer in action during the morning hours:
I also tried out the more common two diode mixer with trifilar toroid. (In this one, the VFO turns both diodes on, then turns both of them off). The results were similar to what I got with the other two diode mixer.
We are trying to develop four circuits -- bandpass filter, mixer, variable frequency oscillator, and audio amplifier -- that will be simple enough for construction by high school students, but not so simple as to compromise performance. We want the receiver to work well.
So far, my conclusion is that the best results come from the diode rig mixer with two trifilar toroids. Here is a short video showing the diode ring in action on the morning of November 9, 2022:
Take a look at the simple little mixer above. I think I first saw it in SPRAT. Thinking that it was really just a simplified version of the two diode Doug DeMaw mixer that I had been using for years, I couple of years ago I built it into a little Direct Conversion receiver. It worked great. But later, I began to have doubts about it. In the words of young James Clerk Maxwell, I started to wonder about "the particular go of it."
You see, the way the DeMaw mixer is set up, both of the diodes are simultaneously on and off. This has the effect of "chopping up" the incoming RF at a rate set by the VFO frequency. Boom. Fournier. Mixing. Great.
But look at the mixer at the top of this post. Here the VFO signal is coming in on the wiper of the 1k pot. The same signal is hitting both diodes at the same time. The diodes are not being fed differentially. So D1 and D2 are NOT both simultaneously tuning on and off. Instead, when the wiper goes positive, D2 turns on while D1 is off. On negative swings of the voltage at the wiper, D1 turns on while D2 is off. For me, this made it a "mystery mixer."
This reminded me of the sub-harmonic DC receiver I built earlier in the year: The VFO runs at half the operating frequency, but the diodes are set up to switch on and sample the RF TWICE each VFO cycle. This is the equivalent of having the VFO at the operating frequency.
Could it be that this was just a sub-harmonic mixer with the VFO at the operating frequency? (I should note that Doug DeMaw published a design that actually made this mistake. See: https://soldersmoke.blogspot.com/2011/07/doug-demay-and-polyakov.html ) I knew that this would sort of work, but it would not work very well. And the mystery mixer seemed to work very well. Hmmm.
Our beloved book, Solid State Design for the Radio Amateur (SSDRA) has an explanation of this circuit on page 74. But this explanation didn't seen to work for me. Check it out. YMMV.
Bottom line: I still couldn't figure this circuit out, so left it alone for while.
The other day I woke up and looked at it with fresh eyes. Suddenly it hit me. Although the VFO was hitting the diodes in the same non-differential way as is done in the sub-harmonic mixer, the RF (signal) is entering the mixer in a differential way. This means that the two diodes are taking turns sampling the upper side of L2, then bottom side of L2, via L1 and L2. This results in a complex repeating waveform that is similar to that of diode ring mixer. Within that complex repeating waveform, there are sum and difference frequencies. I did some noodling on this:
The key difference between this mixer and the sub-harmonic mixer is the way L2 is positioned: In the sub-harmonic mixer, there is no differential feed of the RF. Both diodes get the same polarity of RF. The VFO switches on D1, then D2. The RF is sampled at twice the VFO frequency. But in the mystery mixer that had me scratching my head, the RF is fed to the diodes in differential form. So while the diodes here are -- as in the sub-harmonic mixer -- being switched on and off sequentially, they are taking turns sampling the top and the bottom of L2. That provides the complex repeating waveform that we need to get the sum and difference frequencies. In a DC receiver the difference frequency is audio.
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?
This would be good mixer for a school project. It is simpler than a mixer with a tri-filar toroid.
Here is a progress report on Direct Conversion Receiver developments.
Dual Tuned Circuit, Diode Ring with Diplexer, PTO VFO from Farhan's Daylight rig, two stage 49 db BJT AF amp with a transformer.
It works very well.
I discuss: Shielding of the VFO -- necessary or not?
Why brass in the PTO?
Do we really need 100db in a receiver, especially with ear buds?
Sourcing the AF amplifier's transformer.
Using W7EL's diplexer. (I think it has solved my Radio Marti breakthrough problem).
Developing a DC RX circuit that can be built by students.
I end with a bandsweep of 40 meters that includes CW, FT-8, SSB, and AM
A deeper look into how the Diode Ring detector works: "the particular go of it."
Here I rely on a wonderful diagram from the RSGB. This diagram clearly shows how
in this circuit, the switching action of the diodes -- controlled by the VFO -- results in sum and difference frequencies at the output. This is amazingly illuminating.
I then tried to build this actual circuit. It works, but I am also getting a lot of AM breakthrough from a local AM station (WFAX) and Radio Marti at 7335 kHz. I will try again.
In any case, the diagram shows how the diode ring does its thing!
I need to beef up the Band Pass Filter.
I tuned around a bit on 40 meters -- you can listen.
Students at a local high school have been trying to get the DC receiver I loaned to them going -- they may be confused by the intricacies of SSB tuning. I will see them next week.
Here is the RSGB diagram that reveals the secrets of the Diode Ring. (Now that could be the title of a book or movie. I claim the rights to that!) Click on the image for a better view.
I took a new look at the receiver, did some measurements, and decided to take out the RF amplifier and the AF pre-amp. The receiver works fine without them, another indication that they were unnecessary.
Line-up is now: Band-pass filter, Mixer, VFO, AF amp. I think there is only 49 db of gain in the entire receiver, but it is useable with an un-amplified speaker, and is a bit too loud on ear buds.
Not bad for just 4 transistors. And I think we could do this with just 3 (no need for the VFO buffer).
It is inhaling nicely but some improvements are still pending. Click on the video above to see and listen to the bandsweep done on 40 this morning.
-- The front end consists of capacitive divider input impedance matching circuit, followed by one LC circuit and an FET RF amp.
-- The VFO is a super-simple Colpitts design by Farhan. The two feedback capacitors do double duty in the LC tan circuit.
-- I am using an old variable capacitor instead of the PTOs that we have been experimenting with.
-- The mixer is singly balanced using one trifilar toroid and two diodes. We have found out that even with these three simple devices, there is significant variation in how people connect them to VFO, RF in and audio out. I think we have found the best way to do this: Be sure to put the VFO on the primary of the transformer, and let this signal turn the diodes on and off.
-- For the AF amplification, I have one FET, followed by two BJTs. I have a small audio transformer between the speaker and the final AF amp. There is plenty of audio.
You may wonder why, after all the SSB superhet transceivers, I am building a simple Direct Conversion receiver. Well, we hope to help a bunch of high school kids build one, so we need to be really familiar with how it works. And I find that as simple as it is, there is still a lot to learn in a project like this.
There I was, minding my own business, when suddenly I was dragged into the construction of Direct Conversion receivers.
Here is a video about my latest effort. But I feel the urge for more simplification -- I may go back to the seminal DC receiver designed by Wes W7ZOI and presented in the November 1968 issue of QST. It is on page 15 here: https://worldradiohistory.com/Archive-DX/QST/60s/QST-1968-11.pdf
Michael AG5VG built a Sub-Harmonic Direct Conversion receiver. But then he took it a step further and moved it up from the 80/40 meter version that I had built, and used the same concept to run it on 20 meters using an oscillator on 40 meters (after some re-winding of the front-end coils). Using a station from Puerto Rico transmitting on 20 meters as an example, he starts out showing how well the receiver works in sub-harmonic mode (with the oscillator on 40), then quickly switches to normal Direct Conversion mode with the oscillator also on 20 (but using only one diode as the detector) -- he can still hear the Puerto Rican station in that mode. Very cool.
Good Evening Bill,
I built the Polykov and I attached a picture of it. I also used Pete's pre audio driver circuit from his jessystems.com site. Then I used an lm386 as the main audio driver. I could hear ft8 on the 40m band. Then I hooked the output of my lm386 circuit to a conventional set of computer speakers to really hear it. I am currently using an indoor wire antenna along the ground so it's certainly not optimal. Very fun build and I'll be learning more about it. When I have a better antenna system I'll hook it all up and send a video of it.
73s
MIchael
AG5VG
Bill,
I am just using a standard signal generator at 1 vpp output. The volume gets louder with every 100 millivolt I go up, but so does the noise. 0.8vpp was a little low for me so I bumped it up a bit.
The indoor antenna actually did surprising well but I'm looking forward to putting a wire up into a tree I have here. I just recently moved so I have to setup my outside antenna. I live in the San Antonio, TX area.
I am currently using three stages of audio amplification to be able to really hear it. 1st stage is Pete's pre audio driver, then an lm386, then a standard set of computer speakers.
I did plenty of playing around with it last night and the doubling function is so cool how it works. When I was around 3.538 MHz, with the variable cap tuned for the 7Mhz area, I was actually listening to 7.76Mhz, the FT8 frequency for 40 meters. I agree with You and Pete in a podcast you did a bit ago, that FT8 is great for seeing if the band is open and checking receivers with!
The next project is the art of the 3.5 - 4Mhz analog VFO and use it with the Polykov. I am very dependent on the Arduino/Si5351 pair as the code is available and easy to hook up.
Nice video, but I'm afraid it is a bit of an old-fashioned fantasy. It would be nice to think that our beloved analog mixers and direct conversion receivers still have a place in the SDR world. That may have been true a few years ago when we were using soundcard-based SDRs. But today we just put an Analog to Digital Converter at the antenna, do "Direct Sampling," create a digital stream, and sent it to the CPU for processing, right?
Sometimes we think that we can show younger people how our older tech (Direct Conversion receivers) is STILL relevant in the age of SDR radio. But I can just hear them scoffing at this notion, pointing out that I,Q-to-soundcard front ends have gone the way of the dinosaurs, and all we need now is an ADC and a CPU.
But hey, I am an HDR guy. Am I missing something here?
This is the first in a series of four podcast that include Echolink conversations with Andy ZS6ADY about old tube radios (boatanchors) in South Africa. Click on the YouTube link above to listen.
January 2, 2008 SPECIAL NEW YEAR'S EDITION AA1TJ's circuitry and poetry. Homemade tubes. Book Review "Early Radio" by Peter Jensen. The Vatican's antennas. Google Earth flight simulator. Mixer madness continues (now in LTSpice). Mars-asteroid collision? Bollywood: The BITX-20 connection. BANDSWEEP: Straight Key Night at WA6ARA. ECHO-GUEST: Andy, ZS6ADY, South African Boatanchor fan. MAILBAG: Jake N4UY(NOVA QRP), Steve G0FUE (Bath Build-a-thon), Nigel M0NDE
Our old friend "Vasily" sent in a very insightful comment about the Polyakov receiver. It was so good that it merits a blog post of its own. Here it is. Thanks Vasily!
Thanks Bill. My own experiments at HF with subharmonically pumped Schottky diode mixers show clearly that almost every mixer parameter we measure is worse than our classic balanced mixer topologies. Definitely 2LO-RF isolation was better than other unbalanced mixers without the need for a transformer.
I guess it's appealing for low-complexity receiver builders. For zero IF receivers, I like and run my LO at 1/2 RF frequency and then use a doubler -- that's a great advantage for a DC/ Zero-IF receiver and a built-in feature for the subharmonic mixer.
The SH mixer becomes quite appealing at SHF to mm-wave lengths where making a quiet, temp stable LO gets rather expensive and tricky.
Subharmonically pumped mixers can also work at odd integers if the mixer LO/RF drive is balanced and designed to produce distortion that for example, triples the LO frequency. Rohde & Schwarz had a 40.1 GHz spectrum analyzer with one --- and if the LO was 13 GHz while the RF was 39.5 GHz, this gave an IF output of 500 MHz in 1 particular circuit. Really amazing design work. Here's an interesting URL:
The SH mixer has been around for > 4 decades. The oldest SH mixer paper I've got in my library is from Schneider and Snell from 1975. I don't think they invented the SH, but this pair helped popularize it for the world and design work continues today.I've seen optical SH mixers with I/Q outputs in research papers.
Here's the abstract and citation:
Harmonically Pumped Stripline Down-Converter
M. V. Schneider, W. W. Snell Published 1 March 1975 Physics, Engineering IEEE Transactions on Microwave Theory and Techniques
A novel thin-film down-converter which is pumped at a submultiple of the local-oscillator frequency has given a conversion loss which is comparable to the performance of conventional balanced mixers. The converter consists of two stripline filters and two Schottky-barrier diodes which are shunt mounted in a strip transmission line. The conversion loss measured at a signal frequency of 3.5 GHz is 3.2 dB for a pump frequency of 1.7 GHz and 4.9 dB for a pump frequency of 0.85 GHz. The circuit looks attractive for use at millimeter-wave frequencies where stable pump sources with low FM noise are not readily available.
"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
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