How exacly 80m ARDF works?

Garyl · Aug 5, 2017

All tests below have been done without any antenna and with 12V (12.31V measured) power supply.

betwixt said:
1. Can you measure the voltage between the negative (ground) wire from the power source to TP1 and TP2 please.

Input voltage: 12.31V

TP1: 8.04V (so the voltage regulator is working, I think...)

TP2: 10.43V (It's the audio amplifier supply voltage, I think...)

betwixt said:
2. Please tell me the voltage at pin 5 of the LM386.

5.15V (I tried changing the volume potentiometer but it's still around 5.15V, no more than 0.05V deviation...)

betwixt said:
3. with the volume at maximum setting, if you touch TP3 can you hear anything in the loudspeaker? (crackles, buzzing...)

Yes, the static sound changes to something different and louder, like a constant noise, vibration..

betwixt said:
4. still with the volume on maximum, if you touch pin 10 of the MC3361 can you hear anything in the loudspeaker?

Yes, the static sound changes to a louder and more disorted version of the constant noise/vibration from point 3.

PS: I have made those tests with the SSB/CW switch enabled and then disabled, but the results are very similiar....

betwixt · Aug 5, 2017

All good news!

What I was doing was starting at the loudspeaker and working towards the antenna to make sure all the audio stages were working properly. When you touched TP3 you injected random interference to the amplifier and heard it through the loudspeaker, when you did it at pin 10 you injected at the input of the MC3361 internal pre-amp which is why it was louder. It proved all the output stages are working fine.

Conclusion: either the oscillator isn't working or it is off frequency -or- the antenna signal isn't reaching the mixer.

You must bear in mind the MC3361 is not intended to be used as an AM receiver, in fact most of it's internal circuits are not being used in this design at all. It is really designed as the middle stage of an FM receiver with other circuits ahead of it.

How it works is signals in the range 3.5 to 4.0 MHz pass through the input filter to pin 16. The components between pins 1 and 2 form a tuned circuit which should also tune from 3.5 to 4.0MHz. Inside the IC there is a mixer stage that subtracts the two frequencies and sends the result to pin 3 where it is amplified and fed to the loudspeaker. So for example, if you wanted to hear a CW signal at 3.600MHz, you would tune the oscillator to 3.601MHz and the result would be a tone at 1KHz which you could hear.
So if nothing seems to be coming out of the mixer it means the oscillator isn't running or is at the wrong frequency or the antenna signal never reached the mixer stage.

Do you have access to an oscilloscope or frequency counter?

Brian.

Garyl · Aug 6, 2017

Thanks for explanation, it makes things more clearer now!

I have an old analog oscilloscope and I can try to measure things, altough I did not use this one much and most likely I'll have to look into the manual, or maybe you could tell me as well how to set it.

PS: I'm not sure about this "not reaching signal", because the sound from speaker is clearly different when:
- I have no antenna (low volume noise)
- I have antenna (higher volume noise + strange sounds while tuning)
- I touch the antenna (sound changes as well)

- - - Updated - - -

I have tried to use my old oscilloscope to measure frequency generated by the test signal generator.
The schematic:

I have set oscilloscope to DC, used channel 1, set time scale to 0.1us and somehow managed to get the trigger.
Here is the setup image:

and the scope image:

I have counted that signal has around 2.8 squares phase, which equals to 0.1 * 2.8us = 0.28 us, which is around 3571kHz.
The used crystal value is 3.686MHz = 3686kHz, so I have abs(3571-3686)/3686)*100 = 3.1% measurement error.

betwixt, I'm not really used to analog oscilloscope, can you check if my measurement and calculations shown above are correct, and then, tell me what exacly should I measure?

betwixt · Aug 6, 2017

Well done! The measurement technique is exactly right. Don't worry about 3% inaccuracy, that is actually quite good when you take into account the timebase errors in the oscilloscope and reading the exact timing off the screen.

To check the receiver, you have to do the same test with the oscilloscope ground clip to the receiver ground and the probe to pin 2 of the MC3361 but if possible use a "x10" probe or if your probe has a switch on it, set it to the x10 position. If makes the trace 10 times smaller but also reduces the effect the probe has on the circuit it is connected to. When dealing with that kind of circuit, the probe itself can stop it working properly but using x10 will minimize its effects. To overcome the smaller trace you can increase the Y gain control on the oscilloscope.

You should see a similar oscillation on the screen and the frequency should be adjustable with the receiver tuning. Ideally the tuning should cover 3.5 to 4.0MHz and maybe a bit more on each side.

Brian.

Garyl · Aug 6, 2017

I have a probe with 1x - 10x switch.

It was a bit hard to tune the oscilloscope (Or maybe I just don't really know how to use analog ones) but I managed to get such results:

0.1us timebase, 1x probe (but it was very similiar for 10x as well), upper frequency limit (I adjusted only the main pot)

0.1us timebase, 1x probe (but it was very similiar for 10x as well), lower frequency limit (I adjusted only the main pot)

I am not sure about those measurements, I connected probe at the capacitor pin which is connected to pin 2 of MC.

0.3us means 3333 kHz
and
0.27us means 3703 kHz

The receiver description says it works in range "3495kHz – 3805kHz", so maybe I'm kinda of it, but really not much?

So, I should use the two trimmer pots to adjust it to the "Ideally the tuning should cover 3.5 to 4.0MHz and maybe a bit more on each side." range?

But still, I have half of band covered as it is right now...

betwixt · Aug 6, 2017

Always best to use x10 so it has less effect on the tuning but it does seem to be in the correct range. When you connect the probe it adds a little extra capacitance to the tuned circuit which makes the frequency drop slightly so I would guess without the probe it is tuning as it should.

The only other thing that could stop it working is the filter at the antenna. Please try this:

1. Temporarily, lift the leg of the 1nF capacitor from the junction of the 270pF and 680pF capacitors (leave it connected to pin 16) and connect the antenna to the end you lifted.
See if you can now hear stations, they may be distorted but you should hear something.

if that works:

2. reconnect the 1nF capacitor and lift the left (nearest antenna) side of the 10pF capacitor. Attach the antenna to the end you lifted. Try again to hear any stations.

Brian.

Garyl · Aug 6, 2017

betwixt said:
1. Temporarily, lift the leg of the 1nF capacitor from the junction of the 270pF and 680pF capacitors (leave it connected to pin 16) and connect the antenna to the end you lifted.
See if you can now hear stations, they may be distorted but you should hear something.

.

The sound is louder but I can't hear any radio stations, no matter how I tune nor how I set the SSB/CW switch.
Altough I can hear hissing or something like that (but nothing really special?).

I tried two cases:
- antenna wire hanging outside window
- antenna wire hanging between corners of the little balcony

PS: I checked the antenna wire for connectivity with multimeter and it is all right...

Garyl · Aug 10, 2017

Bump, I'd also like to mention that I tried to replace the MC* chip before but it didn't change anything.
Any other ideas?

betwixt · Aug 11, 2017

To be honest, no!

It appears everything is working electrically so why nothing is received is a mystery. I checked here to see if I had any MC3361 in stock but unfortunately, I only have MC3357 which is pin compatible but much lower gain so it wouldn't be fair to duplicate it here to compare our results.

As I've said before though, don't be too concerned about the antenna. It will obviously work best if you have a proper impedance matched half-wave dipole but the difference between one of those and a random thrown length of wire isn't really that great. My antenna here is just a long wire stretched between two poles about 2m above ground level and I'm in a valley surrounded by tall trees, I can hear about 20 stations between 3.5 and 3.8MHz at the moment in mid-morning. In the evening when propagation improves, the band is packed from end to end.

Bear in mind it is a direct conversion receiver, the signals you pick up won't be like an ordinary AM radio but you should be able to resolve CW (Morse code) signals and SSB signals if you tune carefully. If you still can't get any results, without complex test equipment, I would be inclined to try a pre-amplifier stage to boost incoming signal strengths.

Where are you located? I will see if I can trace and local signals that should be in range all the time.

Brian.

Garyl · Aug 11, 2017

betwixt said:
It appears everything is working electrically so why nothing is received is a mystery.

I think that I will try to etch a new PCB for this project (this time myself) and solder everything again using 100% new components - I have spare parts so that won't be a problem...
Can you tell me if the tracks pattern really matter for this project? I mean, I will try to design my board almost the same as original, but there might be differences...

betwixt said:
As I've said before though, don't be too concerned about the antenna.

I saw various people using successfully this receiver around the web, including Youtube, but all of them had better antennas.

betwixt said:
My antenna here is just a long wire stretched between two poles about 2m above ground level and I'm in a valley surrounded by tall trees, I can hear about 20 stations between 3.5 and 3.8MHz at the moment in mid-morning. In the evening when propagation improves, the band is packed from end to end.

Are you far from the city? I have several taller buildings around me.

Can a WiFi signal interfere that 80m band stuff?

betwixt said:
Bear in mind it is a direct conversion receiver, the signals you pick up won't be like an ordinary AM radio but you should be able to resolve CW (Morse code) signals and SSB signals if you tune carefully. If you still can't get any results, without complex test equipment, I would be inclined to try a pre-amplifier stage to boost incoming signal strengths.

I realize that the sound quality might be bad because I watched youtube videos of this Lidia 80 tests, but I can't pick anything at all.

betwixt said:
Where are you located? I will see if I can trace and local signals that should be in range all the time.

Can you please receive the "friend request" and let me tell you that in the PM? I mean, just that one thing, for the sake of privacy

betwixt · Aug 11, 2017

Can you tell me if the tracks pattern really matter for this project? I mean, I will try to design my board almost the same as original, but there might be differences...

The original board should be OK but if you are making a new one, it would make sense to add a few extra parts, such as antenna amplifier, provision to use other bands and maybe an extra audio stage. At those frequencies there isn't any need to follow special RF techniques though.

Are you far from the city? I have several taller buildings around me.

Geographically, the nearest city to me is Dublin in Ireland - but that's a different country and across about 100Km of sea!

Can a WiFi signal interfere that 80m band stuff?

Highly unlikely. Wifi frequencies are around 2400MHz and 5300MHz. It is technically possible for desensing to occur in VERY close proximity (<0.5m) but even that is highly unlikely.

I realize that the sound quality might be bad because I watched youtube videos of this Lidia 80 tests, but I can't pick anything at all.

I will try to watch the videos but where I am at the moment the internet is very slow (< 100Kbs) so it may not be possible to see video at all.

Brian.

Garyl · Aug 11, 2017

betwixt said:
The original board should be OK but if you are making a new one, it would make sense to add a few extra parts, such as antenna amplifier, provision to use other bands and maybe an extra audio stage. At those frequencies there isn't any need to follow special RF techniques though.

I would love to do that, but isn't it a bit, well, too early? That's my first radio circuit - in the past I worked only with power supplies and various microcontrollers stuff.

As fair as I know there is a 40m variation of Lidia80 circuit:
English version: http://www.qsl.net/sp5ddj/Lidia 40 DC Receiver/Lidia 40 - DC Receiver.htm
Polish (his original language): http://www.sp5ddj.pl/Lidia 40 DC Receiver/Lidia 40 - DC Receiver.htm

Description by SP5DDJ:

Schematics of "Lidia 40"

Modifications of VFO and front-end are distinguish in black bold and values are in red. New component is capacitor 22pF parallel to captrimmer.

Digital readout of any kind might be attached, therefore FET follower is shown.

So from what I see adding support for other bands would mean making separate input filters...

Can you elaborate on what would you add? How would antenna amplifier stage look like?

EDIT: Here is youtube video of the original author, and his antenna is indeed short http://www.youtube.com/watch?v=UQTPFyd5zj8 , but maybe I will try to improve mine and use 5 meters of wire around the room...
Do concrete walls of house block the signal entirely?

betwixt · Aug 13, 2017

Sorry Garyl, I've been very busy with other things and haven't had chance to look at the videos yet. I need to invent a time machine so I can fit more hours into the day!

Do concrete walls of house block the signal entirely?

The amount signals are absorbed into materials depends on the frequency but concrete is quite poor as a shield at low frequencies, it doesn't start to be significant until several hundred MHz and above so that isn't the problem.

I checked where you are located and you should be surrounded by hundreds of stations so the chance of not picking something up are very low, that unfortunately means something else is wrong with your receiver. I can even pick up stations not too far from you on my receiver here 1500 Km away! My receiver is considerably more complicated than yours but my antenna isn't much better.

The only real test would be to analyze the input filter at the antenna but for that you need a signal generator and RF voltmeter.

Brian.

Garyl · Aug 13, 2017

betwixt said:
The only real test would be to analyze the input filter at the antenna but for that you need a signal generator

Can I simply generate the signal with Arduino or PIC MCU? What kind of signal do I need?

betwixt said:
and RF voltmeter.

Well, I have an osciloscope, wouldn't it be that sufficient?

Tell me more about the test and I will try to get more equipment

betwixt · Aug 13, 2017

PIC and Arduino can't produce high enough frequencies on their own but you can use them to control a signal source.

From your tests it seems like the oscillator in side the MC3361 is working OK and the audio stages are working OK. The conclusion has to be that the radio signals are not entering the MC3361 because they are blocked by the antenna filter components. I'm sure there are plenty of signals from your antenna but they are not reaching pin 16. The coils and capacitors in the filter are there to block signals outside the range you want to receive but it seems they are also blocking the wants you want to hear.

Testing filters like that is not an easy process, what you have to do is sweep a signal generator frequency from a little below the lowest frequency you want to just above the highest frequency and at the same time measure the signal level passing through the filter. Ideally, nothing comes through the filter when the frequency is below 3.5MHz and nothing comes out above about 4MHz but between them the signal passes at full strength. It never happens like that in real life, there is always some signal lost inside the passband and some leakage through the filter from outside it.

You can do a crude test with an oscilloscope but the results may not be accurate. You need to set a signal generator to say 3.0Mhz and feed it to the antenna input point and take a note of the amplitude on pin 16. Set the oscilloscope to quite a high Y gain and set the oscillator output level so you can only just see the waveform. Then take a note of the amplitude. Repeat the test at say 50KHz steps up in frequency and without adjusting the oscilloscope, note each amplitude. Keep repeating until the frequency is about 5MHz then plot the frequency (X axis) against amplitude (Y axis) on a graph. If you join the points you should see a gradual rise up to 3.5MHz then a flat top then a gradual fall after 4.0 MHz. Also, without changing the oscilloscope settings, take a note of the level at the antenna input for comparison.

Brian.

Garyl · Aug 17, 2017

betwixt said:
All have here as an antenna is about 25m of wire stretched between two poles and about 3m above ground. My receiver is considerably more sensitive but it can pick up stations around the whole World on the 80m band.

I will most likely be able to put two poles within the 15 meters distance.
Can you tell me how is your antenna connected?
Is it connected at the one of the poles and the other wire end is just dangling, or do you connect it in the center?
I mean:

[Antenna connection]--[POLE_A]--------------------------------------[POLE_B]

or

[POLE_A]---------------------[Antenna connection]-----------------[POLE_B]

Do you also "ground" somehow the ground of the receiver or add some coils in the antenna?

betwixt · Aug 17, 2017

My antenna is like your first example and I have a ground connection to a 1m (exact length is unimportant) metal rod driven into the ground.

The only difference is that I have to use an ATU (Antenna Tuning Unit) because I live inside a national park and a conservation area where antennas on buildings are not allowed. I have the antenna hidden in trees (not good for reception!) and have to feed the signals to the house through underground ducting along with power and telephone cables to keep them all out of sight and that means using screened cable to block interference. The ATU matches the screened cable between the transceiver and the end of the antenna to minimize signal losses. It is far from an ideal situation but there is nothing I can do about it.

Brian.

Garyl · Aug 18, 2017

Hello again!
Even tough I am on the summer vacation in different country right now, doesn't it mean that I've stopped working on this project.

I have decided that I will etch a new board for this project, build it with 100% new components and compare the results. This way I might determine where I made the mistake why it's not receiving...

Here's my new board with extra 7812 regulator and I would like to ask several question before I etch that.
Please take a look:

View attachment 20170818.zip

I have following concerns:
1. Can I mount the pots on the front panel with the short wires? How much the extra stray inductance will affect the receiver? I am mostly afraid about the VFO pot...
2. Can you please at least partially check my PCB and tell me what can be improved?
3. To be more specific, does the... order of elements in the track matter?
4. How much the bulk / filtering eletrolytic capacitance matter here? I mean, I've already had some bad experiences with "impulse power supplies" that caused certain circuit to fail, while it worked 100% perfectly with classic 50Hz transfoer supply....
5. If everything is okay, I will try to etch this circuit and solder it using 100% new components. Do you think it will work this time?
6. Is there anything else I can improve?

Thanks in advance! I really hope that I will manage to receive anything soon...

betwixt · Aug 18, 2017

Looks OK to me. The only comment on layout I would make is the diode is marked as 1N4148 which would be under-rated but if you use a 1N400x type it needs wider pad spacing.

The VFO pot isn't an issue, it only has DC on it and there is a 100nF capacitor to ensure it doesn't pick up RF or interference. However, I would not mount the antenna input on the PCB at all. It would make more sense to wire the antenna input directly to the pot and only wire the ground and top of the track back to the PCB. Maybe use X3 as the input from the pot rather than from the antenna, it also fixes the missing track problems around C21 and C22.

Brian.

Garyl · Aug 19, 2017

Okay, I have improved board as you said.
What about the bulk capacitors? Do they have a big impact on reception?

Also, will the VFO circuit work without the MC chip in order to just test different varicaps with oscilloscope?

How exacly 80m ARDF works?

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