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Saturday, January 21, 2023

A Call for Builders! Please help us Test this Receiver! Please Build this Receiver!

 

This is the Direct Conversion receiver that Dean and I have built.  We plan to have students at a local high school build it, starting in early February.   We would like to have some others build it, to make sure that the design is re-producible without problems.  

Please build this receiver!  But we ask that you build it exactly as per the schematic above and below. Innovation can come later -- for now we just want to make sure this thing works, that there are no errors in the schematic, and that it can be built by the students with minimum woe.  Thanks in advance! 

Dean or others with 3D printers may be able to supply the plastic form for the PTO inductor.  

We know of one other builder, but he is having some trouble.  We would like to confirm that this design is sound.  

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Above is the screenshot of the LTSpice model of the 40 meter Direct Conversion receiver that Dean KK4DAS and I have been working on.  I will post a larger scale version of the picture below.  Click on the images for a better view.  Comments welcome.  Please let us know if you find any errors or mistakes.  Realize that we wanted to keep this all simple, discrete, and entirely analog. 

Here (I hope!) is the net list for the LTSpice model: 


First, one of the surprising things about the LTSpice model: IT IS ALIVE!  I never had a VFO or PTO actually turn on for me in LTSpice.  This one did!  So I just connected the PTO to the Mixer and the receiver works in LTSpice.   I just put an RF signal at the receiver input, and you can see the resulting AF across the 8 ohm resistor at the audio amp output.  I was even able to calculate the precise frequency of the PTO:  7078 kHz.  As in the real world, in an effort to stabilize the frequency, I changed the capacitors to NP0 in LTSpice.  Very cool.  Dean joked that all we need is a way to get RF in and audio out and we will have made an SDR receiver.  

About the receiver:  

--  Four stages that will be built by students Manhattan-style on four copper clad boards: Bandpass filter, diode ring mixer, Permeability Tuned Oscillator (PTO),  AF Amplifier.  

-- The bandpass filter is a simple dual-tuned circuit device based on the info on the QRP Labs site.  (Thanks Hans!)  We out a 10k pot as an RF gain control between the antenna and the filter. 

-- The mixer is a standard diode ring.  We included a diplexer at the output using a circuit from the famous W7EL  Optimized transceiver. (Thanks Roy!) 

-- The Permeability Tuned Oscillator is a very simple and very stable Colpitts design developed by Farhan VU2ESE.  We added a simple FET buffer using the circuit in Farhan's Daylight Again rig.  (Thanks Farhan!) 

-- The AF amp is a very simple three transistor amplifier based loosely on designs from Forrest Mims and from the Herring Aid 5 receiver. Both these designs use just two stages -- we added a third and put an AF gain pot between the first and the second stages. There is an impedance mismatch between the diode ring and the AF amp, but we found that most of the proposed solutions were more trouble than they were worth, so we left it as is.  

--Thanks to Wes W7ZOI for his November 1968 QST article on the solid-state DC receiver. Wes's article inspired our efforts.  

Dean and I have both built these receivers.  They work very well.  Dean has even decoded FT-8 with his. We used Radio Marti at 7355 kHz to test for AM breakthrough -- with the diode ring, the diplexer, and the RF gain control we were able to bring the AM breakthrough down to acceptable levels. You can see many videos of my receiver in action over on my YouTube channel:  (355) SolderSmoke - YouTube

Here is a larger image of the schematic (click for a full view): 


And here is a nicer schematic done by our friend Walter KA4KXX: 



3 comments:

  1. I'm interested; I've been following this project for a while now and have modest building skills likely on par with the students; maybe more experience soldering and reading schematics, but less experience building Manhattan-style.

    Two things that would help, though: A BoM to make sourcing parts easier (bonus points for a CSV so I can import into Tayda and/or Mouser), and a link to the files for the PTO. I have a printer, just not sure if I've seen the model / STL file.

    Also, how soon is the student build, so I know what time pressure there might be?
    - Sam, AI7PR

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  2. Hi Bill,

    I'm reluctant to try to build this receiver, and I hope there'll be a few tweaks before 'general release'.

    It is difficult to meaningfully critique someone else's electronic circuit design unless you know all the criteria under which the design was carried out, and the end goals of the process.

    The final audio stage is my main concern. Q1's dissipation seems excessive, and the model for the output transformer 'looks wrong'. Some indication of the actual transformer device used would help! My choice here would be the LT700: primary inductance around 500mH with about 60ohms resistance.

    The AC load presented ( 8 x (11.486 ^ 2)) to the collector is around 1000 ohms. A swing of 20mA would bring the device close to signal saturation. A quiescent current of 100mA seems excessive.

    Regards,

    Peter.

    p.s. you may get more feedback on your LTSpice simulation if you shared the .asc file.

    ReplyDelete
  3. Hey Peter: I have an updated schematic using a different first audio stage that Farhan suggested, so if you would like a copy, see QRZ.com for my email and contact me. --Walter KA4KXX

    ReplyDelete

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