Listen to Latest SolderSmoke Podcast

Sunday, January 28, 2018

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


Sunday, January 21, 2018

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!

Saturday, January 13, 2018

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

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




Saturday, January 6, 2018

Steve Murphy N8NM on QSO Today

Picture
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)

Monday, January 1, 2018

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. 




Designer: Douglas Bowman | Dimodifikasi oleh Abdul Munir Original Posting Rounders 3 Column