AM/FM on one carrier

Hi.

Yes.

More interesting questions would be "How much of the AM signal will the FM demodulator output have?" and vice-versa. The FM demodulator should do well if you have enough signal to produce hard limiting before the detector. The AM demodulator may have some trouble depending on how flat the AM frequency response is. By "10KHz front-end", do you mean 10 KHz of AM response, or a 10 KHz RF bandwidth? If it's the latter, plan on seeing FM signal from your AM demodulator.

You're welcome.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Not very well.

John

I suggest you use a simulator such as PSPICE to superimpose an AM signal and FM signal with different modulation frequencies and see for youself how the amplitude envelope of the composite signal changes.... That will give you an indication of how well the AM demodulator will respond to only the AM signal.....

Then build a jperfect limiter and simple HI-Q tank to use as an FM demodulator (easy to do since you can specify damn near infinite Qs) and see how the amplitude output of the tank changes, which will be an indication of the independence of the FM signal.

This , in my opinion, is one of the ways in which PSPICE (and other simulators) can shine. You can model almost any modulation and demodulation, and, using a transient simulation, see what happens...

My gut feel is that the two signals will not be demodulated independently.... The superposition of two independent signals will probably result in a mutually generated amplitude composite as well as a mutually generated phase composite..... It is easier to visualize if both modulating signals are exactly the same.... However, my gut feel has failed me before, and , if I were really curious, I would use the simulator.....

Good luck... when you find the answer, why not post it back here?

Andy

I was thinking that, in the frequency domain, the AM has lots of sidebands, and the FM has, well, lots of sidebands, and they're going to get a lot more tangled than they simplistically appear.

Consider the case where the AM is at 100% modulation... there's no FM signal at the negative peaks of the AM mod! That can't be good. Similarly, modulated FM adds sidebands that the AM detector will decode.

John

--
Since the amplitude of the carrier will be varying because of the AM
and there will be a constant amplitude offset due to the FM, I think
the envelope detection used to demodulate the AM signal will be
successful.

However, since the AM will generate sidebands which will vary in
frequency and mix (?) with the deviating FM, I think demodulating the
FM will be difficult to impossible.

Andy replies:

Absolutely ! One has to use realistic models and simulations to correctly model real world circuits. That comes with experience. And mistakes....

But my understanding of a LOT of theoretical stuff and mathematical stuff exploded when I was able to feed a perfect waveform thru a perfect circuit and see exactly what happened, as the purely mathematical treatment which was taught to all of us back in school predicted...

I also found that a lot of stuff that I thought I understood just wasn't so..... and became the better man for it....

As time went by, I started doing this more and more and simulators and models got better and better...... I still don't trust predictors of things like third order intermod or compression points, tho some people do. I guess they have either done a better modelling job or have better transistors to use (grin). But it is still fun as hell to do....

When I retired, I made damn sure I had my student copy of PSPICE running at home......

Andy

--
The broadcast _can\'t_ be pure AM because the process of modulation
creates sidebands which deviate from the carrier center frequency by
the frequency of the modulating waveform.

That is, for a 1MHz carrier being modulated by 1000Hz, three signals
will be broadcast: f1, the 1MHz carrier, f1 + f2, the carrier plus the
1000Hz modulating signal (the 1.001MHz upper sideband) and f1 - f2,
the carrier minus the 1000Hz modulating signal (the 0.999MHz lower
sideband).

That's a good point, Howard, --- I had forgotton about the VOR system.

That system has a reference signal which is a 9960 AM 30% subcarrier . The

9960 is then frequency modulated at 30 hertz with a deviation of 480 hertz.

The "variable" signal is produced with a rotating antenna. The transmitter VOR signal is sent to a cardiod radiation pattern that rotates at 30 hz. A receiver located in space would pick up a 30% AM component of 30 hz as the radiation pattern passes round....

So, the receiver gets a 30 hz AM signal which is a function of it's location relative to the transmitting station, and a reference signal which, when demodulated in an FM demodulator, goes thru zero when the antenna pattern is pointed North ( or east, .. I forget which..)

The phase of the ref 30 hz is then compared to the phase of the variable 30 hz to determine the bearing to, or from, the receiver to the transmitter......

And, this stuff passes thru the receiver and the AM and FM are demodulated without crosstalk. So, maybe pdru's idea has a chance after all......

I hope he tries it out and feeds it back to us.....

Andy

PS In my youth, I designed VOR/ILS nav systems for Bendix Avionics in Ft Lauderdale...... It was fun as hell.....

This is not correct. In all AM modes that do not use a form of suppressed carrier (such as ssb), the carrier is constant. !00% modulation only says that the combined power of the sidebands is 50% as much as the power of the carrier.

Don

Tjhat is not correct. The carrier amplitude is constant with modulation.

Don

If the carrier amplitude is constant, it is not being amplitude modulated. This is so obvious as to be tautological. "AM" = "amplitude modulation".

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Hello John,

True. Especially since real-world IF filters will have a finite slope and the passband won't be level. Unless you spend big bucks on a crystal filter.

A much better solution would be FM with a subcarrier. This used to be done on FM radio for distributing background music for stores. I don't know if they still do that. Another option is dual sideband or multiplex.

Regards, Joerg

Hello Andy,

But resist the temptation to place an ideal filter. That's the danger of Spice. It can show you a rose garden and then on the breadboard it's all weeds.

Regards, Joerg

In fact, FM stereo is a subcarrier scheme. The FM modulated channel carrier consists of a single (Left + Right) signal; the AM modulated

38Khz subcarrier consists of (Left - Right).

If you're in FM mono mode, you get only the demodulated FM sum (Left + Right) signal (this is rather uncommon now, except when the channel signal is too weak to get a good subcarrier signal). This was invented for compatibility with existing mono FM channels, to avoid obsoleting all existing mono FM radios, and promote acceptance of stereo by broadcasters, who were understandably nervous about adopting a new signal no existing listeners could use :-).

If you're in FM stereo mode, both signals are demodulated, and the difference signal is split and one copy inverted. The left channel is (Left + Right) + (Left - Right), and the right channel is (Left + Right)

- (Left - Right).

There's also a subcarrier above 38KHz (57KHz?) for distributing canned elevator style music. (Wasn't this MUZAK?)

John Perry

Is this digital or analog modulation? 100% AM modulation would imply that no power is being emitted for a certain percentage of the time. Therefore you would lose FM information. FM data could get lost if it was a digital system or FM analog could sound crappy if it was audio. But it real depends on the type of information that is being transmitted and how you are mixing the two together.

Thomas

I beleive I heard of something like this to do with stero AM.

you can sometimes pick up AM transmissions on FM receivers but quite faintly, either the broadcast isnt pure AM or the receiver isnt so good at rejecting AM.

Colin =^.^=

Single sideband is effectively simultaneous amplitude and phase modulation. Frequency is the derivative of phase. Amplitude and phase represent two degrees of freedom. In theory, one should be able to resolve the two degrees with the appropriate method of synchronous detection. However, practical implementation may be more difficult.

The familiar cartesian (rectangular) form of "I" and "Q" implemented in many RX'ers contains all the information, but in the wrong form. You are asking for the polar form of "A/theta" where A is the amplitude portion and theta is the integrated frequency. These have the obvious mathematical transformation and relationship. You need to implement "A/theta" rather than IQ, although A/theta can be derived from IQ in principle. Also, the large carrier in "regular" AM represents a DC offset. You'll need to get rid of this DC if you transmit the large carrier.

I doubt envelope detection will work for extracting only the AM portion (I suspect some form of synchronous detection is required), although limiting and ratio detecting for the FM portion may work to some extent.

I think this is possible but tricky. The RX'er will have some complexity, so what is the advantage over something like SSB, which can carry two channels in the same total bandwidth?

Thanks for adding an opinion on the subject. Your assumption is obviously in error.

Don

Not exactly fitting your example but-

a) IIRC VOR navigation systems use a signal that has both AM & FM components.

b0 Some M/W radio relay links use a QAM modulated carrier which is FM'd to (cheaply & easily) implement the site to site orderwire voice & data streams. The QAM will be in the order of megabits/second while the ordedwire only has a modulation bandwith of a few 10s of kilohertz, it's so "slow" it doesn't affect the QAM to any major degree.

H.