Dave Richards AA7EE

July 29, 2012

Half Of The ZL2BMI DSB Transceiver – A Simple 80M Direct Conversion Receiver :-)

A few weeks ago, Jason NT7S mentioned the ZL2BMI DSB transceiver as a rig I might be interested in building. He was right – I had seen it in SPRAT but for some reason hadn’t seriously considered making it into a project. The mention from him somehow got me to take another look at it and, well, it was such a simple design, it didn’t seem as if I had anything to lose by giving it the old college try.

The great thing about building a transceiver is that if the transmitter part doesn’t work, you’ve still got a receiver. That’s what happened to me (or at least, until I figure out my problems with the transmitter.) Everything was working fine up to and including the building of the driver stage. Once I added the BD139-16 final, I started experiencing problems with a constant residual carrier being transmitted when no modulation was present.

However…….once I had finished the first part of the build, which consisted of building the receiver, I took a little time to enjoy listening to the receiver and generally being surprised that such a small collection of parts would allow me to listen to the ragchewing on 75M. Little things like touching a wire to the antenna terminal and hearing atmospheric noise coming out of the speaker always give me a kick.

Here’s the schematic for my (only ever so slightly different) version of the receiver part of the ZL2BMI DSB transceiver:

There are no RF or AF gain controls in this schematic.  The circuit is still on a board, not yet in an enclosure, and in the experimental stage. If I ever get it into a box, I’ll add an RF attenuator pot in the antenna input circuit. This is even easier to build than a receiver with a VFO, as there are fewer toroids to wind. In fact, there is only only one – the single antenna input inductor.  Coverage was about 3911 – 4009KHz, so I didn’t bother winding a rubbering inductor, figuring that 100KHz of coverage was already pretty good for such a circuit.

The frequency drift wasn’t very encouraging. I was expecting a little better from a ceramic resonator VXO, being around 200 – 300Hz/hour upward drift after an initial 15 minute warm-up period.  The free-running VFO in my 40M NE602 CW DC RX had a better stability – on 7MHz! This board wasn’t in an enclosure though, whereas the 40M receiver was.  I wonder if that could have made the difference?

However, the receiver sounded pretty good, and there wasn’t much to it:

Here’s another view:

Looking at the schematic, you’ll see that as well as the 0.1uF coupling capacitor from pin 5 of the NE602 to pin 3 of the LM386, there are 2 x 0.1uF decoupling capacitors – one from pin 5 of the NE602 to ground, and the other from pin 3 of the LM386 to ground. I saw a wonderful looking version of the ZL2BMI transceiver built by a ham in the Czech Republic, but looking at his schematic, saw that he left out these 2 decoupling capacitors. There was a 0.1uF coupling cap from the output of the NE602 to the input of the LM386 and that was it. I thought that perhaps he was onto something, so I also left out these 2 caps. Well, they are quite important. The 3 capacitors between them form a kind of simple diplexer – as far as I can tell from my limited knowledge. With just the coupling cap, I was hearing stations, but was also hearing breakthrough from nearby strong in-band signals. Adding the 0.1uF from the output of the NE602 to ground cut out the breakthrough as well as cutting down on some of the higher-frequencies in the audio. Adding the second 0.1uF cap from the input of the LM386 to ground helped shape the audio a bit more and cut down on some more of the higher frequencies, making the receiver more pleasant to listen to for long periods.

I shouldn’t admit this in public, but my first thought on seeing the 3-capacitor network that connects the two chips was “How can a 0.1uF cap bypass RF to ground, when the same value is also coupling audio to the input of the next circuit? If the 0.1uF coupling cap passes audio to the next stage, why doesn’t the 0.1uF bypass cap short all the audio to ground?”

After a bit of thinking, I realized that the capacitors form RC filters with the circuit impedances which determine which frequencies they pass and which they don’t. Imagine you’re an audio signal coming out of the NE602 and heading towards the coupling cap for the input of the LM386. You are going to see that 0.1uF capacitance as well as the input impedance of the LM386, which is about 10K. The 3dB cut-off frequency of this high-pass filter is given by:

But what about those 0.1uF caps to ground? Well, they also form high-pass filters, and the impedance in this case is the impedance of the connection to ground which, if the capacitor is connected properly to the ground plane, should be very low. Therefore, the cut-off frequency of this simple filter is much higher. Audio frequencies are blocked and subsequently passed on to the LM386 AF amp, while RF is bypassed to ground.

While pondering the really nice-looking ZL2BMI rig that had been built by the the Czech ham, I decided to do some testing of my own.  Maybe he had a reason for leaving out those 2 coupling caps?  I decided to replicate this in my circuit and found very quickly that without at least one of those 2 bypass caps, the circuit experiences breakthrough from strong stations on nearby frequencies.  I made a recording with and without one of the bypass caps, and here are the results:

If I were going to build this as a simple little receiver to listen to the ragchewing and general chat on 75 and 80M, instead of using a ceramic resonator, I would use a varactor-tuned free-running VFO as with my Hi-Per-Mite DC RX so that I could cover a wider portion of the band. I’d also use a double-tuned bandpass filter for the antenna input and include an RF attenuation pot as well as possible an AF gain pot.

Naturally, these simple receivers have their limitations, and it doesn’t stop me from dreaming about owning a Ten-Tec RX-340, but I get a real kick out of receiving good-sounding signals from a handful of parts.

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39 Comments »

  1. I would not care if it it doesn’t work. Just seeing your build is a joy and a work of art!!!

    Comment by Rogier — July 30, 2012 @ 2:32 am | Reply

    • Thank you Rogier. I think that part of the appeal is the close-up nature of the pictures. To be honest, my projects don’t look quite as nice in real life, but I’ll happily accept your compliment!

      Comment by AA7EE — August 1, 2012 @ 2:23 pm | Reply

  2. Greetings from Sweden! Ditto the above comments… Have you looked at this version of the ZL2BMI trx? It has a few mods that make a lot of difference to the usability.

    http://www.gqrp.com/ZL2BMI_80m_%20DSB_TRX_mods.pdf

    73s de sm0mfc

    Comment by Anil — July 30, 2012 @ 11:09 am | Reply

    • Anil – Indeed I have seen the mods by G3XIZ. In fact, I incorporated his microphone pre-amp into the TX section of my ZL2BMI transceiver. I also tried feeding the driver through a capacitor the way that he does, instead of the transformer arrangement that ZL2BMI uses, but reverted back to the original arrangement. I’ll decide which method to use when I get the TX section working. Thanks for your suggestion.

      Comment by AA7EE — August 1, 2012 @ 2:29 pm | Reply

  3. What a beauty Dave! It’s such a treat to see the circuit laid out like you’ve done, manhattan-style. Everything is visible and the routing is easily seen, easily worked on. I’ve put this project on my list of things to do.

    Comment by poweredbynature — August 1, 2012 @ 9:24 am | Reply

    • poweredbynature – I think the appeal is the simplicity of the circuit – and it does work, as you can hear from the video. Let me know how it goes when you build it please!

      Comment by AA7EE — August 1, 2012 @ 2:30 pm | Reply

  4. A .1 microfarad cap shunt to ground exerts a low pass filter effect

    Comment by Jan Bjorgenson — August 18, 2012 @ 4:11 pm | Reply

  5. 22uH for the choke of the driver transistor is too much. May be 2,2 uH. If realimentation persist, shunt a 1 K resistor in paralell with this choke.
    Regards. Edgardo Maffia LU1AR – Buenos Aires – Argentina

    Comment by Edgardo Maffia — December 12, 2012 @ 3:16 pm | Reply

    • Thank you Edgardo. This project has been sitting on a shelf for the last few months, but I will try your ideas when I start working on it again. 73!

      Comment by AA7EE — December 12, 2012 @ 4:24 pm | Reply

  6. I am presently building this transceiver. I have gotten the receiver to work pretty well and I have got the transmitter to give some output but it was self oscillation. Still working to debug it. I have been using a carbon mic like the original but it is not working out too well either. Think I will try the mic circuit you show.
    Regr=ards, Steve N4IOR

    Comment by Steve — March 14, 2013 @ 9:11 pm | Reply

    • I had the same experience as you Steve – got the receiver part working well, but found that my transmitter was self-oscillating. I shelved the project but really need to get back to it and attempt to solve the problem. There are quite a few working ZL2BMI transceivers around, so the issue can certainly be resolved!

      Dave
      AA7EE

      Comment by AA7EE — March 16, 2013 @ 5:23 pm | Reply

  7. Dear Dave: You are’n takin advantage of the balanced nature of the NE602 and LM386 IC’s. If you put both ends of the input coil to 1 & 2 pins and decouple to ground one of them tru a 100 nf capacitor and connect pin 4 of 602 to pin 2 of the 386 using same scheme of the pins 5 and 3, you gain 20 dB of common mode regection to AM powerfull stations.
    On the other hand, you can balance better the remaining carrier adding a 500 k trimpot with the wipper grounded, thru a couple of 100 K resistors to the pins 1 & 2 of the 602. This don’t affect in eception. Disbalancing one pin 1 or 2 of the 602 to ground thru 22 K resistor, you generate full power carrier, very usefull to adjust. You mus decouple the end of the resistor with 10uF electrolytic to avoid hum in the carrier.
    Don’t forget the rejection ratio is only 30 dB. In SSB the carrier is in the side of the filter and this is not very trouble. In DSB you must play with the polarization of the driver stage, to be non linear for small signals. Like a squelch. Small signal no output. A bit more give you full power.
    I hope this will be usefull.
    I enjoy making small QRP equipment. Look at my site http://www.lu1ar.blogspot.com.ar
    Regards.
    Edgardo Maffia LU1AR
    Buenos Aires – Argentina

    Comment by Edgardo Maffia LU1AR — March 18, 2013 @ 1:13 am | Reply

    • Great input Edgardo – thank you. I love your blog too – that 50MHz transceiver looks very interesting!

      Dave
      AA7EE

      Comment by AA7EE — March 18, 2013 @ 1:54 am | Reply

  8. what is that MVI662 on the schematich diod or ???? Cant find it in romanian sops

    Comment by dan — April 30, 2013 @ 8:37 pm | Reply

    • You can use any varactor that has a fairly wide capacitance swing Dan. Look at this webpage for test data on the MV1662 – http://www.hamelectronicsmagazine.com/MV1662.html It looks like it has a capacitance of about 430pF at 2V and 180pF at 8V. If you can’t find a suitable varactor, try a variable capacitor instead. That would make the circuit simpler, as you could leave out the 100K resistor, the 0.1uF capacitor and the 10-turn pot (as well as the varactor of course). The maximum operating voltage of the NE602 is 8V, so the regulator should probably be for a lower voltage – say, 6V.

      Comment by AA7EE — April 30, 2013 @ 8:57 pm | Reply

  9. and the ceramic rezist for 4 MHZ can it be replaced with a one of 4,5 or 5,5 ????

    Comment by dan — April 30, 2013 @ 8:41 pm | Reply

    • Yes, but it will affect your operating frequency.

      Comment by AA7EE — April 30, 2013 @ 8:57 pm | Reply

      • it vhill be biger or smaller the operating frequency. i think biger

        Comment by dan — April 30, 2013 @ 9:24 pm

      • question my ceramic resist has 3 pins how do i mount it??????

        Comment by dan — May 2, 2013 @ 8:40 pm

  10. the support for the coil cant be any tip, or specific t 50 2 type. i will try to use one from a power suply for start until i locate the one needed

    Comment by dan — April 30, 2013 @ 9:29 pm | Reply

    • You can use other types of core, but the coil needs to be resonant on your operating frequency.

      Comment by AA7EE — April 30, 2013 @ 10:11 pm | Reply

      • when it is don i am going to send you some pictures of my small project the idea is that i want to us it for field work so it is going to run of a 12 v 20 A battery , on the part of the audio amp i decided to repaced with a TDA 2822 it is stereo runs from 1,8v to 12 V

        Comment by dan — May 1, 2013 @ 4:27 am

  11. sory for these questions i know they sound stupid but is the first time i built a receiver of these tip i am used building other tips that don t include integrated circuits

    Comment by dan — April 30, 2013 @ 9:34 pm | Reply

    • Check the datasheet for your particular CR but usually, the center-pin is ground.

      Comment by Dave Richards AA7EE — May 2, 2013 @ 8:45 pm | Reply

      • hai ..nice rx circuit …i want to build this project can i use sa612 /ne612

        Comment by Daniel — June 24, 2013 @ 1:16 pm

      • Yes you can Daniel. They are interchangeable. It’s a fun little receiver. If you want to build it for 40M, hamshop.cz have 7.2MHz ceramic resonators. You’d just have to wind the input coil for 40M instead.

        Dave
        AA7EE

        Dave
        AA7EE

        Comment by AA7EE — June 24, 2013 @ 3:37 pm

  12. I am having big problems the rig it is built but it does not work . The amp part works very well but no signal from the NE602. There are on the market a number of pint tips for the NE602?? If it is a yes then please send to me the models you know.

    Comment by dan — September 26, 2013 @ 6:47 pm | Reply

    • Dan – I’m sorry but I don’t understand your question. I had the exact opposite problem – everything in my rig worked except for the driver and PA. Actually, they did work but were suffering from RF feedbakc/parasitics which I think was due to the driver and PA not being sufficiently shielded from the rest of the rig.

      Is your receiver working? Getting the receiver to work should be the first thing. If it’s not working, check to see if the oscillator works (by listening for it on another receiver). Let me know how it goes. I’m not that great at troubleshooting though, I’m afraid

      Dave
      AA7EE

      Comment by AA7EE — September 26, 2013 @ 7:03 pm | Reply

  13. I am thinking I did the connections all wrong on the NE602 but that is a maybe that is why I am asking if there are other pin configurations to the NE602 . I am thinking to put a standard 4 MHz crystal in place of the ceramic resist I am having big problems locating on of 4 MHz only one of 4,5 MHz

    Comment by dan — September 26, 2013 @ 7:12 pm | Reply

  14. Nop I am from Romania Dave I am in central Europe i need to find the parts here so best of luck to me

    Comment by dan — September 26, 2013 @ 7:56 pm | Reply

    • I am sorry Dan – I forgot that you are in Romania. Your 80M band is 3.5 – 3.8MHz isn’t it? How about this 3.68MHz resonator from hamshop.cz in the Czech Republic? It should tune a little above the center freq and quite a lot below it. At a very rough guess, you should get something like 3.6 – 3.7MHz tuning range.

      http://hamshop.cz/crystals-c11/keramicky-rezonator-3-680-mhz-i166/

      Dave
      AA7EE

      Comment by AA7EE — September 26, 2013 @ 7:57 pm | Reply

  15. Give me your email or a link where i can upload some foto of my work

    Comment by dan — September 26, 2013 @ 8:15 pm | Reply

    • I’m good on QRZ.com or aa7ee at arrl.net should work.

      Comment by AA7EE — September 26, 2013 @ 8:18 pm | Reply

  16. well I managed to get a varicap diod from an old tuner bloc I had it is a BA and it should do the job I managed to do something whit the coil I wound it on a carcass with ferrite bed from the tuner I found I added 28 turns to the secondery I will keep you updated of what I am modifying and maybe you can aply them to your tuner if they are good updates 2 this project

    Comment by dan — September 27, 2013 @ 6:19 pm | Reply

  17. Glad to see it’s getting comments!

    I have lots of develpoments since the first article.
    Eric Sears ZL2BMI

    Comment by Eric — October 4, 2013 @ 11:38 pm | Reply

    • Eric – I have learned from my experience with this build and realized that I need to provide better isolation between the driver/PA and earlier stages of the TX. Thank you so much for the design! I am currently working on VK3YE’s Micro 40, which incorporates elements of your design too (as you know). I have an idea to rebuild yours at some stage as a xtal-controlled rig on a single freq for my daily net on 40M.

      I’m looking forward to seeing news of your developments.

      73

      Dave
      AA7EE

      Comment by AA7EE — October 5, 2013 @ 12:12 am | Reply

  18. good news it is starting to work but not just yet I changed the ferit ring with a ferit rod also I found a ceramic resist that i order. I stead o radio amator band a lot of local radios I changed the diod again with 1N4148 it is inly 4 pF so 2 or maybe 3 in paralele a have to do a lot of modifying until it will work until then I will use my old trusty tuner wit 2 tranzistors for 80 m Yo10dan out

    Comment by dan — October 5, 2013 @ 6:17 am | Reply


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