Did You Hear of Such an AM Receiver? For DSB-Suppressed Carrier.

[coppice]

Octave is like Matlab, just that Octave is free.  If you can't download Octave from where you are, try using Tor Browser (not torrents, though a torrent client might work too), and visit the Octave download page using the Tor Browser instead of whatever browser you may be using now.  Tor behaves like a VPN, it accesses the Internet as if you were from somewhere else.

Note where the OP is. People in Syria, Iran, and North Korea are restricted in various ways the rest of the planet are not.

[KerimF]

Octave is like Matlab, just that Octave is free.  If you can't download Octave from where you are, try using Tor Browser (not torrents, though a torrent client might work too), and visit the Octave download page using the Tor Browser instead of whatever browser you may be using now.  Tor behaves like a VPN, it accesses the Internet as if you were from somewhere else.

https://www.torproject.org/download/
https://octave.org/download

I was able to open the page of https://octave.org/download , thank you. It directed me to other sites since my OS is W7, 32 bits.

I bought my today/old laptop a few months before March 2011 after which we had no mains AC, fuel and water for many years (we had just daily missiles over the streets and buildings at random). Now, we have 4/24 mains AC at best, and enough fuel to warm one room for a couple of hours (fortunately we are allowed to have fresh water) while almost all private businesses in the city (including mine) are paralyzed.

I am 75, so I think it is a bit late for me now to learn how to use professionally a versatile tool as Octave (which likely needs other resources) while I live in the actual living conditions.

Anyway, thank you again for your care.

[KerimF]

The simple trick that lets PLL be locked despite the polarity reversal of the carrier is the frequency doubler (see U2 and U4, XOR gates).

[KerimF]

In every PLL circuit, its crucial part to be designed properly is its LPF (R9, R10 and C6 in the uploaded example) to have a stable and fast response.

[KerimF]

R7 and C5 forms a delay circuit to let the e-switch of CD4066 (U7) in phase with the suppressed carrier when its frequency equals to half of the VCO mid-frequency.
Although it will be out of phase at higher and lower frequencies, the phase difference is relatively small (this reduces the gain of the synchronous detector) even when the carrier is at the limits +/- 30 kHz.
 

[RoGeorge]

Note where the OP is. People in Syria, Iran, and North Korea are restricted in various ways the rest of the planet are not.

You must be kidding when saying the rest of the planet is not under censorship/restriction.  It is, just that maybe not that severe restriction, but the restrictions are increasing fast, haven't anybody else noticed?  In fact the UK levels of censorship increased at alarming rates in the recent years.  Similar for where I live, EU/Romania.  I can't visit certain websites.

Don't want to go into politics, but FIY, the level of censorship and propaganda in the western world right now is bigger than the one I've seen under Ceausescu dictatorship here in Romania, about 35+ years ago.  I'm not a nostalgic for communists regimes, or for dictators long gone, just that 30 years ago I would have never thought the western world would catch up with those dystopian levels of propaganda and censorship we used to have during the Ceausescu's regime.

I was very happy when Romania escaped the soviet influence, and joined to EU and to the western world, except now it turned into a clown world, lying in your face, all day long.  To give an example, last month our supreme court canceled the presidential elections in the middle of the process, because allegedly Tik-Toc influences.  No evidence, just that they canceled peoples votes.  Everybody here in Romania is outraged.  Same like everybody else in EU is outraged for being invaded and displaced by illegal immigrants with 10 times the birth rates of EU nations.  Same as people in UK is outraged for being arrested over misgendering people on twitter and such.

The censorship in the west is higher than ever, and increasing.  In this rhythm, won't be long till we catch up with North Korea.  We've already reached the levels I know from the former Communism/Socialism during Ceausescu. 

Sorry for the offtopic slip, but pretending everything is fine at this point is suicidal for the west.

As for the Tor Browser (Onion network) don't know if it can be used from where the OP is, might work, or might not work, worth a try.  Same for the torrent network, might work or might not, worth a try to install qBittorent, or alike torrent client with a search plugin included.

[KerimF]

By adding a LPF after VCO_in, the FM modulating signal can also be received.
In theory, this FM modulating frequency can be made high. So, one may think that the AM and FM modulating signals could be of a stereo system, left and right (having 9 kHz bandwidth each, for example). In this case, the FM-AM bandwidth of the transmitted signal would be... who can guess it

[radiolistener]

On our dear planet, Earth, humans have been forced, since always in human history, to live in different, if not divided, worlds (after all, this is how the world is designed, and no one can change it).

Just FYI, I'm living in the country where currently is one of the most bloody war of century with more than 1 millions deaths, I survived military assault on my city with everyday shelling and bombing for about 6 months long, and living in this hell for almost 3 years. Because there is no way to leave it for civilians, it is unofficially prohibited (in violation of the requirements of the constitution and laws). So if you know what I'm talking about, you should know that I'm living in one of the restricted place of the world at the moment. But I don't see the reason why you cannot use GNU Octave, especially if you have access to the internet.

This is even more confusing, since you're using LTSpice.


Here is example of AM, DSB and FM modulation written in Octave:

Code: [Select]

Fs = 384000;    % Sample rate
N = 2^24;       % Sample length

A = 1;          % Carrier amplitude
M = 0.8;        % Message amplitude
%m = M / A;      % AM modulation index m = M / A

wc = 7000 * 2*pi;   % Carrier frequency in radians
wm = 1    * 2*pi;   % Message frequency in radians
wd = 1000 * 2*pi;   % FM deviation in radians

message = M * sin(wm * (0:N-1)/Fs);
carrier = A * cos(wc * (0:N-1)/Fs);
   
% AM modulated signal
signal_am = (1 + message/A) .* carrier;   

% DSB modulated signal
signal_dsb = message .* carrier;

% FM modulated signal
phase_fm = cumsum((message * wd + wc) / Fs);
signal_fm = cos(phase_fm);

and here is the spectrum of this FM signal:


Do I understand correctly that your modulation scheme is a usual DSB modulation where you trying to use a special signal instead of carrier which is represented as FM modulation of 455 kHz carrier with 300 Hz message with 30 kHz deviation?

If so, I'm not sure what is the reason to use such complicated signal as a carrier? Is it some kind of attempt to encrypt the message?

[KerimF]

But I don't see the reason why you cannot use GNU Octave, especially if you have access to the internet.

Please note that I answered it in my reply #26.

This is even more confusing, since you're using LTSpice.

I am using its rather very old version LTspiceIV. I couldn't download any of its newer versions after LT was acquired by Analog Devices (which, unlike LT, has to obey the rules of its system). To my big surprise I was able to join its forum (of AD) to talk about this demodulator. A few days later, I got an email with an apology that AD had to ban me to avoid breaking the rules. It was in Holidays season, so I wish the sender a Merry Christmas and Happy New Year Eve).   

Here is example of AM, DSB and FM modulation written in Octave:

Code: [Select]

Fs = 384000;    % Sample rate
N = 2^24;       % Sample length

A = 1;          % Carrier amplitude
M = 0.8;        % Message amplitude
%m = M / A;      % AM modulation index m = M / A

wc = 7000 * 2*pi;   % Carrier frequency in radians
wm = 1    * 2*pi;   % Message frequency in radians
wd = 1000 * 2*pi;   % FM deviation in radians

message = M * sin(wm * (0:N-1)/Fs);
carrier = A * cos(wc * (0:N-1)/Fs);
   
% AM modulated signal
signal_am = (1 + message/A) .* carrier;   

% DSB modulated signal
signal_dsb = message .* carrier;

% FM modulated signal
phase_fm = cumsum((message * wd + wc) / Fs);
signal_fm = cos(phase_fm);

Do I understand correctly that your modulation scheme is a usual DSB modulation where you trying to use a special signal instead of carrier which is represented as FM modulation of 455 kHz carrier with 300 Hz message with 30 kHz deviation?

If so, I'm not sure what is the reason to use such complicated signal as a carrier? Is it some kind of attempt to encrypt the message?

I also cannot understand why you insist to use Octave other than finding the effective FM-AM signal bandwidth which could be deduced from the separate study of AM and FM modulations.

[KerimF]

If so, I'm not sure what is the reason to use such complicated signal as a carrier? Is it some kind of attempt to encrypt the message?

I guess you likely missed reading my reply #7.
 

[radiolistener]

Please note that I answered it in my reply #26.

I bought my today/old laptop a few months before March 2011 after which we had no mains AC, fuel and water for many years (we had just daily missiles over the streets and buildings at random). Now, we have 4/24 mains AC at best, and enough fuel to warm one room for a couple of hours (fortunately we are allowed to have fresh water) while almost all private businesses in the city (including mine) are paralyzed.

We also had a period when AC mains was delivered very rarely - several hours per several days or something like that, but it don't prevent to use computer from batteries. Usually it happened after next big bomb blast. I have 222 Wh battery for this case, which allows to run a tablet for about 24 hours or my small desktop for 7 hours. Usually internet is going offline after 3-4 hours of AC outage, but you can do some programming or development offline.

Missiles and drones still flying over my city and I hear air strike alarm and/or blasts sounds every hour, now it happens more rare, but still happens and everyone here is already used to it and doesn't pay attention. Of course there is a risk that it can hit some civilian building, and many building was damaged in that way (for example yesterday we have civilian building damage with bomb), but you cannot do anything with this.

I also cannot understand why you insist to use Octave other than finding the effective FM-AM signal bandwidth which could be deduced from the separate study of AM and FM modulations.

Octave allows to generate signal, estimate its parameters and quality and generate test signals which can be used in simulator for testing. You can do all things in the math first and then implement it in circuit and can test it in simulator. In other words you can make sure that all things works as expected and estimate it's quality even with no need to build a real circuit.

I am using its rather very old version LTspiceIV. I couldn't download any of its newer versions after LT was acquired by Analog Devices (which, unlike LT, has to obey the rules of its system). To my big surprise I was able to join its forum (of AD) to talk about this demodulator. A few days later, I got an email with an apology that AD had to ban me to avoid breaking the rules. It was in Holidays season, so I wish the sender a Merry Christmas and Happy New Year Eve).   

You can try simulator in KiCad. It also has circuit simulator, it is similar to LTSplice, but a little bit different. It is open source and I think it is probably available with no issue for you.

[KerimF]

Octave allows to generate signal, estimate its parameters and quality and generate test signals which can be used in simulator for testing. You can do all things in the math first and then implement it in circuit and can test it in simulator. In other words you can make sure that all things works as expected and estimate it's quality even with no need to build a real circuit.

What could I add?
I am fortunate that my old laptop has a genuine Excel (2007) by which I was (still am) able to solve all my design problems which are no more related to communications. Since year 1975, I used to produce various controllers which were needed by the local consumers in every period of time.

I just did this modulator as an MS thesis. Then, I used it for many years in the 80's to scramble my short-range RF voice links between home and workplace (3 km) till I got a phone line at home.

It happens that LTspice and Excel complement each other. For example, LTspice has FFT function (and THD) and Excel solves non-linear equations.

[KerimF]

I forgot to add that I do have Kicad though a very old version. I got it before the Kicad doors started to close too. This old version is good enough for the local old PCB services I print the PCB layout as PDF and send it to the PCB maker.

For instance, before year 2011, I used to send the designed schematic only to China where the Chinese engineers drew its layout, made its PCB, assembled its components and test the final ordered boards before exporting them to my place. This sort of easy life is now a thing of the past.
 

[dustooff]

Been following this thread as best I can, (not radio engineer).

By some coincidence this article came to life a few days ago,
https://www.allaboutcircuits.com/technical-articles/understanding-the-square-law-modulator-for-generating-am-signals

Initially I was curious as to the benefit of DSB-SC, it was not clear to me that the suppressed carrier was due to zero signal.
So it may be the main benefit is keeping a transmission path going without continuous carrier, thus making it hidden most of the time. (if necessary)

[radiolistener]

is this is what did you mean?

Code: [Select]

Fs = 1000000;   % Sample rate
N = 2^24;       % Sample length

A = 1;          % Carrier amplitude
M = 0.8;        % Message amplitude

wc = 455000 * 2*pi;   % Carrier for FM-modulated sub-carrier [frequency in radians]
ws = 300    * 2*pi;   % Message for FM-modulated sub-carrier [frequency in radians]
wd = 30000  * 2*pi;   % FM deviation for FM-modulated sub-carrier [frequency in radians]
wm = 1000   * 2*pi;   % Message frequency [frequency in radians]

subMessage = cos(ws * (0:N-1)/Fs);                      % 300 Hz sine
subCarrier = cos(cumsum((subMessage * wd + wc) / Fs));  % 455kHz FM modulated with 300 Hz

message = M * sin(wm * (0:N-1)/Fs);  % message 1 kHz sine

signal = message .* subCarrier;      % sub-carrier DSB modulated with message

%audiowrite("test.wav", [signal], Fs, "BitsPerSample", 32);



I tried to receive it with DSB, AM and FM demodulators, but it sounds like buzz. You can listen flac file in attachment on how it sounds on a usual receiver (it split on a three parts with different demodulators - DSB, AM and FM).

The main issue is that it requires 60-80 kHz bandwidth to transmit 1 kHz sine and it is unclear for me on how to reconstruct the carrier frequency and phase

[KerimF]

Been following this thread as best I can, (not radio engineer).

By some coincidence this article came to life a few days ago,
https://www.allaboutcircuits.com/technical-articles/understanding-the-square-law-modulator-for-generating-am-signals

Initially I was curious as to the benefit of DSB-SC, it was not clear to me that the suppressed carrier was due to zero signal.
So it may be the main benefit is keeping a transmission path going without continuous carrier, thus making it hidden most of the time. (if necessary)

Hi,

You got it. During silence periods there is no RF signal. This also applies on single sideband suppressed carrier system, SSB-SC.
The benefit of SSB is its reduced bandwidth (half of DSB), so the number of RF channels is doubled in a specific RF spectrum. But this has a price. The suppressed carrier cannot be restored from one side, it has to be generated in the receiver. This solves partially the recovery of the modulated signal since the generated carrier cannot be made in phase with the transmitted suppressed carrier. For this reason, the conventional envelop detector (with its limitation about the signal strength) has to be used to recover the modulating signal (also not in phase, but our ears are not sensitive to the phase variation of the audio signal).

The DSB-SC system was not recommended for two reasons. It needs twice the bandwidth of SSB (this is the main disadvantage of DSB). And it needs more power for the same receiver RF sensitivity. But this gives two extra bonuses. The carrier can be restored, frequency and phase, from the two symmetrical sidebands. So, the recovery of hi-fi audio signal can be achieved by using synchronous detection. The second benefit is that a relatively low strength of the RF signal doesn't distort the recovered signal as in the case of envelop detector.

There was (I say, there was, not there is) a third reason for which the DSB-SC wasn't recommended. Its known demodulators were more complex and less reliable than of the SSB-SC demodulators. The known topologies for DSB-SC demodulation were two. The first one is of Costas Loop which needs I-Q signals (in quadrature), not simple to build. And the second one is presented in your article (your URL above) which needs an LC selective filter which is somehow bulky.

In this thread, I presented a third topology (I knew and used it in real since the 80's, as explained earlier) which combines the previous two known topologies while being simpler and more reliable. It is simple because it doesn't need I-Q signals or an LC selective filter. It is more reliable because it locks to the suppressed carrier in a relatively wide spectrum (in my example here, Fc varies between 455-30 kHz and 455+30 kHz). Naturally, it can also recover the FM modulating signal (that is it could be seen as a double detector that recovers two signals at the same time).

But... there is always a BUT... the believers of 'Science' cannot add this third topology (which could be called 'Harmonic Loop', since 'Kerim Loop' is out of question, mainly in these days ), into their set of knowledge, before the today's representatives of 'Science' approve it officially.

[KerimF]

I tried to receive it with DSB, AM and FM demodulators, but it sounds like buzz. You can listen flac file in attachment on how it sounds on a usual receiver (it split on a three parts with different demodulators - DSB, AM and FM).

The main issue is that it requires 60-80 kHz bandwidth to transmit 1 kHz sine and it is unclear for me on how to reconstruct the carrier frequency and phase

Sorry, but I have to say it clearly to all readers here. Any AI robot has also no idea, at all, on how the presented FM-AM DSB-SC demodulator here could be made. I am afraid it is a novel solution that the world didn't recognize it universally yet.

Yes, a conventional AM detector, not my novel DSB-SC demodulator (discussed in this thread), detects the FM-AM signal of post #1 (an exaggerated example) as of a noisy interference.

[coppice]

Note where the OP is. People in Syria, Iran, and North Korea are restricted in various ways the rest of the planet are not.

You must be kidding when saying the rest of the planet is not under censorship/restriction.  It is, just that maybe not that severe restriction, but the restrictions are increasing fast, haven't anybody else noticed?  In fact the UK levels of censorship increased at alarming rates in the recent years.  Similar for where I live, EU/Romania.  I can't visit certain websites.

Where did I say the rest of the world was a paradise of freedom? I said those three countries have specific restrictions placed on them, which nowhere else does.  Lot of things are restricted for export to lots of places, but those three countries have a total ban on anything being supplied to them that has not been given explicit permission by an appropriate government. For example, see https://www.state.gov/iran-north-korea-and-syria-nonproliferation-act-sanctions-inksna/

[KerimF]

Fortunately, we all are allowed to talk together here as friends who care of each other, despite all sort of divisions in the world created (for money and power) by the powerful rich ruling systems.

[KerimF]

To those who have time to study the ‘Harmonic Loop’ demodulator, uploaded on reply #9, I repeat here its important points I mentioned ealier:

[1] There is a simple trick added to its conventional PLL that lets the PLL be locked to the suppressed carrier, frequency and phase, despite its polarity reversal at the zero crossings of the modulating signal. It is the frequency doubler (see U2 and U4, XOR gates). It could be used in other applications, for example, to double the frequency of a crystal.

[2] In every PLL circuit, the crucial part to be designed properly is its LPF (see R9, R10 and C6 in the uploaded example) in order to have a stable and fast response. On request, more details on how to calculate their values could be given, in case someone has no PC program (tool) made for such study/calculations.

[3] R7 and C5 are added between CD4013, U3, and CD4066, U7 (e-switch), to delay the phase of the recovered carrier and let it be in phase with the suppressed one when its frequency equals half of the VCO mid-frequency.
Although the two will be out of phase at higher and lower frequencies, their maximum phase difference is relatively small when the carrier is at the limits +/- 30 kHz. Please note that the phase difference reduces the gain of the synchronous detector.

[4] By adding a low pass filter at VCO_in, also the FM modulating signal can be recovered.
In theory, this FM modulating frequency can be made high (much higher than 300 Hz). So, one may think that the AM and FM modulating signals could be of a stereo system, left and right (having 9 kHz bandwidth each, for example).

This leads us to a question which was never asked at schools and universities
What is the effective bandwidth of an RF channel that uses an FM-AM signal (the carrier is AM and FM modulated at the same time)?
I can't access any AI robot. May someone ask one about it? Thank you.

[KerimF]

If this simple reliable DSB-SC demodulator was known in the early 80’s (soon after I knew it) and the new FM receivers, at that time, started using it, adding the pilot 19 kHz (for FM stereo) would be no more needed. This removal increases the power efficiency of the FM transmitters. Anyway, most transmissions are now made digital.

When I was in need, I took advantage of it (being unknown by the world) in the 80’s by transmitting the voice signal on 32768 Hz suppressed carrier (instead of 38 kHz of FM stereo) on the FM baseband. The conventional FM receivers detect just a silent channel during a conversation. And the lack of a pilot made impossible for an RF hacker (local or foreigner) to recover the voice signal properly. I used this technique for many years without being noticed

[KerimF]

If you want to simulate the modulation and demodulation you can look into GNU-radio that is a freeware. Together with a SDR tv dongle you can demodulate almost anything.

If someday you will find a tool, hardware or software, which can demodulate the FM-DSB-SC signal (described on post #1), I hope you will let me hear of it too because, to my knowledge, it doesn't exist yet to the world (other than the topology which I described in this thread). Thank you in advance.

[KerimF]

For instance, among the designs I rejected while I was looking, in 1979 as an MS thesis project, to build a simple reliable DSB-SC demodulator, it was the topology which was 'patented' for someone many years later (United States Patent 4430620). Its idea could be summarized:
“A DSB-SC signal could be demodulated without reinsertion of the carrier at the receiver. The received DSB-SC wave is amplified, demodulated by a full-wave rectifier. The rectifier output reversed in polarity at instants of successive zero crossings of the recovered signal.”

I couldn't accept it after I noticed that when the modulating signal becomes very close to zero before going up again (fake zero crossing), the output rectifier is fooled, and the output is reversed. In this case, the AC and DC voltages of the recovered signal are distorted.
And to determine when the reversing is needed or not, to avoid this sort of distortion, cannot be achieved by a simple circuit (the main purpose of the project).

I wonder who will be able to patent the topology of the DSB-SC 'Harmonic Loop' demodulator which we are discussing here. I will be glad to hear that someone will also take advantage of it, as I did in the 80's. Or it will be patented after I die

[KerimF]

Although no one here seems to be interested, for one reason or another, in my simple DSB-SC demodulator (non-existent yet to the world), we will see, in my next thread, how its topology could also be updated to receive/demodulate two signals in quadrature (yes, AM stereo).

Cheers

Note: Useful technical posts about my DSB-SC demodulator are at #1, #9, #14, #15, #40, #44,

[SteveThackery]

Although no one here seems to be interested, for one reason or another, in my simple DSB-SC demodulator (non-existent yet to the world).....

I don't think people are uninterested, it's just that what you are discussing might be seen as a "curiosity" of limited relevance in the real world. Yes, it is interesting and intriguing, but if you are hoping to see the world of broadcast radio adopt this technology en masse, you will surely be disappointed. Another way to express it is that it looks a bit like a solution looking for a problem.

Personally I do find it interesting although I can't follow the theory and mathematics so I cannot make any kind of useful contribution to this thread. For example, earlier on I asked why the carrier is frequency modulated, but I simply didn’t understand the answer.