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Building the Simple 80 Metre DSB Minitransceiver by VK3AJG?

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So when it is all working, you make sure no audio enters the mic input then adjust the two potentiometers for minimum (ideally zero) output.
I still don't have the output stage (transistor, mic, etc) soldered, but I will remember that. Could I just connect MP3 player output instead of mic and then try to transmit signal few meters across the room?


The oscillator should run no matter where the control is set, if it is stopping it is probably at the extreme of minimum value. You could try adding a small capacitor across it to prevent the value reaching so low (~5pF) or you could try changing the transistor for one with higher gain.
I've added 15pF capacitor and it's working all the time now.


When you hear a station near 3.58MHz, adjust the 'can' transformer for maximum signal. It is the equivalent of the bandpass filter on your Lidia receiver but because the tuning range is so narrow on this one, a peaking filter is more appropriate to use.

Okay, first I wanted to try testing this receiver with test signal generator, but then I realized I am already picking something with my crude "cable from roof" antenna.
Here's what I managed to receive during this evening:
View attachment 2017_09_08_20_34_46_VK3AJG_is_this_CW.zip
View attachment 2017_09_08_20_38_23_VK3AJG_is_this_CW.zip
View attachment 2017_09_08_21_13_13_VK3AJG_is_this_CW.zip
I'm not sure if it's CW or maybe that RTTY...
... still, it's good that I have received something, because my crude "cable from roof" antenna here is so bad that even my Lidia receiver fails to receive anything understandable...
 

Well done!

All of those are CW stations with a little SSB voice in the background on the first one. I couldn't quite read the CW but I think the second one is from an Italian station calling "CQ DX" (request for a distant contact).

Brian.
 
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All of those are CW stations with a little SSB voice in the background on the first one. I couldn't quite read the CW but I think the second one is from an Italian station calling "CQ DX" (request for a distant contact).
I have really no idea how are you able to tell that there is a SSB voice! I am sure that I am younger than you, but it seems that your hearing is indeed very much better! :D


I've been thinking on improving this schematic and I decided to try using VFO from hs8jyx design
(see full article here: https://hs8jyx.blogspot.com/2013/08/ssbcw-7-mhz-fine-tune.html)

I have modified two schematics but I am unsure if I am feeding the VFO signal correctly to the transistor base, can you check?

This is the VFO module:
vfo.jpg
and this is the main topic design modified for this VFO:
dsb-minitransceiver-modified.png

Things I am unsure about:
1. do I have to power this VFO with 8V or can it be just 12V? I think it can be 12V.... that 8V were forced by NE602, which is no longer on the schematics...
2. Is the main idea of connecting this to transistor base correct?
3. Is the transtistor-base restistor value correct? Maybe it would be better aroudn 1k or so...
4. Is the emitter-to-ground resistor still needed there?
5. Futhermore, that VFO was designed for 40m and not 80m, so I will have to adjust the inductor value and maybe capacitors, but how much? Twice?
 

1. Stay with 8V, the voltage should be stabilized or any variation in it will cause a shift in tuning. 8V is probably the most you can reliably stabilize from a 12V source given the overhead needed by the regulator IC.

2. It should work fine but the signal must be fed to the base of the BC549C via the 10pF output capacitor on the VFO.

3. Keep R5 and R6 connected as before, it needs them both to bias the transistor correctly. You should remove C1 and C2.

4. Keep the value of R2 as it is for now. It might be possible to bypass it with a capacitor (10nF ceramic) to increase the VFO injection but leave it out for now.

5. The formula is 1/(2*pi*sqrt(L * C)) where L is in Henrys, C is in Farads and the frequency is in Hz. You can probably get enough frequency coverage by just changing the inductor and leaving the capacitors as they are but you will have to experiment as there are so many variables. 80m will need more turns than 40m, try with twice as many and adjust if necessary. Removing turns reduces the inductance and makes it tune to a higher frequency.

Brian.
 

Okay, I will try soon as you said.

Most likely first I will test that VFO on separate board to see which values are needed to generate 80m signal, and then I will update the PCB of entire transceiver.

Now then, it seems that I also need to add a proper antenna filtering, right? Would it make sense now to get the rid of IF can filter and add a proper 80m signal filter?
 

Now then, it seems that I also need to add a proper antenna filtering, right? Would it make sense now to get the rid of IF can filter and add a proper 80m signal filter?
You certainly need a filter but first try to understand how they work. I will explain as simply as I can:

There are two types of applicable filter, the first is a bandpass filter which allows ALL signals within set limits to pass through but has relatively high losses at all those frequencies. They have much greater losses at frequencies outside the bandwidth of course. The second is a tuned filter, that is one which has a narrow bandwidth, ideally to let just one station pass through, it has much lower losses and helps to eliminate adjacent signal breakthrough.

The drawback to adding a VFO is by receiving a wider range of frequencies you are forced to use a bandpass filter -or- to retune a narrow filter for each frequency you want to receive. An ideal situation would be to link the VFO and antenna tuning together so as you adjust one, the other 'tracks' it at the same frequency and that method is used occasionally in a kind of receiver called a "TRF" (Tuned Radio Frequency) but they are notoriously difficult to build.

On a receiver with a 'pulled' ceramic resonator to set the VFO frequency, the tuning range will be very narrow anyway so although it isn't optimal, even a fixed narrow band filter will be adequate. It will (if adjusted properly) give peak performance at the center of the tuning range but won't have degraded enough at the edges to cause problems.

When the VFO range is wider you really have no practical choice but to use a bandpass filter.

So there is nothing wrong with the 'IF can" filter, it will work better than the circuit using more coils and capacitors but unless you can adjust the core at the same time as the VFO you will get worse performance. You might think of filters as being like windows (the ones you look through, not the operating system!). A wide window is like a bandpass filter, it lets you see a wide view but possibly see things you don't want. A narrow window is like a tuned peaking filter, it can't see as much unless you can somehow move it from side to side. The problem compounds when you have more than one narrow widow, one behind the other and you have to move them at the same time or they don't line up and nothing can be seen at all.

Brian.
 
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Well, it makes sense now!

I just started designing a clear schematic for this with accordance to your hints, and I realized that this transceiver has a 8.2V supply voltage stabilized by Zener diode.
Would just:
a) using that 8.2V voltage for VFO
b) replacing Zener with 7808 and using that voltage for VFO
bring any advantages?
You know, I don't see the point of keeping BOTH zener and stabilizer on board.


Btw, if I replace the filter with bandpass one, and add a wider VFO, how good would this transceiver work (in your opinion) compared to the MC3361 one?
 

I would just use the 7808, as you point out, they do pretty much the same thing. The 7808 probably has slightly better temperature stability.
Btw, if I replace the filter with bandpass one, and add a wider VFO, how good would this transceiver work (in your opinion) compared to the MC3361 one?
Its very difficult to tell. There is more gain in the MC3361 so it should be more sensitive but it probably has poorer balance in the mixer. There probably wouldn't be much difference overall.

Brian.
 
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I've been thinking about it again and I am now considering creating VFO on a separate board which would be plugged directly into the transceiver board.
I mean, no wires at all, just two standard pin headers (male and female).

The 7808 would be on main board and not on VFO board.
Here's what I have so far:

20170912VFO.png
20170912VFO2.png
20170912VFO3.png

CAD files for Eagle:
View attachment vfo_bc547_fineTune6.zip

I've tried to keep ground plane solid, but I don't know about the other things... I will most likely also push compontents apart in the final version. Any more suggestions?
 

I can't use EAGLE files so I'm working from the images, I presume they are the same.

Suggestions:

Take the top of R2 & R15 to the regulated 8V line instead of the incoming 12V. It will slightly reduce the tuning range but you can recover that by shorting out R2 and R15 and maybe (probably not necessary) increasing the values of C12 and C14 slightly. By using the regulated voltage for the tuning supply, you remove the risk of the frequency drifting if the 12V isn't stable.

Make provision to switch the inductor so it can cover different bands. Probably the easiest method is to use a small change-over relay and use the NO and NC contacts to ground the bottoms of two inductors and connect the C (common) pin to ground. There should just about be space on the board to fit one. Wire a diode (1N4001 will do) across the relay coil and ground it's cathode and one side of the relay coil. The other side becomes a point where you can apply relay coil voltage (suggest you use a 12V one) to switch bands.

The board connection pins are not really rated for RF signals but at 3.5/7 MHz they should be adequate. If you move the pins right up to the edge of the board you can make them reach far enough to plug directly into the other board. Remember that connecting wires between the boards are not a good choice because of the inductance they introduce. If you do intend to link the boards with wired connectors, use a screened co-axial cable for the RF output.

Brian.
 
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I can't use EAGLE files so I'm working from the images, I presume they are the same.
Yes, they are the same, I just uploaded them just in case.



Take the top of R2 & R15 to the regulated 8V line instead of the incoming 12V. It will slightly reduce the tuning range but you can recover that by shorting out R2 and R15 and maybe (probably not necessary) increasing the values of C12 and C14 slightly. By using the regulated voltage for the tuning supply, you remove the risk of the frequency drifting if the 12V isn't stable.
The "VDD" on the schematic is supposed to be regulated 8V. The regulator on the schematic is the 7805. It was that way all the time. By the way, is that 7805 needed there? I'm kinda not sure but it's the design from: https://hs8jyx.blogspot.com/2013/08/ssbcw-7-mhz-fine-tune.html


The board connection pins are not really rated for RF signals but at 3.5/7 MHz they should be adequate. If you move the pins right up to the edge of the board you can make them reach far enough to plug directly into the other board. Remember that connecting wires between the boards are not a good choice because of the inductance they introduce. If you do intend to link the boards with wired connectors, use a screened co-axial cable for the RF output.
Speaking of of co-axial cables, I've been recently wondering how should I deal with antenna connections. Right now I have four simple (but working) receivers build, but I don't have any antenna cables/sockets setup. Are they standarized, or can I just RCA sockets and cables recycled from waste? I'm not even sure how are they called, are they the same as "TV aerial antenna plug"?
 

The "VDD" on the schematic is supposed to be regulated 8V. The regulator on the schematic is the 7805. It was that way all the time. By the way, is that 7805 needed there? I'm kinda not sure but it's the design from: **broken link removed**
Sorry! My mistake - so many schematics...

I can't see any advantage in using a regulator just for the drain supply to the VFO when the whole supply is already regulated. It's probably wise to keep the voltage at 5V but making R11 say 470 Ohms and using a 5.1V Zener diode instead of the 78L05 would work just as well and save some board space.

For a simple receiver application you can use almost anything for the antenna socket but 'pro' users would opt for 50 Ohm BNC plugs/sockets or PL259 /SO259 screw connectors. Note that the size of co-axial cable and plug has to match as there are variants for different cable diameters. Ideally use 50 Ohm co-axial cables, the ones for audio work are not really suitable except for very short lengths (<15cm) and low frequencies.

Brian.
 
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Hey, are you the one of the site authors? Do you know if it's possible to get just the CAD files of PCB?

I am considering building this radio in the future (at least receiver part for now), especially because I already have most of the components at hand. I just need PCB. With the CAD files I could etch it myself at home.
 

Hey, are you the one of the site authors? Do you know if it's possible to get just the CAD files of PCB?

I am considering building this radio in the future (at least receiver part for now), especially because I already have most of the components at hand. I just need PCB. With the CAD files I could etch it myself at home.

No, I have never been in Australia..:-D

If I were you, I draw my own PCB to learn more about..It will be a good experience for you.
 

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