FM-AM DSB-SC Demodulator

[Kerim]

To pass my time (since most people around me are just busy lately in predicting where and when the 3rd neutron bomb, planted underground, will be triggered,) I also modulated, on the LTspice schematic, the frequency of the suppressed carrier (please refer to the previous thread: “DSB-SC versus SSB-SC Demodulation, Continued”).

The frequency deviation is +/- 20 KHz (435Kz to 475 KHz). The FM modulating signal is a 300Hz sinewave.

I added the block A1 “Modulate” for FM and AM, also a simple RC LPF (R1=220K, C1=1n5) at VCO_in (for the FM LF output). I also removed the block X4, “AM IF Mute”, and its CD4066 switch, U2, between nodes PC1 and PC1m.

As you will see, by using the simple “Harmony Loop” (or Kerim Loop, if you like ), the DSB-SC demodulator can recover both the FM and AM demodulating signals, here the 300 Hz and 6000 Hz signals respectively.

For instance, whoever had time to read my previous posts about this demodulator may recall that I deliberately varied (in the 80’s) the frequency of the suppressed carrier at a rate of 6 Hz in about 60 KHz bandwidth, so that radio listeners on MW band couldn’t know that the detected noise/interference by their radio sets was actually of a voice RF transmission.

On the LTspice plot window, a close look at the trace ‘Raudio’ (the received 6000 Hz) shows that its amplitude is not constant as of the transmitted ‘Audio’ is. Fortunately, this won’t be noticed during a conversation. The output gain variation is due to the phase variation between the carrier and the VCO signal (to maintain the lock state), while the simple RC delay circuit, R5 and C7, is fixed (for about 45 degree). But, when necessary, this delay could also be made active and let it be controlled by VCO_in (PLL LPF output). In this case, the delay during lock could vary between 0 and 90 degrees (not 0-180, since F_mid = 2*Fc) when the VCO frequency varies between the lower and upper frequency limits.

The overall performance of this PLL demodulator depends, to a great extent, on the LPF between nodes PC1 and VCO_in. A professional analysis of its response is not trivial and, naturally, it has to be related to the application of interest and the chosen components, mainly if the demodulator is used in a combined FM-AM system.

Cheers,
Kerim

 

[Tony Stewart]

That's an interesting hybrid that I would appreciate more in my youth when my neurons fired faster.

 

[Kerim]

That's an interesting hybrid that I would appreciate more in my youth when my neurons fired faster.

I am not sure if the actual two strong earthquakes are artificial or natural. But, in both cases, businesses in the region are paralyzed to a great extent with a continuous fear of the near future. Even now, once a while, a weak or medium earthquake occurs during which the external screen of my laptop starts dancing for a few seconds on the desk

 

[rjenkinsgb]

Re. the above video - it's a scam, an edited version of a 2015 Russian video.

No, this video does not show a nuclear weapon being used in Yemen

A Facebook post shares a video of a huge explosion followed by the formation of a mushroom-shaped cloud and claims it was the result of a “tactical nuclear bomb” targeting civilian homes in Yemen. The explosion was actually the result of a 2015 airstrike using conventional weapons by the...

factcheck.afp.com

 

[Kerim]

Re. the above video - it's a scam, an edited version of a 2015 Russian video.

I used to believe what I observe personally on the ground (while being aware that I may be also fooled sometimes by artificial facts made much like magicians do ).

And to find out the hidden truth(s) in universal tales made for adults (some may call these tales 'sky fairies' ) I just apply the hint "The real nature of hidden roots could be known when their fruits show up" (similarly, it was said: "You will know them by their fruits"). So, I usually wait for the fruits of such universal tales to show up clearly, after being created and spread worldwide, though this may take several months, if not years. For instance, I look for truths just for my own knowledge, so that I won't see a naive person who follows some other's beliefs blindly (social, scientific, religious or political) every time I look at a mirror.

 

[Kerim]

Since the DSB-SC demodulator above could be used to recover two separate signals, this let me recall an idea (implemented in some transceivers) which was presented on the ARRL Handbook of 1983 (the last one I was able to order). It is titled ‘NarrowBand Voice Modulation’, on page 14-33:34 and figures 50, 51.

In brief, the speech signal of two audio spectrums are transmitted; 350 to 600Hz and 1000 to 2500 Hz. Here, the lower band could be transmitted by using FM. Also the spectrum of the upper band could be shifted (usually with inversion) to DC so that it occupies a bandwidth of 1500Hz (2500-1000) and could be transmitted by using AM. The bandwidth of the two combined transmissions (FM and AM) will be about 2*1500Hz which is about the typical one for SSB-SC transmission. In this case, the frequency deviation needs to be limited to 900Hz (beta_FM = delta_Fc / F_audio = 900/600 = 1.5) since the FM bandwidth = 2*(beta_FM + 1)*F_audio = 2*(delta_Fc + F_audio).

Be tuned

Kerim

 

[Kerim]

On the thread 'Effective Bandwidth of an AM-FM DSB-SC Signal' we found out that:
BW_fm_am = 2 * (F1 + F2 + dFc)
where:
F1 is the highest frequency of the AM modulating signal.
F2 is the highest frequency of the FM modulating signal.
dFc is the greatest frequency deviation.

So, if the licensed bandwidth is 4 KHz (=BW_fm_am), F1= 1500 Hz and F2=600 Hz (as on the previous post):
dFc = BW_fm_am/2 – F1 – F2 = 4000/2 – 1500 – 600 = - 100 Hz
This means that even if dFc / F2 (=Beta_FM) is close to zero, the needed bandwidth exceeds the 4000 Hz.

But there is a chance to transmit voice signal if we let F1 = 1000 Hz (from 1500 to 2500 Hz in the speech spectrum) and F2 = 250 Hz (from 350 Hz to 600 Hz). This needs balanced modulators and filters at low frequencies (not hard to do).
In this case:
dFc = 4000/2 – 1000 – 250 = 750 Hz
So,
Beta_FM = dFc / F2 = 750 / 250 = 3 which is not bad.

For instance, if this AM-FM DSB-SC is implemented in a transceiver system between two radio amateurs, their link will be a real secure one till some others will follow them.

Kerim

 

[rjenkinsgb]

For instance, if this AM-FM DSB-SC is implemented in a transceiver system between two radio amateurs, their link will be a real secure one till some others will follow them.

For information, you need to be wary of saying things like that, which could be misconstrued - in some countries (UK & I believe Europe, at least), any deliberate encryption or similar is illegal.

eg. From the UK licence conditions. section 11 (2)(b):

Messages sent from the station shall not be encrypted for the purposes of rendering the Message unintelligible to other radio spectrum users.

However if it's purely for the purpose of experimentation and research in improved modulation techniques (rather than content concealment), it is of course in line with purpose of the licence

... for the purpose of self-training in radio communications, including conducting technical investigations;

 

[Kerim]

For information, you need to be wary of saying things like that, which could be misconstrued - in some countries (UK & I believe Europe, at least), any deliberate encryption or similar is illegal.

Thank you for your clarification.
I guess what you pointed out applies in all Free countries (usually known as of the Western World).
And I may say that I am fortunate for being born in a country supposed not free :

For many years in the 80's, I linked my apartment and workplace (about 3 km air distance) by using first the FM-AM DSB-SC on the MW band, then the AM DSB-SC on the FM band (on 32KHz suppressed carrier and without a pilot) till I was able to get a phone line on both sides. Fortunately, in countries known as underdeveloped, no one expects real innovative ideas in communications for example (even the local authorities didn't believe me), so where I live there was (still is) no need for rules to regulate them