AM/FM on one carrier

[[email protected]]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

Any comments?

Thank,

pdrunen

 

[Larry Brasfield]

Hi All, Hi.
Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?
Yes.

Any comments?

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.

Thank,

You're welcome.

 

[John Larkin]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

Not very well.

John

 

[[email protected]]

Andy writes:

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

 

[Joerg]

Hello John,

Not very well.

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

 

[Joerg]

Hello Andy,

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...

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

 

[Thomas Magma]

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

 

[John Fields]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

---
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.

 

[colin]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

Any comments?

Thank,

pdrunen

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 =^.^=

 

[John Fields]

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.

---
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).

 

[Dr. Sisyphus Frankenstein]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

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?

 

[John Larkin]

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

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

 

[Howard Eisenhauer]

Hi All,

Given that I have a carrier which I can AM modulate and FM modulate, AM
is max at 100% and FM deviation is +/-5KHz. The FM side and the AM side
are different signals BW limited to 2.5KHz.

On the other end is a AM only receiver with 10KHz front-end
and an narrowband FM only receiver tuned to the carrier frequency.

Will the receivers be able to demodulate with the other modulation
riding the carrier?

Any comments?

Thank,

pdrunen

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.

 

[Andy]

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

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

 

[Andy]

Andy writes:

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.....

 

[Joerg]

Hello Andy,

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

Try Linear's LT Spice. It is quite good and free.

Regards, Joerg

 

[John Perry]

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.

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

 

[Don Bowey]

Is this digital or analog modulation? 100% AM modulation would imply that no
power is being emitted for a certain percentage of the time.

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

 

[Don Bowey]

Since the amplitude of the carrier will be varying because of the AM
and

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

Don

 

[Larry Brasfield]

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

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