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Showing posts with label DeMaw--Doug. Show all posts
Showing posts with label DeMaw--Doug. Show all posts

Wednesday, April 26, 2023

Retro QRP Rigs of the 1960's, 70's, and 80's -- Video by Mike WU2D


It is time to put aside (again!) all of the heated ideological arguments about the power level that defines "low power."  Just sit back and enjoy this wonderful trip down QRP memory lane. 

Mike's description of the simple, single-transistor QRP transmitter was really nice.   I recently made something similar: https://soldersmoke.blogspot.com/2023/02/first-qso-with-high-school-receiver-100.html  And Mike does a nice plug for our beloved Michigan Mighty Mite.  Go CBLA! 

The modular idea:  words to live by my friends. 

40673!  TT2!  And G3RJV's PW Severn - indeed, bow your heads!

Wow, the Ten Tec Power Mite (or Might!) -- I still want one.  Same for the Argonaut -- what a great name (sounds like a "magic carpet), and with SSB to boot!  I want to join the Argonaut cult! 

I have both the HW-7 and HW-8 (the HW-8 is heading to the Dominican Republic).  This video makes me want to fire up the HW-7.  Maybe on 40.  

My 40 meter homebrew rig (Digi-Tia) has in it the filter from that old Yaesu FT-7 rig.  The filter was given to me by Steve "Snort Rosin" Smith.  https://soldersmoke.blogspot.com/2015/05/bitx-digi-tia-build-update-2-installing.html  


Thanks to Mike for including me in the credit roll at the end.  What a great group of people -- it is a real honor to be listed with those folks. 

Thursday, April 13, 2023

The Franklin Oscillator: A Super-Stable VFO. Why No Attention? Why So Little Use?

My Franklin VFO

Lee KD4RE of the Vienna Wireless Society has been talking about the Franklin oscillator. He has been telling us that it is very stable, and capable of stable operation up through the ten meter band.  Lee wants to build an direct conversion receiver for all of the HF bands with one of these circuits. 

I was skeptical.  First, I'd never heard of this circuit.  I'd grown up in ham radio on a steady diet of Hartley and Colpitts and Pierce.  Vackar or Clapp were about as exotic as I got.  And second, I'd come to accept that it is just not possible to build a good, stable, simple,  analog VFO for frequencies above around 10 MHz.  For example,  in his Design Notebook, Doug DeMaw wrote, "VFOs that operate on fundamental frequencies above, say, 10 MHz are generally impractical for use in communications circuits that have receivers with narrow filters."  DeMaw was known for resorting to variable crystal oscillators. 

But then this month Mike Murphy WU2D put out two videos about his use of the Franklin oscillator circuit in a direct conversion receiver at 21 MHz.   The VFO was shockingly stable.  I began to believe Lee.   I fired up my soldering iron and built one.  

WU2D's Franklin Oscillator

Lee was right,  it is in fact remarkably stable, even at higher frequencies. My build (see picture above) was a bit slap-dash and could be improved a bit, but even in these circumstances here is what I got.   This was with a stable 6 Volt Supply and with only a cardboard box covering the circuit: 

Local time                  Frequency

0543                           19.1114 MHz  (cold start)

0636                           19.1116 

0804                           19.1117

1034                           19.1118

1144                           19.1117

I started digging around for references to the Franklin.  There was nothing about it in Solid State Design for the Radio Amateur, nor in Experimental Methods of RF Design.  Pat Hawker G3VA (SK) did discuss it in his Technical Topics column in RADCOM, February 1990.  Pat gave a great bio on Charles S. Franklin (born in 1879 and a colleague of Guillermo Marconi). But tellingly, Pat writes that, "Despite its many advantages, the Franklin oscillator remains virtually unknown to the bulk of American amateurs."  

QST "How's DX" August 1947

It wasn't always unknown.   In the 1940s, we see articles about the Franklin oscillator circuit. There is a good one in the January 1940 issue of "Radio." 
 The author W6CEM notes that the circuit "is probably familiar to only a few amateurs."  It shows up in the "How's DX" column (above). And the 1958/1959 issue of Don Stoner's New Sideband Handbook we see a lengthy description of the Franklin oscillator.  Stoner wrote: "The author's favorite oscillator is the 'old time' Franklin, and it is believed to be the most stable of them all! This rock-solid device can put a quartz crystal to shame! Because it represents the ultimate in stability, it is the ideal VFO for sideband applications."  And we see a PTO-tuned Franklin oscillator in the July 1964 QST. And it is in the fifth edition of the RSGB Handbook (1976). 

Here is the January 1940 "Radio" with the Franklin oscillator article on page 41 by W6CEM: 
Here is the July 1964 QST article: 

There was an article about the Franklin oscillator in 73 magazine by W4LJC in February 1999: 
The author notes that: 


Much more recently (2022), Mike WN2A, modified his Mousefet transmitters (seen in QRP Classics in 1992) to include the use of the Franklin VFO circuit.  Mike's documentation is really excellent.  Kostas SV3ORA has a Franklin oscillator in his Super VFO circuit.  Hans G0UPL has one on his site. 


Look, there may be reasons why the Franklin oscillator has been ignored.  But the circuit sure seems to present a lot of advantages.  Stable operation beyond the 10 MHz barrier is the big one.  Simplicity is another.  If there are problems and shortcomings, let's hear about them. But it seems as if the Franklin oscillator may provide the opportunity for us to build stable VFOs beyond 10 MHz without resort to complicated PLL stabilization techniques, and without opting to go with an Si5351 or other complex digital devices.   

So let me ask:  Why hasn't the Franklin oscillator been given more attention, and why haven't we seen more use of this circuit by hams or even by manufacturers?  

Tuesday, April 11, 2023

Arnie Coro: Jaguey Rig Designed in 1982, More info on the Rig

Jaguey, Matanzas, Cuba

Dxers Unlimited's mid week edition for 23-24 October 2007

By Arnie Coro
Radio Amateur CO2KK
...

My own personal experience with the original JAGUEY direct conversion 
transceiver, designed way back in 1982, is that when used with a well 
designed front end input circuit, those receivers provide amazing 
sensitivity, with signals as low as 1 microvolt easily detected but, 
they do have one drawback, their selectivity or ability to separated 
between stations is very poor. The direct conversion radio receivers are 
used for picking up CW Morse Code Signals , Digital Modes and Single 
Side Band, but they are not good for receiving AM signals, and can't 
pick up FM modulated signals at all...

The original JAGUEY 82 Cuban designed single band amateur transceiver, was tested against a sophisticated and really expensive factory built 
transceiver. The tests showed that our design was at least as sensitive 
as the very expensive professional equipment, registering a measured 
sensitivity of less than one microvolt per meter, producing perfect CW 
Morse Code copy of such a signal. Adding well engineered audio filtering 
to a direct conversion receiver can turn it into a really wonderful 
radio by all standards amigos. 

Radio is a fun hobby, and believe me amigos, there is nothing more 
magical than listening to a radio receiver you have just finished 
building !!!

-----

Peter Parker VK3YE Found a nice description of the Jaguey by Cuban radio Amateur Jose Angel Amador from the BITX40 Facebook Group: 

A translation.  This was apparently in response to someone who thought they'd found a Jaguey schematic: 

"That's not an original Jaguey, that was a simple, single band, unswitched, 5 watt, DSB, kit for beginners with no gear and needing something to put on the license.
Carbon microphone direct to balanced modulator, two stages with 20 dB gain, W1FB/W1CER style feedback, and final with 2 x 2N2102 class B.
The receiver was more like that of the schematic, with a TAA263, easy to get from the FRC in 1978, and headphones. No need for an RF stage: the mixer was overloaded at night with European broadcasts above 7150.
The VFO is also inspired by Solid State Design for the Amateur Radio, a Colpitts with 2SC372 and a low gain feedback buffer with two 2SC372s.
Binocular ferrites were taken from Soviet TV baluns. The conditions of Cuba 1978.
Today I would make an SSB rig with polyphase networks, mixer with 4066,  and VFO Si5351.
The big complication of BitX is the crystal filter, they either get it made, or stick to a recipe, but few have what is needed to measure and tinker with crystal filters.

Friday, December 2, 2022

But why? Why Can't I Listen to DSB (or AM) on my Direct Conversion Receiver?

I've said this before:  I just seems so unfair.  We just should be able to listen to DSB signals with our beautifully simple homebrew Direct Conversion receivers. I mean, building a DSB transmitter is a natural follow-on to DC receiver construction.  And we are using AM shortwave broadcast stations (Radio Marti --I'm looking at you)  to test our DC receivers for AM breakthrough.  But when we tune these stations in, they sound, well, awful.  So unfair!  Why?   Unfortunately it has to do with laws.  Laws of physics and mathematics.  Blame Fourier, not me.  

Over the years there has been a lot of handwaving about this problem.  From Doug DeMaw, for example: 

In his "W1FB's Design Notebook," Doug wrote (p 171):  "It is important to be aware that two DSSC (DSB) transmitters and two DC receivers in a single communication channel are unsatisfactory.  Either one is suitable, however, when used with a station that is equipped for SSB transmissions or reception. The lack of compatibility between two DSSC (DSB) transmitters and two DC receivers results from the transmitter producing both USB and LSB energy while the DC receiver responds to or copies both sidebands at the same time."

That's correct, but for me, that explanation didn't really explain the situation.  I mean we listen to AM signals all the time.  They produce two sidebands, and our receivers respond to both sidebands, and the results are entirely satisfactory, right?  Why can't we do this with our Direct Conversion receivers?  I struggled with this question before: https://soldersmoke.blogspot.com/2015/07/peter-parker-reviews-dsb-kit-and.html  You can see in that post that I was not quite sure I had the answer completely correct. 

It took some discussion with a fellow Vienna Wireless Society member, and some Googling and Noodling for me to figure it out.  But I think I've got it: 

Imagine a station transmitting a DSB signal at 7100 kHz with a 1 kHz tone at the AF input.  There will be signals at 7101 kHz and at 7099 kHz.  Assume the carrier is completely suppressed. 

We come along with our DC RX and try to tune in the signal. 

Remember that they heart of the DC RX is a product detector, a mixer with the VFO (or PTO) running as close as we can get it to the suppressed carrier frequency (which we can't hear).  

Lets assume that we can somehow get our VFO or PTO exactly on 7100 kHz.  The incoming signals will mix with the VFO/PTO signal.  We are looking for audio, so we will focus on the difference results and ignore the sum results of the mixing.  

The difference between 7101 and 7000 is 1 kHz.  Great! And the difference between 7099 and 7000 is 1 kHz also.  Great again, right?  We are getting the desired 1 kHz signal out of our product detector, right?  So what's the problem?  

Here it is: SIDEBAND INVERSION.  Factoring in this part of the problem helps us see the cause of the distortion that plagues DSB-DC communication more clearly. 

Remember the Hallas Rule:   Whenever you subtract the modulated signal FROM the unmodulated signal, the sidebands invert.  So, in this case, we are subtracting that 7099 "lower sideband" signal FROM the 7100 VFO/PTO signal.  So it will invert.  It will become an upper sideband signal at 1 kHz.  We will have two identical 1 kHz signals at the output.  Perfect right?   Not so fast. Not so PERFECT really.  

The perfect outcome described above assumes that our VFO/PTO signal is EXACTLY on 7100 kHz.  And exactly in phase with the suppressed carrier of the transmitter.  But if it is even SLIGHTLY off, you will end up with two different output frequencies, signals that will move in and out of alignment, causing a wobbling kind of rapid fade-in, fade-out distortion.  You can HEAR this happening in this video by Peter Parker VK3YE, starting at 6:28:

And you can see it in this LTSpice simulation.  


This LTSpice model just shows two diode ring mixers.  The transmitter is on the top, the receiver is on the bottom. The transmitter has RF at 7100 kHz at L1 and audio at 1 kHz at R1.   The receiver has the VFO at 7100.001 L7,  DSB from the transmitter at L12 with audio appearing at R4.  It is instructive to watch the output as you move the VFO frequency.  If you move the VFO freq away from the transmit carrier osc frequency you will see the distortion.  Here is the netlist for the LTSpice simulation: 


On paper, using simple mixer arithmetic, you can tell that it will be there. With the VFO/PTO just 1 Hz (that's ONE cycle per second) off, you will end up with outputs at 1.001 kHz and at .999 kHz.  Yuck.  That won't sound good. These two different frequencies will be moving in and out of alignment -- you will hear them kind of thumping against each other.    And that is with a mere deviation of 1 Hz in the VFO/PTO frequency!  We are scornful when the SDR guys claim to be able to detect us being "40 Hz off."  And before you start wondering if it would be possible to get EXACTLY on frequency and in phase, take a look at the frequency readout on my PTO.  

Now consider what would happen if the incoming signal were SSB, lets say just a tone at 7101 kHz.  We'd put our VFO at around 7100 kHz and we'd hear the signal just fine.  If we were off a bit we'd hear it a bit higher or lower in tone but there would be no second audio frequency coming in to cause distortion.  You can hear this in the VK3YE video:  When Peter switches to SINGLE Sideband receiver, the DSB signals sound fine. Because he is receiving only one of the sidebands. 

The same thing happens when we try to tune in an AM station using a Direct Conversion receiver:  Radio Marti sounds awful on my DC RX, but SSB stations sound great. 

My Drake 2-B allows another opportunity to explore the problem.  I can set the bandwidth at 3.6 kHz on the 2-B, and set the passband so that I will be getting BOTH the upper and the lower sidebands of an AM signal. With the Product Detector and the BFO on,  even with the carrier at zero beat  AM sounds terrible.  It sounds distorted.  But -- with the Product Detector and BFO still on --  if I set the 2-B's  passband to only allow ONE of the sidebands through,  I can zero beat the carrier by ear, and the audio sounds fine. 

There are solutions to this problem:  If you REALLY want to listen to DSB with a DC receiver, build yourself a synchronous detector that gets the your receivers VFO EXACTLY on frequency and in phase with the transmitter's oscillator.  But the synchronizing circuitry will be far more complex than the rest of the DC receiver. 

For AM, you could just use a different kind of detector.  That will be the subject of an upcoming blog post. 

Please let me know if you think I've gotten any of this wrong.  I'm not an expert -- I'm just a ham trying to understand the circuitry. 

Wednesday, November 2, 2022

Understanding a Very Simple Two-Diode Mixer

 

Take a look at the simple little mixer above.  I think I first saw it in SPRAT.  Thinking that it was really just a simplified version of the two diode Doug DeMaw mixer that I had been using for years, I couple of years ago I built it into a little Direct Conversion receiver.  It worked great.  But later, I began to have doubts about it.  In the words of young James Clerk Maxwell, I started to wonder about "the particular go of it." 

You see, the way the DeMaw mixer is set up,  both of the diodes are simultaneously on and off.  This has the effect of "chopping up" the incoming RF at a rate set by the VFO frequency.  Boom.  Fournier.  Mixing.  Great.  


But look at the mixer at the top of this post.  Here the VFO signal is coming in on the wiper of the 1k pot. The same signal is hitting both diodes at the same time.  The diodes are not being fed differentially.  So D1 and D2 are NOT both simultaneously tuning on and off.  Instead, when the wiper goes positive, D2 turns on while D1 is off.  On negative swings of the voltage at the wiper, D1 turns on while D2 is off.  For me, this made it a "mystery mixer." 

This reminded me of the sub-harmonic DC receiver I built earlier in the year:  The VFO runs at half the operating frequency, but the diodes are set up to switch on and sample the RF TWICE each VFO cycle.  This is the equivalent of having the VFO at the operating frequency.  


Could it be that this was just a sub-harmonic mixer with the VFO at the operating frequency? (I should note that Doug DeMaw published a design that actually made this mistake.  See:  https://soldersmoke.blogspot.com/2011/07/doug-demay-and-polyakov.html ) I knew that this would sort of work, but it would not work very well.  And the mystery mixer seemed to work very well.  Hmmm. 

I was loaning the DC receiver with the mystery mixer in it to a local high school.  I worried that I was loaning them something that I didn't really understand. I remembered that I'd been trying to figure out this mixer since early 2021:  https://soldersmoke.blogspot.com/2021/02/some-thoughts-on-singly-balanced-mixers.html  

Our beloved book, Solid State Design for the Radio Amateur (SSDRA) has an explanation of this circuit on page 74.  But this explanation didn't seen to work for me.  Check it out. YMMV. 

Bottom line:  I still couldn't figure this circuit out, so left it alone for while.  

The other day I woke up and looked at it with fresh eyes.  Suddenly it hit me.  Although the VFO was hitting the diodes in the same non-differential way as is done in the sub-harmonic mixer,  the RF (signal) is entering the mixer in a differential way.  This means that the two diodes are taking turns sampling the upper side of L2, then bottom side of L2, via L1 and L2.  This results in a complex repeating waveform that is similar to that of diode ring mixer.  Within that complex repeating waveform, there are sum and difference frequencies. I did some noodling on this: 


The key difference between this mixer and the sub-harmonic mixer is the way L2 is positioned:  In the sub-harmonic mixer, there is no differential feed of the RF.  Both diodes get the same polarity of RF.  The VFO switches on D1, then D2.  The RF is sampled at twice the VFO frequency.    But in the mystery mixer that had me scratching my head, the RF is fed to the diodes in differential form.  So while the diodes here are -- as in the sub-harmonic mixer -- being switched on and off sequentially, they are taking turns sampling the top and the bottom of L2.  That provides the complex repeating waveform that we need to get the sum and difference frequencies.  In a DC receiver the difference frequency is audio. 

What do you guys think?  Do I have this right?  How would you characterize this mixer:  Is it multiplying by 1 and 0?  Or is it multiplying by 1 and -1? 

This would be good mixer for a school project.  It is simpler than a mixer with a tri-filar toroid. 

Friday, September 16, 2022

Fixing Up An Old Homebrew Rig -- Barebones Superhet and VXO 6 Watter

 
I'm not exactly sure why I pulled this old rig off the shelf, but I'll write up what I did -- I often use this blog as a kind of notebook.  I can look back and easily see what I did on my last encounter with the rig. 

The receiver is Doug DeMaw's Barebones (aka Barbados) Superhet.  This was my first superhet receiver. I built in in 1997.   The transmitter was my first real homebrew project -- it is the VXO 6 watter from QRP classics.  I built it in the Dominican Republic, probably in 1993 or 1994.  I built the power supply so that I could say that the entire rig is homebrew. 

This rig is getting a bit long in the tooth:   The receiver is built with 40673 Dual-Gate MOSFETs, an some of the transistor cans have gotten rusty.  The frequency readout on the receiver is the top of a coffee can fitted onto the reduction drive behind the tuning knob from a Drake 2-B (not MY 2B!). 

Here are two 2013 videos that I did on this receiver: 

-- I put the crystal filter back in CW mode.  I had widened it so that I could listen to 20 meter SSB, but I decided to go back to its original configuration.  When I built the receiver in 1987, I didn't characterize the crystals -- I just used the capacitor values that Doug DeMaw had in his article.  I pretty much did that again this time, just putting caps that are close in value to what Doug had.  DeMaw used color burst crystals at 3.579 MHz.  So I guess this would be a GREAT receiver for the Color Burst Liberation Army!  

-- I used My Antuino (thanks Farhan!) to check the passband.  Here is what it looks like.  I just put the Antuino across the 10k resistors on either side of the input and output transformers.  The coil cores had become very loose -- I just tried put them in the right place.  I may need to put some wax in there to allow them to better stay in place. I think they could have used toroids instead -- that would have been easier. One of the transformer connections was open -- they don't work well that way,  once I fixed that, the passband looks like this: 


-- Each of the horizontal divisions is 500 Hz.  The passband is not pretty, but it is OK, and I  didn't feel like doing too much work on this to get it in better shape. 

-- The filter peak was a bit lower in frequency than expected.  I found that trimmer cap C3 in series with the BFO crystal would not allow me to lower its frequency sufficiently.  So I moved C3 to a position in parallel with the crystal.  With this mod, I could get the BFO frequency to 3578.69.  This produces a 690 Hz tone when the received signal is at the peak of the IF passband.  Opposite sideband rejection is quite good. 

March 2013 Rebuild of the VXO 6 watter

-- I didn't have to do any real work on the transmitter.  The RF amplifier in the transmitter had served for a time as the RF amp in by 17 meter DSB rig (I had added a bias circuit, which I removed when I put the amplifier back in Class C).  Some time ago I rebuilt the oscillator circuit (which had been literally cut off the board when I used the amplifier in the DSB rig). 

-- I did have to reconfigure the muting circuit -- the T/R switch in the transmitter switches the antenna and also -- through a two wire circuit -- cuts off 12 V DC to the transmitter when in receive mode. 

-- For sidetone I just put a small piezo buzzer through a 1k resistor between 12 V DC and the key line. 

It all worked fine -- I talked to three stations on the high end of  the 20 meter CW band. 

Tuesday, July 19, 2022

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: 

 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!   

Monday, February 7, 2022

SolderSmoke Podcast #235 NE-602, Azores Rig, Spur Problems, SSB Rigs, Peashooter, HB Filters, MAILBAG


SolderSmoke Podcast #235 is available for download: 

http://soldersmoke.com/soldersmoke235.mp3

Travelogue:

6 weeks in the DR for Bill
One contact on uBITX. More SW listening.
Repaired my Chrome Book in Santo Domingo!
Christmas Present for All: James Web Space Telescope launch

Bill's Bench
-- Understanding the NE-602 (see blog post)
-- Thinking about a 17/12 dual bander.  Looked at old G3YCC Tx for circuits... 
-- Using Spurtune08. WB9KZY found it.  In the LADPAC zip file here:
-- Then Put G3YCC Acores SSB TX back in operation
-- Now working many stations with this old "split" TX/RX

Pete's Bench
-- Dean's VWS build of your DC RX
-- Homebrew Crystal Filters
-- The shrinking of the PSSST

Bad Dead Soldering Stations
-- My X-Tonics 4000 dies.   But it left behind a great box with ample socketry. 

Mailbag: 
AA1TJ Mike Rainey --- Again in the Hobbit Hole! 486 kc RX
Thomas K4SWL -- Radio Astronomy and the Raspberry PI 
WC8C Dennis Invite to the L'Anse Creuse ARC   FB  Fun
Todd K7TFC got boosted at Tektronix Beaverton Ore.  
What happened to Chuck Adams K7QO?  His work taken off the net.
W1MJA ex WN2RTH 
N7DA worked W7ZOI in Sweepstakes.  FB
Kirk NT0Z formerly of ARRL HQ
Farhan VU2ESE was up in the Pench Forest, trying to spot a Tiger! 
We spoke to Farhan's Lamakaan ARC in Hyderabad.  QO-100 beam down! 
Dean KK4DAS's 16 watter.   On SS blog
Scott WA9WFA  Bad 6U8s?   Ordering 6EA8s
Bruce KC1FSZ Peppermint Bark gift box
Bob Scott KD4EBM So many good ideas and links 
Rogier PA1ZZ sending parts packages -- Thanks Rogier

Tuesday, December 7, 2021

Junk Box Sideband from the Azores (2004 QST Article)


About 20 years after I first built it, I find myself working on and using this SSB transmitter.    I recently added some impedance matching to the Swan 240 crystal filter;  several years ago I replaced the PA with a "JBOT" amplifier designed by Farhan VU2ESE.  I now have it on the air, using it with a highly modified Doug DeMaw, Barebones "Barbados" superhet  receiver. I had my first (recent!) QSO with this station yesterday, with Les 6Y6Y on the beach in Negril, Jamaica. 

More on this project in due course. Lots of soul in this machine. 

I'd forgotten about this article -- thanks to Pete Eaton for reminding me. Click on the images for a better look at the article. For an even clearer view, download the images and then open them on your computer. 


Tuesday, November 30, 2021

Putting a Barebones Superhet on 17 Meters with an NE602 Converter (Video)


Armed now with a NanoVNA, I took a look at the passband of the 5 MHz filter in my Barebones Superhet (BBRX)  W4OP built it on a Circuit Board Specialist Board.  He put a 5 MHz CW filter in there;  I broadened the passband for phone by changing the values of the capacitors. Here is what the passband now looks like in the NanoVNA: 


This is what DeMaw would call an "LSB filter."  You would get much better opposite sideband rejection by using it with an LSB signal, placing the BFO/Carrier Oscillator slightly above the passband, in this case near 5.002 MHz. 

When I first built the down converter to get the 18.150 MHz signal down to the 7 MHz range (where I had the receiver running) I used an 11 MHz crystal for the NE602's local oscillator.  But this created a big problem:  18.150 - 11 =   7.150 MHz.  That is in the 40 meter band, but note:  NO SIDEBAND INVERSION.   Then in the BBRX  7.150 MHz - 2.150 MHz = 5 MHz  (the filter frequency) but again:  NO SIDEBAND INVERSION.   The signal started as a USB signal and remained a USB signal. 

I briefly tried shifting the BFO frequency to the other side of the filter passband.  If I could get it to around 4.985 MHz, it might work, but because the filter passband was so large, and because the crystal frequency was so low, I was unable to shift the crystal frequency that far.  In any case the results would have been less than ideal because of the "LSB" shape of the filter.  Back to the drawing board. 

I decided to cause one sideband inversion. 

At first I put a 25.175 MHz crystal module in my down converter.  This shifted the 17 meter phone band down to the 40 meter CW band.  It worked, but I cold hear strong 40 meter CW  signals being picked up by the wiring of the receiver (the box is plastic!).  I went back to the module jar in search of frequency that would move 17 meter phone to the 40 meter area (so I would not have to re-build the BBRX front end) but outside the actual 40 meter band.  

I ended up using a 25 MHz crystal in the down converter. 25 MHz - 18.150 MHz = 6.85 MHz WITH SIDEBAND INVERSION.  After checking on the NA5B Web SDR to see that there are no strong signals in the 6.835 to 6.89 MHz range, I retuned the output circuit on the converter and tweaked the input capacitor on the Barebones.  I shifted the VFO frequency down to 1.835 to 1.89 MHz and put the BFO at 5.002 MHz.   The receiver was inhaling on 17 meter SSB.  

One more change to the BBRX:  in his June 1982 QST article, DeMaw warned that trying to get speaker level audio out of the 741 op amp that he used would result in audio distortion.  And it did.  So I put one of those little LM386 boards I have been using into the BBRX box.  I just ran audio in from the wiper of the AF gain pot.  It sounds good.  

In effect this is my first double-conversion receiver.  I usually prefer single conversion, but this project has highlighted for me one of the advantages of double conversion for someone like me who eschews digital VFOs:   Starting with a crystal filter at 5 MHz,  with double conversion I could keep the frequency of the LC VFO low enough to ensure frequency stability.  That would have been impossible with a 5 MHz IF in a single conversion 17 meter rig.  But if I were starting from scratch for a 17 meter rig, I could stick with single conversion by building the filter at 20 MHz,  keeping the VFO in the manageable 2 MHz range. 

Now, on to the SSB transmitter.   The Swan 240 dual crystal lattice filter from the early 1960s needs some impedance matching. 

Saturday, November 6, 2021

M0NTV's "Crystal Filters for the Fearful" (video)


I really liked Nick M0NTV's approach to making a crystal filter (see video above).  He really simplifies a process that desperately needs simplification. I remember when I was building my first superhet receiver,  I came across Doug DeMaw's schematic for a crystal tester that would allow me to properly build the filter.  But the piece of test gear was far more complicated than the receiver I was building.  I never built Doug's device. 

Nick's technique is simpler even than the G3UUR method that many of us have been using for years.  Nick dispenses -- wisely I think -- with the need to calculate motional parameters, Q,  and equivalent series resistance.  This also eliminates the need to fidget around with the design software such as Dishal or AADE.  

Nick uses the Cohn topology (good choice) and uses kind of an "informed cut-and-try" technique to come up with the capacitor values.  

Filter impedance is determined with series trimmer resistors and the NanoVNA to watch the resulting passband.  Nick says this is a Charlie Morris ZL2CTM suggestion.  It obviously works very well -- the ripple that would result from impedance mismatch is eliminated.  

Nick's determination of the best turns ratio for the impedance matching transformers is brilliant.

Nick apologizes for what he says is a long video.  But it is only 30 minutes or so long, and if you are going to build your own superhet or SSB filter rig, it is well worth watching.   

Three cheers for Nick and for Charlie!  Thanks guys!   

Tuesday, July 27, 2021

QST Recognized Error on Sideband Inversion, But Continued to Make the Same Mistake

 

I don't really know if this is good news or bad news.  It's good that in November 1985 they recognized the error, but then they allowed the same error to be repeated by the same author in the 1989 article "A Four-Stage 75-Meter SSB Superhet," and again in 1990 in W1FB's Design Notebook.   It also made it into the 2002 ARRL Handbook.  

Thanks to Chuck WB9KZY for alerting us to this Feedback piece. 

Monday, July 26, 2021

QST Repeatedly Got Sideband Inversion Wrong

It kind of pains me to do this. These articles are from a long time ago, and the author is an esteemed  Silent Key,  but the myth about the origins of the USB/LSB convention is still out there, and as a homebrewer of SSB gear I feel obligated to point out these examples of the error that that myth is based on.   

Last Friday, Pete WB9FLW and I were talking about homebrewing SSB rigs.  I recommended a series of QST articles by Doug DeMaw.  "Beginner's Bench:  The Principles and Building of SSB Gear" started in QST in September 1985. There were at least five parts -- it continued until January 1986. (Links to the series appear below.) I hadn't looked at these articles in years, but when I did, a big mistake jumped right out at me:  In the first installment, on page 19, Doug  makes the same mistake that he made in his Design Notebook:

"Now comes the conversion section of our SSB generator.  We must move (heterodyne) the 9-MHz SSB signal to 3.75-4.0 MHz. Our balanced mixer works just as it does in a receiver. That is, we inject the mixer with two frequencies (9 MHz and 5 MHz) to produce a sum or a difference output frequency (9 - 5 = 4 MHz, or 9 +5 = 14 MHz) If we are to generate 75 meter SSB energy, we must chose the difference frequency. We could build an 20-meter SSB transmitter by selecting the sum of the mixer frequencies. The RF amplifiers and filter (FL2) that follow would then have to be designed for 14-MHz operation.  In fact, many early two-band homemade SSB transmitters were built for for 75 and 20 meters in order to use this convenient frequency arrangement.  The use of upper sideband on 20 meters and lower sideband on 75 meters may be the result of this frequency arrangement (the sidebands become inverted when switching from the difference to the sum frequency.) " 

Those last two sentences are incorrect.  They repeat the "Myth," or the "Urban Legend" about the origins of the LSB/USB convention.  Contrary to what many hams now believe, with 9 MHz filter and a 5.2 MHz BFO it takes more than just switching from sum frequency to difference frequency to invert one of the sidebands. 

There are two conditions needed for sideband inversion to take place:  

1) You have to be taking the difference product (DeMaw got that right) 

2) The unmodulated (VFO or LO) signal must be larger than the modulated signal. (DeMaw and the ARRL obviously missed that part.  Repeatedly.) 

This is another way of stating the simple, accurate and useful Hallas Rule:  Sideband inversion only occurs when you are subtracting the signal with modulation FROM the signal without modulation. 

For DeMaw's claim to be correct, one of the SSB signals going into the balanced mixer would have to invert, and the other would have to not invert.  Let's see if that happens: He has the sideband signal being generated at 9 MHz and the VFO running around 5 MHz. 

9 - 5 = 4  But we are not subtracting the modulated signal FROM the unmodulated signal.  SO NO INVERSION

9 + 5 = 14   We are not subtracting at all.  SO NO INVERSION.  

Doug's convenient frequency scheme WOULD work if he'd just switch the frequencies of the filter and the VFO.  With a sideband generator on 5.2 MHz and a VFO around 9 MHz you do get the happy 75  LSB, 20 USB arrangement without the need to switch the carrier oscillator/BFO frequency.   That is what happened in the Swan 240, and that is what I have in my Mythbuster rig. I am listening to both 75 LSB and 20 USB without changing the carrier oscillator/BFO frequency.  My filter/BFO/product detector is set up for USB.   With this arrangement the 75 meter LSB signals DO invert, and the 20 meter USB meter signals do not, so both are able to make use of my USB BFO/product detector without shifting the BFO frequency. 

This error shows up again in DeMaw's the May 1989 QST article "A Four Stage 75-meter SSB Superhet" (reprinted in the ARRL's QRP Classics book).  Here he writes: 

"Should you want to cover both the 75- and 20-meter bands you can build a 20-meter version of FL-1 and band switch the two filters. As with the 75 meter only version, an IF of 9.0 MHz (Y1) is required. With this arrangement the 20 meter band will tune backwards from the 75 meter band, but upper- and lower-sideband reception will occur, as required, without changing the BFO frequency (Y2). This two band scheme with a 5-MHz VFO is an old one!"   NOTE: FL1 is the bandpass filter, not the IF filter.  

Doug's mistakes in this area may simply be due to the fact that he was more of a CW guy.  And this is something that is quite easy to confuse:  9 and 5 will get you to 75 and 20, but you have to make sure the VFO is at 9 if you want to make use of sideband inversion and avoid having to shift the BFO/ carrier oscillator.   I've made this mistake myself: 

In October 1993 I wrote to DeMaw about his Four Stage 75 meter SSB Superhet.  I think I was looking for details on how to put it on 20 meters.  As I recall, Doug wrote back telling me to just pick 20 meter values for the input bandpass filter.  Had I done so, I would have discovered that  -- for the reasons cited above -- this just wouldn't have worked on 20.  His BFO and filter were set up to receive LSB signals. That's fine for the incoming 75 meter LSB signals.  But on 20 -- contrary to DeMaw's thinking -- there would be NO sideband inversion. I'd be trying to listen to 20 meter USB signals with a receiver set up for 20 meter LSB.  

Did anyone else notice these errors.  Were there ever errata notices in QST on this?  

This is a reminder that you should take all technical articles and schematics with a grain of salt.  Many contain errors. We are all human, and this is a complicated subject with lots of details. 








Thursday, July 22, 2021

The Unicorn! A 75 LSB /20 USB Receiver (That Can't Work)

 

Don't get me wrong -- I'm a huge fan of Doug DeMaw.  His books and articles are a treasure trove for ham radio homebrewers.  Also, Doug was an honest guy who admitted in the preface to his QRP book that at times he did not fully understand the circuits he was building; that kind of honesty is rare,  and is very helpful to amateurs who struggle to understand the circuits we work on.  

But everyone makes mistakes, and Doug made one in his "W1FB Design Notebook."  I present it here not as a "gotcha" effort to nitpick or sharpshoot a giant of homebrew radio, but because this error illustrates well the depth of the 75 LSB/20 USB myth, where it comes from, and how important it is to really understand sideband inversion.     Here is the mistake: 



That's just wrong.  A receiver built like this will not allow you to listen to 75 LSB and 20 USB "without changing the BFO frequency." (Am I the first one to spot this error?  Didn't anyone build this thing, only to discover that it, uh, doesn't work?)

Here's a little drawing that I think illustrates why the mythical scheme will not work: 


All confusion about sideband inversion could be avoided with the simple application of what I think we should call "The Hallas Rule"

"Sideband reversal occurs in mixing only  if the signal with the modulation is subtracted from the signal that isn't modulated."  

Be careful here:   I think some arithmetic carelessness is responsible for much of the myth. Taking the difference frequency is not enough to produce sideband inversion. Read the Hallas Rule carefully:   For sideband inversion to occur, the signal with the modulation must be subtracted FROM the signal without the modulation.
--------------------------------------------- 
About the Swan 240's SSB generation scheme: 

I first stumbled on this problem when building my first SSB transmitters in the Azores.  I was using a VXO,  and a filter pulled out of a Swan 240 (5.173 MHz).  I started with VXO crystals at around 12.94 MHz.  The rig worked,  but I couldn't pull the VXO crystals very far.  So I switched to crystals at around 23.3 MHz (you can pull higher frequency crystals farther).  But look what happened:  My Carrier Oscillator frequency had been set up to receive USB signals on 17 Meters.  With the 12.94 MHz rocks, that worked fine:   18.150-12.977 = NO INVERSION.  But it all changed when I went to the 23 MHz VXO rocks:  23.323-18.150 = INVERSION!   This had me scratching my head a while.  I had to draw myself little spectrum pictures (like the one above) before I realized what had happened.  To get it to work -- to get it to produce USB on 17 meters -- I had to move the Carrier Oscillator to the other side of the passband. Good thing that Swan 240 came with TWO BFO crystals (5.1768 MHz and 5.1735 MHz). I just had to change the crystal. 

For 75 and 20 meters, the Swan 240 uses the correct 5.173 MHz filter with a 9 MHz VFO to get the happy situation of 75 meter LSB and 20 Meter USB WITHOUT changing the BFO/Carrier Oscillator frequency.  This is the Mythbuster scheme.  Unlike Doug's receiver, it works.  The scheme also works in the Swan 240 on 40 meters because for 40 the Swan rig has the VFO running from 12.073 MHz to 12.513 MHz. Here too, no change in the BFO/Carrier Oscillator  frequency is needed. But the Swan recommended a modification that would allow operation on 20 LSB and 75/40 USB!  It used a BFO/Carrier Oscillator crystal of  5.1765 MHz and a switch mounted on the front panel.  Luckily,  my junker Swan (acquired from HI8P in the Dominican Republic) had the second crystal -- mine was 5.1768 MHz.  It was that crystal that allowed me to get my Azorean SSB transmitter to work using the 23.9 MHz VXO rocks.    

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