N8NM's "20 Dollar Bill" DC Receiver

Steve N8NM wrote: 

I call it "The $20 Bill" because it contains about $20 in junkbox parts and complies with Bill's discreet component, hardware defined radio ethos.

73 - Steve N8NM

https://n8nmsteve.blogspot.com/2018/02/spontaneous-construction-20-bill-or-oh.html

I replied:  

Excellent Steve.   Very nice.   

Your post caused me to fire up my DC RX -- I was listening on 40 earlier today.  I think the world needs MORE 40 meter direct conversion receivers.  

Too bad about the regulator IC chip.  We need to get you an 8 volt Zener so that you can bring that receiver into a state of discrete component purity.  

DISCRETE HDR FOREVER!  

73  Bill N2CQR 

A Wonderful Troubleshooting Story -- Thailand, Mixers, a Simpson 260, Microwaves, and some Black Tape

My old friend was really fortunate to have had such a good Staff Sergeant instructor at Signal School, someone for whom the mixer trig was obviously not enough.  And our old friend obviously also benefitted greatly from having had a dad who set him up with a Simpson 260 and some handmade experimental glass diodes. Wow.  It all came together with some black tape in Thailand... 

Bill,

Enjoyed your latest blog. I remember your asking about mixers years 

ago.  I received much the same explanation from a Staff Sergeant 

instructor at Ft. Monmouth in 1967.  His example was a mixer with 

diodes, noting the need to have them forward biased by the LO supply.  

We worked out much the same waveforms as shown in your Blog and 

the concept became part of my 'intuitive' knowledge.

A few years later I was fighting 120hz hum on the baseband of an IWCS 

microwave system feeding USAF command at the Korat Air Base in

Thailand. The hum was pretty high level and causing inter-modulation 

problems on the 60 channels of signal sideband suppressed carrier 

being applied to the microwave system.

We ended up with a couple of DCA DoD employees being flown in to help, 

to their credit they were prior service and darn good at what they did.  

After three days of testing all parts of the microwave system with a 

very long distance and long duration phone call to the manufacture in 

Calif, they still had not found the trouble.

I had stayed working with the DCA guys all of the time, during the 

testing I noted the hum seem to lessen in strength with someone standing 

directly behind the radio bay.

I went around to the back and took a close look, Yep! the mixer diodes 

for the baseband order-wire were glass and exposed.

Put a length of black tape over them and the hum went away.  Not the 

power supply problem everyone was fixated on, it was diode photo 

sensitivity.  I guess we could have just turned off the florescent 

lights too.

When I was 10 years old my father showed me how to use a Simpson

260 to check diodes and early transistors*. We were on the floor of the 

living room with sunlight streaming in.  I saw the forward resistance change

a lot when the glass diode was in sun light vs shade. It was this memory 

that prompted me to try the black tape.

All the MW systems in SEA later received a MWO to change out the 

order-wire board and I found that the assembly was a non-standard part 

of the microwave system just for military use.  Civilian deployment of 

that microwave system had no need for the order-wire.

Thanks for the quick trip, for me anyway, down memory lane and the 

memory of being an electronics tech hero for all of two minutes. The DCA 

guys made me buy the first round at the club.

73  from an old friend....

HB2HB QSO with KC1FSZ and his Al Fresco Scratch-built BITX

I had some good luck on the ham bands last weekend.  First, I was called by Bruce KC1FSZ -- this time he was on his Al Fresco scratch-built BITX-on-a-board.  FB.  The next day, I called CQ on what seemed like an empty 17 meter band.  I heard someone come back -- it sounded like DX.  I had to swing the Moxon a bit -- oddly, I thought, to the south-east.  FR5FC was calling me from Reunion Island.   TRGHS. 

Here's a follow-up message from Bruce: 

Hi Bill:

Great to catch up with you on 40m yesterday.  I was using the Peppermint II which is a scratch-build of the BITX-40 for the most part, although I did my own digital VFO/BFO and made a few other modifications in order to be able to use it on 80m.  It took about 8 months of noodling to get the thing to work, but it was a great learning experience. 

As discussed, I’m working on a 24V power supply (LM723 + 2N3055) and a push-pull IRF510 final so that I can get some more power.  

I got a few more Williams-Sonoma Peppermint Bark tins off of EBay so I’m ready to start boxing things up as soon as the linear is working. 

73s,

Bruce KC1FSZ

KD4PBJ's Acorn AM Broadcast Band Regen

From Chris, KD4PBJ: 

This is my AM band regen I built during December and early January. 

It uses a 955 acorn tube and is a really hot performer! I can pick up dozens of stations with only a 20 ft piece of wire thrown out my shop window and tied to a nearby tree limb 5 ft off the ground. This is from rural Tennessee where we have no local AM’s. 

It uses a velvet vernier I bought off eBay back around 1999 or 2000 and had saved for a special project like this. 

I’m running filaments off a 6V lantern battery and plates off a type 415 45V battery. 

A nice ham/machinist I met on the Time Nuts list who lives in San Francisco made my insulated shaft couplings. I got the Delrin rod cheap off eBay. He cut them to length, center drilled for 1/4 inch and drilled each end for 2 setscrews.

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

Mike Rainey and Heavy Metal AM Phone

Michael Rainey AA1TJ wrote:  

I can tell you exactly what's going on here. I'd just received a license upgrade from "Novice" to "General." My new license granted radiotelephone privileges and I was eager to try them out.

In the early 1970's no self-respecting amateur radio operator would dream of using amplitude modulation (AM) on wavelengths above 10m. It wasn't illegal, rather, it was frowned upon due to bandwidth issues, among other things.

But in my excitement - and in the time-honored spirit of, "don't ask permission, ask forgiveness" - I tuned my clunky, Heathkit DX-100 to the 40m radiotelephone band and began calling CQ on AM. Everyone that I contacted was very polite, but to the man they all mentioned how "odd" it was to hear an AM signal on 40m. I eventually took the hint, but not before I'd figured out that yakking on a microphone wasn't my thing after all. Morse telegraphy was my first and enduring love.

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

I think Michael's next phone transmitter was that voice-powered rig that he used in an attempt to cross the Atlantic with the only power source being his vocal cords. But even there, he was using his voice to send Morse.  

C'mon back to radiotelephone Mike.  There is more to life than dots and dashes!

SolderSmoke Podcast #202 Cover-Rig, SKN, Pete's Vector Boards, uBITX, K1BQT rig, MAIL

http://n6qw.blogspot.com/

SolderSmoke Podcast #202 is available: 

http://soldersmoke.com/soldersmoke202.mp3

13 Jan 2018

Opening music from Shel Silverstein and Dr. Hook and the Medicine Show.  

On the Cover of the Rolling Stone or  On the Cover of SPRAT magazine.

Travelogue and Weather Report

-- Bill's trip to the Dominican Republic.  SWL on the beach.  Return to the ice box.  

-- Dramatic events in California. 

-- Reading Chinese Sci Fi with lots of radio in it.  "The Three Body Problem" by Cixin Liu

Straight Key Night.  IN QRP MODE THANK YOU.  With nephew Jeffrey.  Annual event -- worked Jim W1PID  friend of Mike Rainey.  

Steve Murphy and Jeff Damm on QSO Today.  FB. 

Oh your rig is homebrew?  "It must be propagation." 

ER Reading about G3UUR

Hams avoiding 60 meters due to 100 watt limit.  SAD. 

Bill's Bench:    Continuing series on the Ceramic Discrete DC receiver.   Described  the oscillator and the AF amp.  Next we will do the Mixer.   The most interesting stage. Nephew John Henry and niece Helena visiting today. 

Pete's bench.  uBITX adventures.   

The K1BQT IC transceiver. 

The Vector Board building technique.  See http://n6qw.blogspot.com/

MAILBAG

Paul KA5WPL  Looking for project with his son.  Sawdust.  Thanks again  Steve Silverman.

Chris Waldrup OM PBJ thanks for the gift

Pete in contact with Sven Johnson

Chuck KE5HPY sent us picture of fashion model with boatanchors in background  Grayson recalled 73 magazine covers...

Pete WB9FLW        Bilateral  SBE-33 ad  "with inherent stability"  That's the best kind!

Bruce KK0S experimenting with AF amps for the DC receiver

John KE5ETX Attempting CBLA

Towers and Antennas in Church Steeples and on Mt. Washington (VIDEO)

Steve Murphy N8NM on QSO Today

It was really nice to sit back and listen to Eric's talk with Steve Murphy. 
Don't miss this one.  Listen here: 

https://www.qsotoday.com/podcasts/N8NM  

My notes: 

-- On  a Stingray bicycle patrol on garbage night, looking for radio parts.  FB Steve! 
-- Started with SWL.  A fine radio pedigree. 
-- Had an R-390A at age 16.  
-- Uses an LC meter to check on toroid windings. 
-- Steve:  Thanks for the kind words about inspiration. 
-- Manhattan And Ugly.    Mugly!
-- Planker!  Better on a Board!
-- Form Factor First, but then it never fits! Al Fresco! 
-- Packages arrive from China faster than they go across Israel. 
- - E-bay as a really good source for parts. 
-- Oh god, not an S-38E.  Stop torturing yourself Steve. 
-- N8NM: Radio Renaissance Man:  Runs a 2 meter repeater network.  Thinking of 900 MHz. 
-- Papa Legba -- I got it from a W9SCH via SPRAT.  He got it from Voodoo.
N8NM is chickenkiller.com   FB. 
-- Moderation?  Ha!  Good luck with that! 
-- An Electromagnetic Playground where Failure has No Consequences.  Well put. 

-Happy New Year Eric and Steve!  Thanks to  both of you. 

Straight Key Night at N2CQR

I did my part for the retro-Luddite-CW-Straight Key cause.   My HT-37 was kind of wheezing.  I may have to go in there to striaghten it out.  At times I had it cranked back to less than 5W (to maintain some semblence of QRP street cred). 

A highlight was the QSO with Jim W1PID.  Jim is a well-known QRPer and a friend of QRP Hero Mike Rainey.  Jim and I talked last year -- same date, same event, same rig. Jim has a really nice web site: http://www.w1pid.com/

My nephew Jeffrey joined me for the last two contacts.   He got a real kick out of it, both the CW and the SSB. 

31 December 2017

17S  P49MR   Martin – Back in the islands!  Like an annual QSO.

40CW  Pre SKN   W1PID on 40 with my HT-37. An annual event!  Jim running 75 watts

40CW SKN KF5RBR Dan in AR.

1 January 2018

40CW SKN W8HOG SKCC 87750 Perry in Ohio

40CW SKN N1CGP Dave in Maine using J37 key

40CW SKN WB4JJJ Al in Fairfax

40CW SKN K8SRB  Stan  in Ohio (with nephew Jeffrey with me)

17SSB WJ2N Andy in Florida (with Jeffrey in the QSO)

Jeff Damm WA7MLH on QSO Today

Happy New Year!    

There was so much wisdom and tribal knowledge in Eric Guth's interview with Jeff Damm WA7MLH. It was almost overwhelming. 

You should all listen to it.  Twice. At least twice: 

https://www.qsotoday.com/podcasts/WA7MLH
  
My notes:

-- I sympathize with Jeff's decision to go solid state and give up on high voltage after an encounter with an undischarged 600 volt capacitor.  
-- I really like the 1700 kHz IF with a 5 MHz VFO for an 80 and 40 meter receiver. 
-- Interesting that EE degree didn't help much in his efforts to understand ham gear.  Better to read Wes's books and Doug's. 
-- Tek Spectrum Analyzers were specially made to fit down a submarine hatch.  
-- Building and measuring just as important as studying the theory.  Inked-up text books. 
-- Learned ugly from Wes as a teenager. 
--Searched for old commercial gear to gut and use as homes for homebrew solid state gear. The enclosures,  panels and controls are very useful.   Great way to avoid metal work.  These rigs are no longer boatanchors!  Again, I sympathize.  I've sacrificed many Heath Lunchboxes and QF-1s.  
-- Jeff Builds the VFO first.  My preference too. But he understands Pete's AF-first approach. 
-- Finger on the input of the AF amp!  Buzz!  Yea! Step your way back to the front end.
-- ALWAYS one stage at a time.  
-- Osh Park Boards for standard circuit modules.  Like Legos. 
-- Cubic Feet of air variables.   Jeff has a lifetime stash.   
--Thinking about what was and should have been his section of EMRFD. Go for it Jeff. PLEASE! 
-- Hesistant about chips. Analog guy.  Would have been a huge time sink.  Analog guy.  
-- Buying parts on e-bay.  Fewer and fewer RF parts at hamfests. 
-- People reading QST Tech Articles for entertainment. Editor apprach: "Nobody will build it anyway." Handbooks giving priority to entertainment and less to information and education. 

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. 

Caribbean Beach Listening Post

Oh man, I wish I was still there.  It was 85F and very nice on Christmas Eve on the Eastern tip of the Island of Hispaniola in the Dominican Republic. I wasn't transmitting -- this time I was just doing some listening with my trusty Sony SW receiver.  But it did occur to me that the palm trees would have been EXCELLENT antenna supports. 

On December 24, 2017 around midday on 17 meter SSB I heard: 

NE2Q, NO4FR, IZ2LSR, N4DIR, PA1CC, N4LO, EA7JR, MI0TLG, VE1IOU, PA3WB, IU5FFM, KD4POK, HK2RMR, KB5VUM AND KG9DW   I was just using the little telescoping antenna that comes with the Sony. 

On December 26, 2017 also around midday I heard on 20 meter CW K8GL AND ZS1C. They were in contact -- I was located between them. 

I am now back in Virginia where it is 15F, dark and windy. 

The Three Body Problem by Cixin Liu

Strongly recommend this sci fi book.   Lots of physics


Some e!ectronics.  And even a bit of Morse.   Ask Santa.

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.