The 40 Meter Direct Conversion Receiver We Have Been Working On -- Comments Welcome

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: 

http://soldersmoke.com/DCRXLTSPICE.net  

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: 

Mr. Carlson's New Lab and Workbench

I am very glad to see that Mr. Carlson is NOT slowing down.  In fact he has built another lab and is ramping up.  FB!  

Another Trivial Electric Motor

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: 

https://soldersmoke.blogspot.com/search?q=Trivial+Electric+Motor

Thanks again Alan! 

The TinySA ULTRA: Audio out! 200 Hz Resolution! Works Up to 6 GHz! Bigger Screen! (Video)

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.   

More info here: https://www.tinysa.org/wiki/

Thanks to Karl K5KHK for alerting us to the Ultra. 

A Homebrew LM386 -- Does Anyone Want to Build It? Help Save Us All from the Indignity of ICs!

 

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.  

Here is link to a higher definition image: 

http://soldersmoke.com/homebrewLM386.jpg

 

Dave writes: 

Hello Bill -

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! 

Watching Shortwave Broadcast Stations on the TinySA Spectrum Analyser

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: 

A 1966 73 Magazine Article on a Homebrew Permeability Tuned Oscillator (PTO)

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.   

Check it out: 
https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1966/73-magazine-04-april-1966.pdf

Page 30.  

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. 

The 2Q -- A Homebrew Solid-State Drake 2-B from 1967

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.   

https://worldradiohistory.com/Archive-DX/73-magazine/73-magazine-1967/73-magazine-10-october-1967.pdf

Page 8

SDR Direct Sampling: The End of Homebrewing (as we know it)

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. 

Amazingly Cool MONTV Video on Direct Conversion Receivers with Glue Stick PTOs

I think this is one of Nick's best videos. And he has made a lot of good ones. 

https://www.youtube.com/watch?v=xl9dnajkYO0

This is a really excellent description of how a Direct Conversion receiver works. But more importantly Nick really captures the joy of building one of these receivers using discrete, analog components, including a Permeability Tuned Oscillator made from our beloved Glue Sticks.  

Extra mojo comes in the form of a mixer designed by Pete Juliano using J310s to simulate a 40673 dual gate MOSFET.  Fantastic.  Icing on the cake comes from a W8DIZ AF amp out of SPRAT magazine. 

There is a grand finale.  I won't spoil it.  Watch the video. Suffice it to say that Farhan would be pleased with this.  

Great stuff.  Thanks Nick!  

A Treasure Trove of Permeability Tuned Oscillator (PTO) Info and Links (Plus Info on Direct Conversion Receivers)

There is really great info on this page, and even more in the links at the bottom of it.  While the page is about PTOs, the links often discuss their use in Direct Conversion Receivers. I really liked the Tin Ear receiver.   And it was great to again come across the work of Alan Yates VK2ZAY.   Alan very admirably admits that laziness caused him to use an LM386 audio amplifier in place of a more virtuous discrete transistor design. 

 https://qrpbuilder.com/pto_mechanism

I bought one of the qrpbuilder PTO kits and I will soon put it together.  I have been having good results with a Glue Stick PTO and with a brass screw PTO form designed by Farhan and 3D printed for me by Dean KK4DAS. 

LET'S GO PTO! 

Dhaka Jack Moves to France

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

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

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!  

73  Bill