Understanding Mixers -- A Free Book Excerpt -- Enlightenment from SPRAT

https://soldersmoke.com/book.htm

     We've recently been talking about mixers, and it has become apparent to me that there is a big gap about what we mean by "understanding" mixers.  Is it enough to embrace at the trig formulas?  Or is it possible to understand these key devices at an intuitive level?   You know, as a boy, when James Clerk Maxwell was trying to understand a device, he used to ask, "What's the go of it?"  If he was dissatisfied with the answer he would ask, "But what's the particular go of it?"   

I struggled for a long time to understand mixers (the struggle continues!).  Here is an excerpt on my efforts to understand mixers from my 2009 book "SolderSmoke -- Global Adventures in Wireless Electronics."  

UNDERSTANDING:  THE MIXER

That dual gate 40673 was relatively easy to understand, but over the years I frequently became aware of the fact that I didn’t really understand what was happening in that mixer circuit.  At times I thought that I understood it, but then I’d dig a bit deeper and find that my understanding was incorrect, or at least incomplete.  Mixers are absolutely key stages in almost all amateur transmitters and receivers, so I knew that as a radical fundamentalist I’d eventually have to really understand how these circuits work.

There are many paths to confusion in this area.  You can be misled by graphical explanations and by “hand waving” verbal descriptions.  And I think that purely mathematical explanations fail to provide the kind of intuitive understanding we are looking for.  Let me describe some of the pitfalls.

When they get to mixers, some books show three nice graphs of sine waves.  They are stacked one over the other.  The top two are input signals, each of a different frequency.  The third graph is the arithmetic sum of the top two.  Moment by moment the signal strengths presented by the top two are added together, and the result is shown on the bottom. 

Soon it becomes clear that the shape of the envelope of the resulting graph is varying at a different frequency. A third frequency.  What is happening is that because the two input signals are of different  frequencies, they are periodically going in and out of phase:  one moment both signals are at a positive peak, and they reinforce each other.  Later, one is at a positive and the other is at a negative peak; here they cancel each other.  It is so simple and easy to see!  You can even count the number of cycles that this new signal is going through.  And—amazingly—the

frequency of this new signal is the arithmetic difference of the first two.  Voila!  This is il terzo suono of Giuseppe Tartini!  Suddenly it seems that you understand mixers and superhets.  You might think that you now understand how Major Armstrong hoped to convert the electrical engine noise from the German bombers down to a frequency at which he could amplify them. 

But you’d be wrong.  Sorry about that.  It’s just not that simple.

It took me a long time to realize that this explanation of mixer action was a kind of children’s fable for mixer theory, a misleading fairy tale with sufficient connection to the related field of acoustics to take on an aura of legitimacy.

The first indication that something was amiss came when I looked for the sum frequency.  I knew that mixers produce new frequencies at BOTH the difference of the inputs AND at the sum of the inputs.  The neat little three-graph presentation seemed to explain the difference frequencies, but what about the sum output?  How do we explain that output using these charts?  It took me a while to realize that you can’t.  Because this is not really the explanation of how mixers work.

In the course of writing this book (in 2007), when I got this point I was reminded that my struggle to understand mixers has been a long battle.  I was reminded of this when I turned to Google in search for insights.  Along with the many learned and highly technical articles that popped up in the search results, I found my own pleas for help going back some ten years.  Here is a typical exchange on this subject posted to sci.electonics.basic USENET group in 1997:

 

September 5, 1997  Bill Meara (wme...@erols.com) wrote:
> Here's a nagging little question that has been bothering me for some
> time:

> I have several Physics and Radio books that give very clear
> explanations of how "beat" frequencies are generated in mixer 
> circuits.  These books have nice little charts showing how the two
> waves combine to produce a third frequency that is the difference
> between the two.  Great!  Very illuminating. 

> But these same books are oddly silent on how the "sum" frequency is
> developed.  Can this frequency be explained in a similarly graphic
> manner?  Any hints? 

An excellent question.

It relies on nonlinear circuit elements and high school trigonometry
(trig) identities.  Ideal mixers have square-law or Vout = Vin^2
characteristics.  This means that if you have two signal of different
frequencies, vin = s1 + s2 where s1 = cos (2*pi*f1*t) and s2 = cos
(2*pi*f2*t), you have vin equal to the sum of two cos, which by trig
identity gives vout equal to terms of cos(f1+f1)/2 and cos(f1-f2)/2.

 

The response in this exchange is typical of what you get when you ask these kinds of “how do mixers really work” questions.  Most experts will immediately come back at you with two things: non-linear elements (like a diode) and trigonometry.  The equations seem to be saying that the sum and difference frequencies that we see coming out of mixer circuits are caused by the multiplication of the two input signals.

What?  How does that work?  At this point many of the books seemed to chicken out on providing non-mathematical explanations.  But for me, the math seemed to cry out for some explanation.  The equation seemed to be saying that some very simple devices—one diode, for example—are somehow able to take two input signals, multiply them together, and spit out new frequencies that are the arithmetic sum and differences of the two inputs.  I found myself thinking, “Diodes are good, but are they that good?  Who taught them the multiplication tables?”  And if we are seeking sum and difference frequencies, why do we eschew addition and subtraction?  Why do we use multiplication? 

The simple explanation using the three charts and Giuseppe Tartini’s Terzo Suono explanation kept putting me on the wrong path.  I kept coming across examples, mostly from acoustics, that showed two frequencies coming together this way to produce a third frequency.  There was, of course, a common experiment in the high school physics lab in which two tuning forks of slightly different frequency are brought together.  You can hear the “beat,” the difference frequency that results.  Where is the “non-linear” element in this case?  Displaying what I thought was a somewhat unquestioning acceptance of what they’d learned in engineering school, some folks told me that for this kind of beating to take place there had to be a non-linear element.  Some suggested that the mixing took place in a non-linear portion of the human ear.  Others hinted that the air itself might have non-linear qualities.

This didn’t sound right to me.  So I built a little circuit that would electrically combine two audio signals.  And I would watch the results on my oscilloscope.  There’d be no air (or ears) involved.  Sure enough, on the scope I could see the beats as the two frequencies came closer together.  There were no non-linear diodes doing multiplication.  I thought I was getting closer to understanding. 

But I wasn’t.

The problem with this kind of mixing or combining is that the resulting beat is not “extractable.”  When I first started seeing that word—extractable—in incoming e-mail messages, I didn’t really understand what it meant.  Tom Holden, VE3MEO, made it clear:  “The beat note that you hear between the two tuning forks is not a new signal—it’s just the period between the constructive and destructive interference due to the superposition or addition of the two signals… You can’t separate the beat frequency signals from the source signals because subtracting one of the source signals from the waveform leaves you with merely the other.  You can’t hear the beat without hearing both forks singing.” 

Real mixing is obviously different from this kind of terzo suono beating.  In a real mixer you want to be able to separate the new frequency from the old ones.  You want to be able to extract it so that you can better filter it and amplify it.  And you want to leave the input signals behind.

OK, back to the drawing boards. 

Back in 1999, I think I kind of came close to a limited understanding of the phenomenon.  Here is another USENET exchange:

 

To:  ianpur...@integritynet.com.au

Ian:  I really like your pages. 

I have a question about the theory behind the mixer stage:  As was done
on your page, explanations of this stage are usually limited to stating
that the active device is operated in the non-linear portion of the
curve and this results in its operation as a mixer. Given that this is
the heart of superheterodyne operation, I've always wished that the
explanations would go a bit deeper.

We recently had a very lengthy and interesting discussion on this in the
sci.electronics.basics newsgroup.  I came to some conclusions about
mixer operation that (I hope) may provide the kind of explanation that I
think is needed for beginners and non-engineers to understand mixers:

--When we say that the active device is operated in the non-linear
portion of its operating curve, we are really saying that we are biasing
it so that each of the two input signals will—in effect—vary the
amount of amplification that the other receives from the device.

-- When this happens, the output of the device is a waveform that
contains sum and difference frequencies.  If we ask WHY this happens, we
have to be satisfied with an answer that points to mathematics: If you
combine two signals in the manner described above, mathematical
principles dictate that the resulting waveform contains sum and
difference frequencies.

Please let me know what you think of this explanation—does it make
sense, is it consistent with accepted theory?

Again, thanks for the great web site.  I will be visiting often. 

73  Bill  N2CQR

 

Bill Meara  <wme...@erols.com> wrote: receives from the device.

Excellent.  This is a very good definition of non-linearity.  Sometimes
"amplification" isn't involved, as when we use a diode mixer, but in that
case each signal varies the amount of _attenuation_ that the other
receives from the device.  I like it, and I think that the explanation is about as useful as any.

For a mathematical analysis, you might want to consider that a
non-linear mixer actually _multiplies_ the two signals, rather than adding
them.  I think I've got this right, anyway...

M Kinsler

So, back in 1999 I seem to have sort of accepted that if you take two signals of different frequency and feed them into a non-linear device, the math tells us that in the output you will get sum and difference frequencies.  I also seem to have been coming close to understanding the need for non-linearity: in order for the signals to really “mix” one signal has to affect how the other signal passes through the device.  My thinking was that if you have one signal in effect varying the bias on a transistor as that signal goes through its cycle, another signal going through that device will see the device as being extremely non-linear.  It will get mixed with the first signal. (In retrospect, my understanding of the role of non-linearity was still quite flaky.) 

Still, I was not satisfied with my understanding of the mixers.  I thought it was a bit of a cop-out to just say, “Well, the math tells us that if you multiply two sine waves, the output will contain sum and difference products.”  Math-oriented scientists and engineers often pour scorn on what they call “arm waving” non-mathematical descriptions.  But I think there is some room for scorn in the opposite direction: I don’t think that memorizing a trig formula means that you really understand how a mixer works.  In “Empire of the Air,” Tom Lewis writes:  “At Columbia, [Edwin Howard] Armstrong developed another trait that displeased some of the staff and would annoy others later in life: his distrust of mathematical explanations to account for phenomena of the physical world. All too often he found his professors taking refuge in such abstractions when faced with a difficult and seemingly intractable conundrum… Time and again as an undergraduate at Columbia, Armstrong had refused to seek in mathematics a refuge from physical realities.”

I guess I still yearned for the clarity and intuitive understanding that had been (falsely) promised by those three nice beat frequency charts.  Time and time again, as I dug into old textbooks and ARRL Handbooks and promising web sites served up by Google, I was disappointed. 

Then I found it.

It was in the Summer 1999 issue of SPRAT, the quarterly journal of the G-QRP Club.  Leon Williams, VK2DOB, of Australia had written an article entitled “CMOS Mixer Experiments.”  In it he wrote, “Generally, mixer theory is explained with the use of complicated maths, but with switching type mixers it can be very intuitive to study them with simple waveform diagrams.” 

Eureka!  Finally I had found someone else who was dissatisfied with trigonometry, someone else who yearned for the clarity of diagrams.  Leon’s article had waveform diagrams that showed, clearly, BOTH sum and difference output frequencies.

Switching mixers apply the same principles used in other kinds of mixers. As the name implies, they switch the mixing device on and off.  This is non-linearity in the extreme.

Not all mixers operate this way.  In non-switching mixers the device is not switched on and off, instead one of the signals varies the amount of gain or attenuation that the other signal will face. And (as we will see) it does this in a non-linear way.  But the basic principles are the same in both switching and non-switching mixers, and as Leon points out, the switching circuits provide an opportunity for an intuitive understanding of how mixers work. 

Let’s take a look at Leon’s circuit.  On the left we have a signal coming in from the antenna.  It goes through a transformer and is then applied to two gate devices.  Pins 5 and 13 of these gates determine whether the signals at pins 4 and 1 will be passed on to pins 3 and 2 respectively. Whenever there is a positive signal on gate 5 or on gate 13, signals on those gaps can pass through the device.  If there is no positive signal on these gates, no signals pass.  Don’t worry about pins 6-12.

RF A is the signal going to pin 4, RF B is the “flip side” of the same signal going to pin 1.  VFO A is a square wave Variable Frequency Oscillator signal at Pin 5. It is going from zero to some positive voltage.  VFO B is the flip side.  It too goes from zero to some positive voltage. 

Look at the schematic.  Imagine pins 5 and 13 descending to bridge the gaps whenever they are given a positive voltage.  That square wave signal from the VFO is going to chop up that signal coming in from the antenna.  It is the result of this chopping that gives us the sum and difference frequencies.  Take a ruler, place it vertically across the waveforms, and follow the progress of the VFO and RF signals as they mix in the gates.  You will see that whenever pin 5 is positive, the RF signal that is on pin 4 at that moment will be passed to the output.  The same process takes place on the lower gate.  The results show up on the bottom “AUDIO OUTPUT” curve. 

Now, count up the number of cycles in the RF, and the number of cycles in the VFO.  Take a look at the output. You will find that that long lazy curve traces the overall rise and fall of the output signal.  You will notice that its frequency equals RF frequency minus VFO frequency.  Count up the number of peaks in the choppy wave form contained within that lazy curve.  You will find that that equals RF frequency plus VFO frequency. 

Thanks Leon! 

Back to the math for a second.  Why do they say that those diodes multiply?  And what do trigonometric sines have to do with all this? 

First the sines.  Most of the signals we are dealing with are the result of some sort of circular or oscillating motion—coils that are being spun around magnets, resonant circuits that behave like a playground swing.  For this reason, the trigonometry of circles can be used to determine the amplitude of a signal at any given instant.  Take the peak value of a sine wave signal, and multiply it by sin[2π(freq)(time)] and you will get the instantaneous value of that signal. 

When we say that mixers multiply, it is important to realize that we are NOT saying they multiply frequencies.  We are saying they multiply the instantaneous amplitudes of the input signals.  And it is that multiplication that results in the generation of the sum and difference frequencies.

Why multiplication?  Again, by looking at switching mixers, it is easier to understand.  Consider one of the gates in Leon’s mixer.  The RF input is a sine wave.  Its instantaneous value varies according to Peak*sin(2πft).  The VFO signal is either positive or 0.  If it is positive, the RF signal passes through the gate.  We can say that if it is on, it will have a value of 1.  If it is 0, no signal passes through the gate.  Mathematically we can say that the output is a multiplication product of the two inputs.  If RF is at 1.2, and VFO is positive (1), the output from that gate will be 1.2x1=1.2.  If RF is at 1.2 and VFO is at 0, the output will be 1.2x0=0

Note that this is very different from the simple summation in the “children’s fable” presented at the beginning.  If addition were at work here, we’d expect the outputs to look like 1.2+1=2.2 or 1.2+0=1.2  But that is clearly NOT what we’d get with a switching mixer.  Clearly multiplication is the operation that best models this circuit. 

Now this doesn’t mean that in every mixer circuit one input with an instantaneous input of 2 volts and another with an instantaneous input of 3 volts will result in an instantaneous output of 6 volts. After all, some mixers are made up of transistors that are capable of amplification, but others use simple diodes, and these diodes can’t amplify.  Different mixing circuits use different kinds of devices, different input levels, and different biasing voltages.  So the outputs will vary quite a bit.  But the shape of the output waveform will resemble the waveform that results when you multiply the instantaneous values of two input waveforms.  We can say that in addition to the multiplication that is the heart of the process, there are also mathematical constants and offsets that result from the particular characteristics of individual circuits.

You can use a simple spreadsheet program to get a feel for this.  Set up two columns each with the formula Peak*sin(2πft).  Assign different values of frequency to each column.  Set up another column for time—make it 1-100 and think of each division as a block of time.  Graph the results.  Then run a third column that multiplies the first two.  And put this third column on the same graph.  You’ll see the mixer action. 

Leon’s switching mixer circuit helped me get a bit more of the kind of intuitive understanding that I’m always looking for.  Later on, through a more careful reading of Experimental Methods in RF Design’s mixer chapter, I think I started to understand how non-switching mixers work, and why non-linearity is an essential element of a mixer circuit.

Jean Baptiste Joseph Fourier (1768-1830) discovered that any complex periodic waveform can be shown to be the result of the combination of a set of sine waves of different frequencies.  Here’s a great illustration of this principle.  It is from ON7YD’s web site.  The darkest line is the

complex signal that results from the sine waves that are shown around it.  A picture is worth a thousand words. 

The key idea here is that if you see a complex periodic (repeating) waveform, you should realize that “beneath” that waveform, there are a number of nice clean sine waves.  And here is where non-linearity as an essential element in mixing comes in.

Let’s consider two devices, both with dual inputs.  One is set up to be very linear.  The other is set up to be non-linear.  Let’s put two signals of different frequencies into each input.  The first input is 1 volt peak at 1 MHz, the second input is .1 volt peak at 10 MHz. 

In the linear circuit, we can think of the stronger 1 volt signal as moving the operating point of the device up and down, up and down along the very straight line that describes the relationship between input and output in this circuit.  As it does so, the weaker 10 MHz signal just sort of rides along.

If we look at the output we can clearly see the two signals, one riding along with the other.  The output waveform is not complicated, and it seems clear that there are only two signals that you could get out of that via filtering: the two input signals.  This is just like the acoustic situation that caused me so much confusion.  The key thing to remember here is that the two signals are not really mixing. 

Now let’s look at the non-linear circuit.  Now the operating curve really is curved. The weaker 10 MHz signal will once again, in a sense, be riding along on the stronger 1 MHz signal, but that 1 MHz signal is no longer moving up and down on that nice straight line.  Now it is on that curve.  Now the two signals really “mix”, mixing almost to the same extent that liquids of two different colors mix in a blender.  You can see how the curved operating characteristic—the non-linearity— causes the two signals to mix.

Out of the non-linear circuit a very complex periodic waveform emerges.  It is a complicated mess, but Fourier tells us that any complex periodic waveform can be seen as being composed of sine waves of many different frequencies.  If we were to dissect this output waveform of this device, we’d find the two original signals, harmonics of these signals, and, most importantly, new signals at the sum and difference frequencies of the input frequencies.  And this complex signal CAN be dissected.  To do this, we make use of “balanced” devices to cancel out the input signals, and filters to shave away the harmonics and perhaps either the sum or the difference output.  We can set things up so that only one frequency emerges from the mix.  That is extremely useful.

 I think (hope!) I’ve made progress in my effort to understand mixing; I think I’ve moved far beyond both hand waving acoustics-based fairy tales, and the almost equally unsatisfactory approach that equates understanding with the ability to regurgitate trig formulas.  I now understand the difference between mixing and adding.  I know why multiplication (and not addition) is the math operation that describes what happens in a mixer.  Most importantly I think, I now know why you need a non-linear device to have true mixing.  Fourier provides the answer: That bend in the operating curve of a non-linear device causes the output to be the kind of complex periodic waveform that contains many different sine waves.  And among those waves are sum and difference frequencies. 

Now I must admit that how it is that among those sine waves there are the exact sum and difference frequencies of the inputs, well, for me that remains a bit of a mystery.  But it kind of makes sense…

  

"The Magic That Only Comes from a Radio you Built Yourself" -- The Many Benefits of True Homebrew

 Receiver on the bottom,built around 1997. 

Transmitter upper right, built in 1993. Power supply upper left, 1998.  

Adventures on the road to HB

Homebrew Radios in the age of the Internet

By Bill Meara, N2CQR

MAGIC

"I listened to the magic that only comes from a radio that you built yourself." In that one sentence (posted to an Internet e-mail group), Mike, VE2GFU, nicely described the feeling that can arise in the midst of a room full of solder smoke... and the reward that awaits those who endeavor to build their own radio receivers. In an age of mass produced, homogenized, high price commercial equipment, there is still magic to be found in the production and use of simple homebrew radios. I recently put together my first superhetrodyne communications receiver - I had so much fun with it that I thought my fellow amateurs might be interested in the project.

I was a frustrated teenage radio builder....

When I put my first homebrew low power transmitter on the air a few years ago, I thought I'd maximized my ham radio satisfaction. I gleefully reported to other stations that "RIG HERE IS HOMEBREW". For a while, I really thought that my fun meter was pegged! But everytime I looked at the commercial receiver that sat alongside my QRP transmitter, I knew in my heart of hearts that I still had some work to do. The truth was that only half my station was homebrew. Until I built my own receiver, I would not be able to enjoy the warm glow of satisfaction that comes from running a completely homebrew station. As a kid, I'd always looked with wonder and envy at the exotic homebrew stations in the DX column of QST magazine. I wanted to do what those intrepid foreigners had done. I decided to finish the job. I decided to build a receiver.

"Receivers are Difficult!!!"

I approached the project with some trepidation. Since my earliest days in the hobby I'd heard that "receivers are difficult." There seemed to be a deeply believed and long-standing bit of conventional wisdom that said that most hams could sucessfully build transmitters, but receivers were somehow beyond our capabilities. During radio club meetings, old timers would share tales of homebrew adventures from days-gone-by. They told of tube transmitters built on chassis fashioned from purloined street signs. There were a lot of great stories, but they were all about transmitters. When I'd ask about receivers, the old timers would look a bit sheepish as they admitted that their receivers were all commercial.

Receivers are difficult. I knew from personal experience that there was some truth in this axiom. As a teenager I had tried to barge into the ranks of the homebrewers with an audacious attempt at reproducing a varactor diode-controlled receiver I'd seen in one of the ham magazines. I never got it to work. As I approached this recent receiver project, I think a desire for vindication - and a desire to finish the job I started in 1974 - was part of my motivation.

Barebones, no frills, one step at a time

The "Barebones Superhet" presented in a July 1982 QST article by Doug DeMaw seemed to be just what I was looking for. As the title imples, it is a very simple, easy-to-understand circuit. Most of the stages were built around discrete solid state components - no mysterious IC black boxes. 

Remembering my bitter defeat in my earlier receiver project, I decided to take a fool-proof approach to this one. I took Doug DeMaw's very simple schematic and made it even simpler by dividing it up into separate stages. I would build each stage one at a time, each on a separate printed circuit (PC) board. For my receiver there would be separate boards for the Radio Frequency Mixer, the Variable Crystal Oscillator (VXO), the intermediate frequency (IF) amplifier, one board for the Product detector/beat frequency oscillator (BFO) and one audio amplifier board. I would test each stage before going on to the next.

Parts acquisition in the age of the Internet

As a teenage wanna-be radio maker, parts acqusition had been a major problem. I'm happy to report that the Internet and Express mail services have largely eliminated the tortuous "waiting for the mailman" vigils that many of us endured back in the dark ages. I kicked off my project with a brief session involving several parts catalogs, my computer and a credit card. A few short days later, the boxes started coming in and actual construction was about to begin.

While the catalog houses provided many of the parts, my junk box, hamfests and fellow hams were the sources for many of the components. I think that this diversity of parts sources adds to the character of the final product. When I look at my receiver, I can see parts that came from my old friend (now SK) Pericle, HI8P. There are components in there that were sent to me by Tom, W1HET and several other ham friends. There is a reduction drive from an old Swan 240 and a grommet from a deceased Heathkit Luchbox. The LM386 audio amplifier chip (a concession to modernity!) came out of a Kanga Kits direct conversion receiver; I didn't have an eight pin socket for it, so I scrounged through my junk box, found a 16 pin socket and cut it in half. Like I said, this approach to parts acquisition gives the radio some character. 

Lunch time PC board design

My "one stage at a time" approach resulted in some special challenges and opportunities. I had to design the PC board patterns myself. For hams accostomed to using ready-made PC boards, or simply reproducing patterns made by others, this might seem like an intimidating task, but since I was dealing with only one stage on each board, it turned out to be easy and rewarding. I was using boards that fit very conveniently in the front pocket of my shirts. I made PC board design a lunch-hour project. I would go to work with my schematic and a couple of index cards in my pocket. I'd cut the cards down to PC board size and used them to plan the layout of the boards. I usually had to do two or three "drafts" before I was satisfied, but I found that I was able to do about one board per lunch hour. Doing the layout myself definitely added to the "I did it myself" feeling at the end of the project.

I set a goal of completing one board per week - most of the design and planning would take place during the lunch hours, most of the construction took place early on Saturday and Sunday mornings.

Testing, testing....

My arsenal of test gear is far from laboratory grade! I have a little (ancient) Eico 435 oscilliscope and an old Heathkit signal generator. I bought the scope for 25 dollars on the Internet. The generator was a 15 dollar hamfest purchase. The 'scope will only read up to about 5 Mhz, but since the IF of my receiver would be 3.579 Mhz, I knew it would be very useful.

Testing the stages was a lot of fun. The VXO and BFO were easy to test - I just listened for the signal on a Radio Shack general coverage receiver. For the IF AMP I used the signal generator to put some 3.579 Mhz energy into board and used the 'scope to make sure it was amplifying.

One of the most difficult parts of HF superhet construction is the IF filter. Doug DeMaw's circuit employed a three crystal ladder filter. Doug described it as simple and easy, but to me it looked a bit intimidating. One of the benefits of homebrewing is that you can really "have it your way". Wishing to avoid a frustrating battle with a complicated filter, I searched through the QRP/Homebrew literature for a simpler approach to IF filtering. I found what I was looking for in another article by DeMaw. In this cicuit he used one crystal with a resistor to ground. I decided to use this simple filter and put off construction of the more sophisticated (and narrow) three crystal circuit until later.

So I redesigned the RF mixer board to accomodate my simplified filter. I wasn't quite sure if this little foray into electrical engineering would be successful (my degree is in economics!) so the testing of this stage was tinged with some anxiety. I set the signal generator for the low end of the 20 meter band. I got the VXO oscillating and put the scope on the output of my simple filter. Slowly I tuned the generator across the 20 meter CW band. All of a sudden, at one very specific point, a big 3.579 Mhz signal popped onto the 'scope screen! Eureka! My mixer was mixing and my filter was filtering!

Holy cow! It really works!

After about a month and a half of this, I had assembled an impressive looking collection of small circuit boards. I couldn't resist putting them all together on the workbench to see if this thing would really receive. Armed with a set of alligator clip test leads I connected inputs to outputs. It was early in the morning and 20 wasn't really open yet, but it was Saturday and I figured there were some folks out there trying to coax the ionosphere into action. As I was checking the test leads, I started to hear - almost imperceptiably at first - CW. At first I thought the sound was coming from my Drake 2-B, but a quick check showed the Drake was completely off. My little creation was actually receiving radio signals!

As late afternoon rolled around I decided to see how my still incomplete device would handle SSB. As luck would have it, my crystal let me tune around 14.200 Mhz. There I found the very melodious tones of EA3OT. Echo Alfa Three Oooold Timer, with his "six over six over six" antenna system filled my shack with beautiful phone signals. My relatively wide, one crystal filter was ideal for reception of Mike's fine signal. There really was something quite magical about looking at my little collection of boards and realizing that they were receiving signals from far-away Barcelona. I was experiencing "the magic that only comes from a receiver that you built yourself..."

Enclosure (sort of)

Now it was time to start putting the radio in a proper enclosure. A few years ago, Paul Carr, N4PC, * (*Described in several editions of the CQ magazine during 1993) built a 40 meter solid state rig on a wooden base. Disliking metal work, I immediately appreciated the wisdom of this approach. Realizing that I'd probably want to add additional circuits later on, I decided to make the chassis about twice the size I really needed. A visit to Home Depot yielded a suitable (16"X11") piece of pine. I also picked up some very light sheet metal that I thought would help with the front panel.

I had three large double sided PC boards in the junk box. The circuit boards were attached to these PC "base" boards with some Radio Shack spacers and 4X40 screws. The Base boards were bolted to the wooden base.

I used the sheet metal to fashon an L shaed front panel. The material was not quite rigid enough, so (in keeping with a very old ham tradition of stealing radio materials frm the kitchen) I put a little "cookie baking sheet" between two layers of the Home Depot sheet metal. The L shaped panel was afixed to the pine base. A smaller L shaped piece of scrap aluminum was attached to the back side of the pine chassis - this would serve as the mounting point for the antenna and power connectors.

My creation was starting to look like a radio. Better yet, it resembled one of those impressive homebuilt rigs that I used to see in the DX column of QST. I felt I was getting close to membership in the that elite group of intrepid hams who had actually "rolled their own." I was starting to feel a kinship with all of those intrepid, creative wackos who build things in their basements or garages. I felt part of the same homebrew tradition that dates from in the early days of ham radio. Just like the guys who build small airplanes in their backyard shops, just like those guys in California's Homebrew Computer Club, I was approaching the point when I could begin sentences with the proud phrase, "I built..."

Debugging

But of course, I was not done yet. Not by a long shot. When you are homebrewing, you have to be patient. You have to start out realizing that you are definitely not involved in "plug and play" radio.  Very few homebrew receivers will work properly the first time you fire them up. The radio needs to be properly aligned. Amplifiers and oscillators need to be tamed. But I think this is one of the most satisfying part of the homebrew experience. It is during this phase that you really get the sensation that you are molding your creation to satisfy your requirements. You are physically molding it by deciding where you want the control knobs and external connectors. And (even better) you are molding it electronically by deciding how you want to to sound. It is during this phase that you really put electronic theory to work.

I had a few very common problems. My audio amplifier would scream like a banshee if I turned the gain up. My variable crystal oscillator was kind of sluggish - it sometimes wouldn't start up right away when I applied power. A preacher from the 22 Meter broadcast band urged me to repent every time fired up my new radio. And worst of all, 80 meter CW signals from the venerable W1AW jumped right over my receiver's front end filters, landing right in my 3.579 Mhz IF frequency. These signals not only appeared to be mocking my technical abilities, but they also seemed to be making fun of my code speed.  Like I said, this was definitely not plug and play.

In my effort to fix these problems, modern technology provided me with resource that was completely unavailable during my earlier (1974) battle with a superhet: the Internet. The 'net puts the radio builder in almost instantaneous contact with a worldwide network of entusiastic solder melters. I found the rec.radio.amateur.homebrew USENET group to be an excellent source of information, advice and moral support.

The internet can turn your homebrew project into a multinational enterprise. Hams from around the world chimed in with helpful hints. It was a lot of fun to encorporate suggestions from distant Australia into my little HB receiver. And it was very reassuring to know that all those far-flung Elmers were available if I got into a real jam.

I was particularly gratified when I got some e-mailed words of encouragement from the guy who had designed the receiver I was building, Doug DeMaw, W1FB. Doug's son had spotted one of my pleas for help in one of the USENET groups and had relayed my message to his father. Doug sent me a very nice and encouraging note. I was saddened to learn that shortly after our exchange he became a silent Key.

Solutions to most of my problems came very quickly - and I learned something with each of them.

The screaming banshee audio amp turned out to be the result of a simple circuit error - I'd failed to ground one of the bypass caps on the LM386 AF amp chip (the only IC in the rig).

The Variable Crystal oscillator was made more obedient by playing a bit with the values of the two capacitors that madeup the feedback network in the Colpitts oscilator.

The preacher and W1AW required a little more effort. I decided that I needed a bit more filtering at the front end of the radio. I could have easily just thrown in one or two more tuned circuits between the antenna and the mixer, but I was concerned that losses in these circuits would adversely affect receiver sensitivity. Roy Lewllan, W7EL, had advised me (via the net) to perform a simple check of receiver sensitivity: I was told to listen to the receiver output while connecting and disconnecting the antenna. If connecting the antenna resulted in a noticeable increase in the noise output of the receiver, there would be no need for additional front end amplification. My receiver was not really doing well on this test, so I was concerned that adding more tuned circuits at the front end would worsen the sensitivity problem. It seemed to me that a stage of RF amplification that included a couple of tuned circuits might help me banish the unwanted preachers and code practice sessions without further degradation of receiver sensitivity.

Doug Demaw's QRP Notebook pointed to a simple, grounded gate FET amplifier with tuned circuits at the input and output. I quickly put this stage together on its own small PC board and put it between my antenna connection and the mixer board. The amp was obviously amplifying, but it seemed to be getting carried away. Whenever I'd tune both the input and output circuits to peak, the amp would begin to oscillate. I turned to the Internet and aske for advice. Help quickly came from afar. A fellow named PK Singh sent me an email with the solution: I had to "tap down" on the toroidal coils in the two tuned circuits. This deliberately introduced impedence mismatches that effectively reduced the stage gain and thus stopped the howling. (A side benefit was a noticeable increase in tuned circuit Q - a big help in my battle with the 22 Meter station). With the tapped down amp in the circuit, my receiver passed Roy Lewellan's noise test with flying colors and I was no longer the subject of harrassment from 22 meters and W1AW. Viva el Internet!

Coffee can frequency readout

My frequency readout scheme needed some work. The tuning capacitor I was using had a little venier reduction drive built into the cap. This made for very smooth tuning, but it made it impossible to work out any kind of frequency readout on the front panel. I had to peer over the panel and look at the variable capacitor to determine where I was in the band. In an age of multidecimal numeric digital readout, I was clearly behind the times.  And my neck was starting to bother me. 

To upgrade, I found a junkbox 365 pf variable cap with no built in reduction drive. This was about twice the capacitance that I needed, so I simply plucked out about half of the rotor plates. I also found a Johnson 6:1 reduction drive in a junker Swan 240 transceiver. With a piece of scrap aluminum, I engineered a little mount for the capacitor. The Johnson drive allows for the attachment of a frequency readout dial. I found that the top of a coffee can (the metal part you always throw away) was ideally sized for my front panel. Soon I had the modified cap, reduction drive and coffee can readout dial mounted on the front panel. A triangular piece of electrical tape provided a sharp looking pointer. A few pieces of masking tape on the coffee can top served as frequency markers. I realize that my "coffee can readout" will seem incredibly crude to those accostomed to glowing numerals, but I get a real kick out of it every time I spin that little homebrew mechanism.

Filter Finale

In a certain sense I was done. I was able to pair my new receiver with my QRP transmitter and was easily able to make QSO's. I was working European stations regularly with 3 watts out. But my simplified crystal filter was a too wide for serious CW work. I could hear several CW signals simultaeously and - worse yet - I could hear the "other sideband" on the stronger signals. So I hadn't really achieved the coveted "single signal reception" status that is - after all - one of the main reasons for going the superhet route.

There are a number of excellent article out there on the design of CW crystal ladder filters. Unfortunately the building of these filters requires the use of some special test gear to determine the electrical charecteristics of the particular crystals that will be used.

Wishing to avoid the construction of test gear that would be more complicated than my radio, I decided to simplify filter construction. I bought a bag of 50 3.579 Mhz TV color burst crystals from Dan's Small parts. I then built a simple Colpitts oscillator circuit on a Radio shack breadboard. I tuned my Drake 2-B receiver to 3.579 Mhz and started plugging crustals into my breadboard oscillator. I screened out those rocks that were signficantly off frequecncy, then I went through the pile again, judging by ear (using the tone from the Drake 2-B) to select three crystals that were very close in frequency. (I know that a frequency counter would have made this easier, but I don't have one so I had to "make do.")

I simply pugged these crystals into the filter circuit described in Doug DeMaw's 1982 article. Essentially I was "hoping for the best", hoping that the characteristics of my rocks would not be significantly different from those employed by Doug DeMaw.

It all worked out very well. The new filter significantly sharpened my receiver's selectivity. I could no longer hear strong signals at two points on the dial. Single signal reception had been acheived!

My filter proved to be far to sharp for confortable SSB reception, so I worked out a little switching arrangment that allowed me to switch between my original (wide) filter and the new, sharp CW filter. MISSION ACCOMPLISHED

I found that my technical skills improved dramatically during the course of this project. I even noticed a marked impovment in manual dexterity. By the time the receiver was finished, I was much more confident about putting together my own circuits. In order to be truly "100 percent homebrew", I needed to whip up a power supply for my station and a sidetone oscillator for my transmitter. These projects were quickly completed and I was soon on the air with a 100 percent HB station.

Homebrew is good for you! It really doesn't matter what band or mode you build for, a homebrew radio will provide a kind of satisfaction unavailable from store-bought units. A project like this will improve your skills, expand your knowledge and will put you in league with all of those intrepid inventors who have turned piles of parts and wires into devices that magically extract signals from the ether.

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More details on this homebrew rig here: https://soldersmoke.blogspot.com/2022/09/fixing-up-old-homebrew-rig-barebones.html

South African Homebrew: ZS4L's "Griffin" 40 Meter SSB Transceiver

 

That's William, now ZL4L, and his homebrew 40 meter SSB transceiver.  He has given the rig a wonderful name from Greek mythology:  The Griffin (see below).  I talked to William this morning on 10 meter SSB.  I mentioned my homebrew rig and to my surprise (this doesn't happen much) he asked for more info.  Then he told me about his own homebrew creation, The Griffin.  FB William! 

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From William's QRZ.com page (https://www.qrz.com/db/ZS4L):  

I have always wanted to build a homebrew transceiver-and recently I completed my pride and joy-a 40m SSB/MCW transceiver-I call it the ZS5WC "Griffin"..

"Griffin"..-well --if you know greek mythology you will find that it defeated much greater adversaries in battle.(to cut a long story short..)
The parallel I am getting at is..Big commercial rigs can be taken on by a rig constructed at home-and with great success!.
Sure, it does not have the bells and whistles of a 1000mp-but the TX audio is good, the RX is great , and the SMILE factor-even with all the little quirks is off the scale!..
Basically it is a single conversion superhet-4 tuned BPF stages,ATT, Gain control stage ahead of 1st Rec. Mixer/Bal. mod (NE612) , Xtal 10mhz homebrew filter,2 transistor feedback amp, second gain control stage,2nd mixer/BFO (NE612)-On TX to PA board-4 transistor pre-amp, IRF510 mosfet PA, LPF and RX /TX relay. ON RX to TL072 audio pre-amp, Spits to AGC/S-meter amp-(741 and BC107's) and audio amp TBA820m.
ALC is done on AGC board as well with BC107 back to back to AGC bc107.
The S-meter drive is developed in the emitter leg of the AGC BC107-simple series pot to calibrate-no zero pot is required..(Works great!)
There is a volt control PCB too, with RX/TX switching.
On the main PCB there is a phase shift osc. for MCW and sidetone. Alc is adjustable from front panel from 1/2 watt to 5 watt.Rit is included in the Hartley osc. circuit and readout is done with a pic and two line LCD disp.(from AADE.com..)
Freq. drift from warm is 200Hz down in frequency then swings round and stabilises close to start freq.
Rit is good for around 5Khz swing.
Amp keying is available on the back panel, as well as an aux. 12 supply-(To run a homebrew noise squasher and amp..)

https://www.qrz.com/db/ZS4L