Electronics > Projects, Designs, and Technical Stuff

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

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KerimF:
Note1: The academic purpose of this thread could be read on reply #72
Note2: More details on replies #9 (LTspice), #14 (AM-FM equation), #40 (DSBSC vs SSBSC), #44 (to gurus in electronics)

The receiver of interest demodulates a special double sideband suppressed carrier (DSB-SC) signal.

The modulating signal is of speech (narrow baseband) or music (wide baseband).
The frequency of the suppressed carrier is 455 kHz (IF, Intermediate Frequency in AM receivers).
The carrier is also frequency modulated by 300 Hz sinewave with a frequency deviation +/- 30 KHz. (It may be called, therefore, FM-DSB-SC.)

To my knowledge, a graduate engineer from any faculty of RF communications wasn’t taught how it could be done, though a hobbyist can build it.
It doesn’t need I-Q signals (Costas Loop) or LC selective filter (Squaring method).
Its reliable simple topology could be integrated as a low-cost IC. This IC doesn’t exist because even an IEEE professional in AM communications has no idea how to do it.

But I suspect a few members here may have heard of it, right?

Happy NEW YEAR 2025

Kerim Fahme

Hint: It is based on conventional PLL.

pardo-bsso:
We had the same conversation some months ago but I can't find the link right now

KerimF:
Sorry for the delay, the battery of my old lap (15 years) decided to die now (right after I started this thread).
I couldn't replace it (or buy a new lap), once every few years, because of the sad events in my region since year 2011.
To avoid a crash now, I have to keep the lap in state of charging all the time. The problem is that we have at best 4/24 mains AC only, so we depend on external inverters/batteries during the rest of the day.

https://www.eevblog.com/forum/rf-microwave/a-simple-reliable-double-sideband-suppressed-carrier-(dsb-sc)-demodulator/

As you see, the previous thread was in the RF forum (around July 2023). I didn't continue it there after I realized that discussing of this DSB-SC demodulator (still unknown universally, though I had the chance to discover its basic topology since fall 1979 as an MS thesis) is related more to electronics than today's RF.

The irony is that the local people (even so-called professors in communications) still tell me that it is impossible for a local engineer (in an underdeveloped country) to know a technical practical idea not known in the developed ones. This doesn't surprise me because, on the internet, even the engineers in the developed countries find really hard to believe a non-familiar topology (or be interested to verify it for their own knowledge in the least) as long it is not approved first by certain top representatives of today's Science.

Naturally, if no one here likes (or is capable) to discuss it (this non-familiar topology of AM demodulation) in depth, electronically (formulas, parameters... for various applications), I agree with you that there is no need to keep this thread alive in this forum too.

radiolistener:
Let's use matlab/octave.

Usual AM can be written as:

--- Code: ---Fs = 384000;     % sample rate
N = Fs*3;       % sample length 3 seconds

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

carrier = A * cos(7000 * 2*pi*(0:N-1)/Fs);
message =     cos(1    * 2*pi*(0:N-1)/Fs);      % Normalized

signal_am = (1 + m*message) .* carrier;

--- End code ---

Could you write formula for your modulation, so we can test it?

SteveThackery:
@KerimF: could you explain more clearly what is the purpose of this thread? I think then you would get more engagement.

The thread title is "Did you hear of...", to which the only answer is "Yes" or "No" - hardly the basis for an interesting discussion. Or is it supposed to be a test for forum members, to see if they can design a circuit that will receive such a signal? Or are you hoping to discuss the benefits of double sideband suppressed carrier? Please explain clearly what you want to discuss.

I learned about various modulation systems several decades ago, and I seem to recall that DSSC was a standard AM signal with the carrier not transmitted. Even more economical for transmitter power is single sideband suppressed carrier.  My understanding is that both carriers contain the same information, so there is no benefit in transmitting both.

So there you go: the sum total of my "knowledge".  :)

My question is this: what is the purpose of frequency modulating the carrier at 300Hz?  And don't forget to tell us what you want from this thread.

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