Putting a Real LC VFO in My Ceramic-Resonator, Direct Conversion 40 Meter Receiver. LC JOVO! (Video)

This is the DC receiver that I built back in 2017-2018. I had used a ceramic resonator in the VFO. That receiver was on the cover of SPRAT magazine. It may not have deserved the honor -- recently Dean KK4DAS and I discovered that the ceramic resonator VFO drifted rather badly. So Dean and I are now building real LC analog VFOs. This is kind of an aside to a Virginia Wireless Society -- Maker Group project. This video shows my receiver working yesterday on 40 using the VFO that was recently thrown together.

More details on the original project (that used the ceramic resonator) here: 

https://soldersmoke.blogspot.com/search/label/DC%20Receiver%20Build

 The VFO circuit comes largely from W1FB's Design Notebook page 36.  I followed most of the conventional tribal wisdom on VFOs:  NP0 caps, often many of them in parallel.  Air core coil (in my case wound on a cardboard coat hanger tube). 

For C1 I used a big variable cap (with anti-backlash gears) that Pete N6QW advised me to buy on e-bay. Thanks Pete.   L1 is on the cardboard tube.  I only built the oscillator and the buffer -- I did not need the Q3 amplifier.  (The water stain in the upper left is the result of a heavy rain in the Azores around 2002 -- water came pouting into the shack.)  

I think the VFO is more stable than the Ceramic Resonator circuit. But I want to go back and give the ceramic resonator circuit another chance...  Miguel PY2OHH has some really interesting ceramic resonator circuits on his site. Scroll down for the English translation: https://www.qsl.net/py2ohh/trx/vxo40e80/vxo40e80.htm

Dean KK4DAS commented that VFO construction is as much an art as a science.  I agree -- there is a lot of cut and try, a lot of fitting the components you have on hand into the device you want to end up with.  You have move both the frequency of the VFO AND the tuning range of the VFO.  Mechanics (in the form of reduction drives) is often involved.  And, of course you have to apply lots of tribal knowledge to get the thing stable. You could, of course, avoid all of this by using an Si5351, but I think that moves you away from the physics of the device, and is just less satisfying. 

So,  JOVO!  LC JOVO!  The Joy of VARIABLE Oscillation!   

SolderSmoke Podcast #203 Winter, Transceivers, Antennas, DC RX, uBITX, Mixers, 'fests, MAILBAG

N6QW in 1959. Building an SSB transceiver

SolderSmoke Pocast #203 is (FINALLY!) available: 

http://soldersmoke.com/soldersmoke203.mp3

24 March 2018

--The reasons for our delay. 
Winter, Computers, College, Family Trees, Lawyers....

-- Winterfest 2018 
-- Pete launches 2018    THE YEAR OF THE TRANSCEIVER
    http://n6qw.blogspot.com/
-- SDR -  Satan's Digital Radio?  
-- Direct Conversion Receiver Projects
-- Mixer Musings 
-- A Thailand Troubleshoot 
-- Nor'Easter knocks out Bill's Moxon -- An appliance replacement? 
-- Homebrew Electret Mics.  Seriously.  
-- uBITX Build with Rogier
-- Civilized Crystal Testing
-- Baofeng! 
-- DRAGNET

-- MAILBAG
KD4PBJ's REGEN 
N6ORS's SDR rig
Mike Rainey's DX-100

WA1UQO's Discrete Ceramic DC Receiver

Armand writes: 

The attached picture is your DC receiver. A little tweaking left to do as the range right now is ~ 7.44Mhz to 7.032Mhz. I used one of Farhan's trifillars and a couple of air coils that you gave me last year.  Listening to the Wisconsin QSO party as I type. 

FB Armand!  The receiver looks great.  I hope others will follow your lead and build this simple little receiver for 40. 

Understanding Switching Mixers (as in the Ceramic DC RX)

W3JDR's Comment on my post about the DC RX mixer got me thinking.   He was right -- my explanation of the mixer action wasn't quite complete, especially as far as switching mixers are concerned.  I remembered that I had written about this in the SolderSmoke book.  Below you can see the part of the book in which I discuss switching mixers.  Realize that the two diodes in F5LVG's mixer play the same role as the two gates in Leon's circuit.  It will be worth your while to sit down with Leon's circuit diagram, his frequency chart,  and a ruler and really go through this so you can SEE and really understand how the two gates (or switching diodes) generate sum and difference frequencies.  

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

I guess I still yearned for clarity and intuitive understanding...  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!  

F5LVG's Glue-Built Mixer Transformer

One thing I forgot to mention:  In Olivier F5LVG's DC receiver article back in SPRAT 100, he casually mentioned building a transformer for his mixer by taking two inductors of the appropriate values and GLUING THEM TOGETHER.   What a great idea!  I had to try it.  I did.  Picture above.  It worked in my Ceramic DC receiver, but the trifilar transformer from Farhan in India worked better.  Perhaps the coupling was tighter.  But hey, it worked.  Three cheers for Olivier.    

Building the Ceramic Discrete Direct Conversion Receiver #4 -- The Mixer

I think the most important stage of a direct conversion receiver is the mixer.   This is the stage that takes the RF energy coming in from the antenna and -- in one fell swoop -- turns it into audio.

It is important to understand how this happens.  I go into this in some detail in the SolderSmoke book.  To summarize: 

1) You have two signals going into a non-linear device.  The way in which the smaller signal passes through the device -- how much it is amplified or attenuated -- depends on the instantaneous value of the larger signal.  We are not just adding the two signals together.

2) The waveform that comes out will be a complicated repeating waveform.  We know from Fourier that any complicated repeating waveform can be broken down into sine wave components.

3) When you analyze the complicated repeating waveforms coming out of the mixer, you will find that the sine wave components include a frequency that is the sum of the two inputs and another that is the difference between the two.

So lets suppose we have a non-linear device.  We send in a signal from our oscillator at 7061 kHz. Coming in from the antenna we have a signal at 7060 kHz.   The non-linear device will produce outputs at 14121 kHz (sum)  and at 1 kHz (difference).  We are interested in the difference frequency.  We can HEAR that one.  We feed it into our audio amplifiers and we can copy the Morse Code coming in.  It will sound like a 1 kHz tone going on and off as the operator at the distant station presses his code key.  (We don't really have to worry about the 14121 kHz signal -- it is easily eliminated by filters and would never make it through our audio amplifiers.  And in any case we could not hear it.)

What can we use as a non-linear device?  In this receiver we will use diodes.  Diodes are  extremely non-linear devices. They can be used as on-off switches, with one of the signals determining if they are on (conducting) or off (not conducting).  When used like this they are "switching mixers." In essence, a larger,  controlling signal from the VFO will be turning the diodes on and off. Thus the signal coming in from the antenna will be chopped up by the switching action of the diode being turned on and off.  This is non-linear mixing at its most extreme.  It will definitely produce the sum and difference products we are looking for.

We could build the mixer with just one diode. You could apply the VFO signal to the diode to turn it on and off, and then feed the signal from the antenna into the same diode.   You would get the sum and the difference product out the other end.   You will see very simple direct conversion receivers intended for use in software defined radio schemes using just one diode. But this kind of circuit has a couple of serious shortcomingsq: it is susceptible to "AM breakthrough" and it is "lossy."

The circuit we are using addresses these problems by using two diodes.  To reduce loss, one conducts during half of the oscillator signal's cycle, the other during the other half.  Here LTSpice is ueful. You can model this mixer and see in the simulator how each of the diodes handles half of the oscillator RF cycle, with both contributing to the AF signal we want at the output (the difference frequency).   (The schematic above is from LTSpice but it is not ready for simulation.  For this you should replace the variable resistor with two fixed 500 ohm resistors, and add two oscillators -- one with the weak incoming RF signal and the other the strong local oscillator signal.)

The AM breakthrough problem is also addressed by the use of two diodes.  Here's the problem:  If you are on 40 meters, there will be strong shortwave AM broadcast signals coming in from your antenna.  Some will be so strong that they will get past your front-end filtering.  If you were using just one diode, that diode might demodulate the AM signal -- the AM carrier would mix with the AM sidebands and you would have an undesired audio signal heading for your AF amplifiers. Many of us have experienced this -- you are trying to listen to ham radio SSB signals, but you can hear China Radio International playing in the background. 

The two diodes take care of this easily. Look at the way an AM signal would reach the diodes. The carrier (and its sidebands) going through the top diode will be 180 degrees our of phase with the signal going into the lower diode. But the output of the diodes are joined together.  They will cancel out.  We say that for the RF signal coming through from the antenna, the circuit is "balanced."  That signal -- in this case the undesired AM signal -- will cancel out at the junction of the two diodes.

But to understand this circuit you must see what is NOT cancelled out.  The signal from the VFO is hitting each diode with the SAME polarity at the same time.  Look at the 1k variable resistor. So the signal from the VFO will NOT be cancelled out at the output.  Nor will the mixing products produced in the diodes.  That last sentence is the key to all of this.  The sum and difference products that result from the mixing of the signal from the antenna and the signal from the VFO SURVIVE.  They are not cancelled out.

We can easily select the one we want.  An RF bypass capacitor connected from the output of the mixer to ground will get rid of most of the VFO signal (7061 kHz) and most of the sum product (14121 kHz) while passing the audio to the AF amplifiers. 

When I built this detector I used a trifilar toroid out of a box of them that Farhan left with me back in May. I used two of the windings  secondary and one of the windings for the primary.  You might want to make a more simple transformer using an FT-43 type core.  I recommend W8DIZ as a source. 

I hope this explanation helps, and I hope I got it right.  Let me know if you see any errors in my explanation.  Tinker with the circuit when you build it.  You should be able to get it going.       

Complete Schematic

Ceramic DC Receiver on the Cover of SPRAT. Happy New Year to All! Straight Key Night.

G-QRP very kindly put my little DC Receiver on the cover of issue Nr. 173.   (Very sorry to see that GM3OXX has become a Silent Key. ) 

As we often say on the podcast, if you are not subscribing to this wonderful magazine, you are missing out on a lot of great ideas and circuits. Information on how to join the club and start receiving SPRAT can be found here: http://www.gqrp.com/join.htm   It is only 22 bucks! 

Reminder:  Straight Key Night is upon us.   It begins at midnight UTC 1 January.  It is a great way to begin the new year.  My HT-37 and my Drake 2-B are warming up now (and are helping to keep the shack warm on a very frigid day).   HNY to all!   73    Bill 

Building the Ceramic Discrete Direct Conversion Receiver - Part 3 -- The Audio Amplifier

Once you have achieved JOVO and have the oscillator covering the frequency range you desire, it will be time to build the audio amplifier stage.  Now, I'm sure many of you will be tempted to just throw an LM386 or 741 op amp in there.  But don't do that!   You want to build the WHOLE receiver.  Don't let that IC manufacturer rob you of the FULL HB EXPERIENCE.

The DISCRETE amplifier circuit we will used is based on a design that several of us used to replace the LM386 in the BITX20 receiver. 

OM Arv presents one version here: https://sites.google.com/site/arvidevans/discretecomponentafamplifier

And here is a tutorial that does a GREAT job of explaining how this circuit works.  I strongly recommend you study the tutorial carefully.  This site will allow you to really understand how your amplifier works.  

http://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html 

It is not that complicated.  But here you have to take care to make sure that your amplifier does not turn into an oscillator.  Keep the outputs away from the inputs.  Keep the leads short.  Some planning is needed here.  Click on the picture below for layout ideas.  The right 1/3 of the board contains the AF amp circuitry. 

After you build the circuit, TEST IT!  If you have an AF signal source and a 'scope, great.  But if not, just hook some earbuds or headphones to the output, connect a 9 V battery and PLACE YOUR FINGER on the input of the first AF amplifier.  If you hear a noticeable increase in hum when your finger makes contact, congratulations, your AF amplifier is amplifying. 

The Complete Schematic

When you get this stage and the oscillator working, you are 3/4 of the way to completion.  Next we will built the mixer, and then the front end filter and amplifier. 

Building the Ceramic Direct Conversion Receiver Part 2 -- Building the VFO -- Our Goal is JOVO!

DC RX VFO and Buffer

I'll put the full schematic at the bottom of each of the posts so that you can easily refer to the big picture. Above you see the schematic of the VFO circuit. 

OK, here we go. Let's build the oscillator.  Our goal is JOVO -- the Joy Of Variable Oscillations. 

At this point you should have a big-enough copper clad board, and you should have given at least some thought to what kind of enclosure you are going to put the board in when you are done.  It pays to think ahead at least a bit, but don't get so carried away with planning that you never get around to building. 

You should plan the allocation of the space on the board.  Think about where you are going to place each stage.  You can mark out the spaces with a pencil or a Sharpie marker.  You might want to look at my board for ideas:     

In the picture above you can see the four stages.   On the left side of the copper-clad board you can see the Front end: the input filter and the RF amplifier (transistor near the top).  Moving toward the center you can see the mixer stage (around the circular 1k trimmer potentiometer).  Below the mixer (near the big round hole in the Bud Chassis) is the Variable Ceramic Oscillator stage and its buffer amplifier. The right 1/3 of the board is taken up by the audio amplifiers.  Note the use of Manhattan pads throughout.    Click on the picture for a closer look. 

Once you know where you will put the VFO, eyeball the schematic and think about where you will need Manhattan pads.  I often start by thinking of three rectangular pads for each transistor, one for each lead.  You can see that there are a lot of parts hanging off each of the terminals.  I sometimes put a long strip across the top or the middle of the board to carry the DC voltage.  

Since this is an oscillator, you don't have to worry too much about keeping the outputs away from the inputs.  You want this one to take of on you. 

For the feedback capacitors (C16 and C17) and the output capacitor (C19)  , get some ceramic disc NPO caps.  I put a 180 pf cap at C17 only becasue I didn't have a second 150 pf cap in the junkbox.  Either value will probably work. 

You will need a ceramic resonator.  I recommend this one from Mouser.  Again, buy a bunch.  They are cheap: https://www.mouser.com/productdetail/520-zta7.3728mt

You can, to start,  build this circuit WITHOUT the two components that allow you to vary the frequency: without L4 and C5.  Just run the left end of the ceramic resonator to ground. See below.  

Connect a 9V battery to the top of C24.  Without L4 and C5 (with one end of the resonator to ground)  you should be oscillating at around 7.168 MHz (the capacitors in the oscillator circuit are pulling the frequency down from 7.37 MHz).

You need some way to find out if it is oscillating.  If you have an oscilloscope, great.  Put the probe at the output and take a look.  But perhaps a simpler and more satisfying way to do this test is with a radio receiver.  Tune the receiver around 7.168 MHz.  You do not need to connect your receiver to the oscillator.  You should be able to hear it.  If you do, congratulations.  If not, check your work.  Be patient.  This is not plug and play radio! 

Once you get the thing oscillating, it is time to make it variable.  Here is an opportunity for variety and experimentation.  Here are some of the options you have: 

Here is what I found.   The frequency stability of these circuits vary.  But all of them are stable enough.  They might drift a bit so that you have to retune the dial every few minutes.  If that realy bothers you you can upgrade to the air core coil with air variable cap arrangement.  

When I put just a variable capacitor that goes from 17 pf to 159 pf between the ceraamic resonator and ground, I was able to tune the oscillator from 7.220 MHz to 7.420 MHz. 

If I put a fixed 8.18 uH coil between the ceramic resonator to ground that moved the frequency to 7.010 MHz.  You could use a toridal core coil for this, but I had best results with an air core coil.  By putting the 17-159 pf variable cap between the coil and ground (similar to the arrangement shown above) I could tune from 7.010 MHz up to 7.367 MHz. 

You could also replace the variable capacitor with a voltage variable capacitnce diode (aka a varactor or a varicap diode).    I had good results with an MV2301.   

You could try using a cheap little polyvaricon capacitor for C5, but my best results came with an air variable. Walter KA4KXX points out that nice variable capacitors are available here: 

https://www.amplifiedparts.com/products/capacitor-365pf-variable  If you can, get one with a reduction drive to slow the rate of tuning as you turn the shaft.  If you can't get one of these, try to get find a reduction drive to slow down the tuning. 

I ended up using a 3 uH air core coil with a variable cap of around 365 pf this allows me to tune from 7.115 MHz to 7.300 MHz (all of the phone portion of the band) with very good stability -- Juliano Criteria levels of Stability.  

One more idea:   As you build this stage, or right after you finish it, go ahead and build a dulicate circuit, perhaps without the variation components.  Why?  Well that second oscillator might be useful when it comes time to peak and tweak the front end input filter of your receiver.  And that second oscillator can become the start of a second version of this project. 

We talked about this project in SolderSmoke Podcasts #199 #200 and #201

Now I'm going to the beach.  I hope the holiday season bring you all joy -- especially the Joy of Variable Oscillations. Send us reports on your progress, your joy, or your tales of woe. 

The Big Picture

Building the Ceramic Direct Conversion Receiver -- Part 1 Introduction, Stages, Parts.

I hope many of you decide to build this little receiver.  With it, you can break into the ranks of those intrepid ham homebrewers who have actually built a receiver.  Today I'll begin a series of blog posts on how you might do this.  Of course, there are many ways of proceeding.  I will describe my method.

FIRST:  ALWAYS look at this receiver as a collection of stages. Understand what each stage does and how they all work together.   Build it stage-by-stage.  Proceed to the next stage only after you confirm that the stage you just built actually works.

I see this receiver as having four stages:

1.  Front end  (RF gain control, input filter, first RF amplifier).

2.  Mixer

3.  Ceramic resonator variable frequency oscillator (and buffer)

4.  Audio amplifier (consisting of four transistors and associated parts).

In the picture above you can see the four stages.   On the left side of the copper-clad board you can see the Front end: the input filter and the RF amplifier (transistor near the top).  Moving toward the center you can see the mixer stage (around the circular 1k trimmer potentiometer).  Below the mixer (near the big round hole in the Bud Chassis) is the Variable Ceramic Oscillator stage and its buffer amplifier. The right 1/3 of the board is taken up by the audio amplifiers.  Note the use of Manhattan pads throughout.    Click on the picture for a closer look. 

I think you should build the oscillator stage first. 

What you will need:   In most cases, you shouldn't buy individual parts for this receiver.  I won't be providing a BOM.  Here is what I think you should do.  If you do not already have a good stock of electronic parts, start developing one.  Buy assortments of parts, or at least several of each part that you will need.  I use e-bay, amazon, mouser, digikey.   The parts are out there.

-- Get an assortment of resistors.  1/4 watt resistors will do.
-- Get a bunch of .1uF capacitors.  You will use a lot of these as bypass caps.
-- Get a bunch of 2N3904 and 2N3906 transistors. 
-- Get a bunch of 2N2222 transistors
-- Get a bunch of MPF102 and/or 2n2819 FET transistors.
-- Get an assortment of small electrolytic capacitors.
-- Get some Zener diodes in the 6-8 volt range. 
You will need some trimmer caps (8-80pf work fine).  Some 1K trimmer pots.  and some other stuff.

Get some copper clad board.  Pete suggest this, or something like it. 
https://www.ebay.com/itm/18-pcs-4-x-6-CEM-1-060-2-oz-Single-Sided-Copper-Clad-Laminate-Board-PCB/311756276147?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Try to avoid the cheap fiber glass boards.   I prefer single-sided, but double sided is OK too. 

You will need to cut the board.  Get some tin shears.  Mine look like big strong scissors.  Use them to cut your boards to size AND to cut the little isolation pads for Manhattan construction.

Crazy glue.  I kind of like Gorilla Glue liquid (not gel).

Small wattage soldering iron.  35 W or so.   Get a small fan to keep the smoke and glue fumes out of your respiratory system.

   I used a piece of scrap wood to get the variable cap into position. 

Here it is with my fancy Archer Dial.  I used a bit of copper clad board to finish the front panel 

and to support the audio gain control. 

Next time I'll write about how you might build the Variable Ceramic Oscillator stage.