Radio and Aliasing

[Alan B]

The responses I've received have confused me.

Sit down, calm down, find a reference book, and read along with the
discussion.

What is the highest frequency that can be received on a 150 khz AM
radio receiver? Is it 150 khz, 300 khz, 75 khz, or 60 khz?

Answer a simple question; are you asking about theoretical possibilities or
administrative possibilities?

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

You need to develop a discriminating mind (pun somewhat intended).

 

[John Larkin]

It may be possible to SEND the modulated signal but ..

The typical demodulation process is a sampling system sampling at Fc and
that you cannot recover data if Fm > Fc/2.

As of right now I believe that though you MAY BE correct in the
transmission side of things (I'm not certain that what you are
describing is modulation as opposed to mixing)

Modulation here means multiplication of the signal (plus a DC offset)
by a sinusoidal carrier. That's what AM is.

Out = ( sig(t)+1 ) * sin(2*pi*150000*t)

where sig(t) is the signal.

but you still could not
demodulate the signal using a typical AM detector.

Right, a cheapo envelope detector doesn't work well with
super-wideband AM, but there are several simple ways to perfectly
recover the signal, a carrier-phase-locked lo and another multiplier,
followed by a good lowpass filter, being one. I think an ideal diode
rectifier or a square-law detector, either followed by a lowpass, work
too.

As long as Fm < Fc, the spectrum is unambiguous, so perfect
demodulation must be possible.

It gets interesting if the receiver has a front-end highpass filter
that passes nothing below the carrier frequency. The result is sort of
a VSB signal (actually, it's SSB+carrier) and all the low-frequency
aliases are discarded. In that case, I think you can use a simple AM
modulator up to Fm = 300 KHz, twice the carrier frequancy!

John

 

[John Larkin]

Wrong. "Aliasing" in this case (AM radio) occurs when the
bandwidth of the modulating signal exceeds that of the carrier,
not 1/2 the carrier - the reason being that the effective "sample
rate" (if you're going to consider AM radio to be a sampling
system) is twice the carrier frequency.

John also missed the boat a little bit, in saying:


Actually, Nyquist DOES apply here, you just have to look at
the "samples" as occuring at the peak (and trough) of each
carrier cycle (which of course define the amplitude "envelope"
of the modulated carrier. It's a bit odd to look at AM radio in this
manner, but the basic criteria DOES hold true here and the
aliasing occurs if the rates aren't right, just as Nyquist predicts.

Bob M.

What boat did I miss? Why would only the peaks of the carrier sinewave
"sample" the signal, and not the entire sinewave? And how does your
theory deal with the fact that the sine-carrier "samples" alternate in
polarity, whereas a real 2F sampler wouldn't? They are nothing like
equivalent.

Look at the spectrum of a sine wave, and look at the spectrum of an
impulse train. They are very different, and, specifically, a sine wave
has no harmonic lines, and an impulse train has all of them, including
the DC term. That's why a linear lowpass filter recovers PAM signals
but can't recover AM.

John

 

[John Larkin]

Well in all fairness, it *is* sci.electronics.BASICS. I suggest you have a
little more patience. Not everyone who asks a question or posts a response
is trying to be perfect, or otherwise infallible. This discussion is
having an immense impact on my orientation into my new career, so let's
keep it civil, folks! So far, so good.

Well, if people state things as facts and speak with authority, they
might consider making some modest effort to be right. It doesn't help
beginners to tell them stuff that's nonsense.

John

 

[John Larkin]

Sorry, no - it's 150 kHz. If you want to approach AM from
the standpoint of sampling theory, you have to realize that there
are two "samples" per cycle of the carrier.

Bob M.

Better yet, realize that the process is time-continuous so there are
an infinite number of "samples" per cycle.

John

 

[John Larkin]

What boat did I miss? Why would only the peaks of the carrier sinewave
"sample" the signal, and not the entire sinewave? And how does your
theory deal with the fact that the sine-carrier "samples" alternate in
polarity, whereas a real 2F sampler wouldn't? They are nothing like
equivalent.

Look at the spectrum of a sine wave, and look at the spectrum of an
impulse train. They are very different, and, specifically, a sine wave
has no harmonic lines, and an impulse train has all of them, including
the DC term. That's why a linear lowpass filter recovers PAM signals
but can't recover AM.

John

Eureka, that's it: the fact that the sinewave carrier "samples"
alternate in polarity means the baseband component of an AM modulated
signal is zero, as contrasted to a truly sampled signal where the
baseband spectrum is preserved intact.

Cool stuff.

John

 

[Don Bowey]

The responses I've received have confused me.

What is the highest frequency that can be received on a 150 khz AM
radio receiver? Is it 150 khz, 300 khz, 75 khz, or 60 khz?

What is the bandwidth of the receiver? That is the limiting factor.

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

 

[Don Bowey]

Sorry. I meant to say what is that highest modulation frequency...

That still depends on the bandwidth of the receiver. It's a receiver design
issue. Is this something you intend to build or buy?

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz station can contain a modulation signal of 300 khz.

Which should I believe??

 

[Radium]

Sit down, calm down, find a reference book, and read along with the
discussion.

Answer a simple question; are you asking about theoretical possibilities or
administrative possibilities?
Theoretical

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

You need to develop a discriminating mind (pun somewhat intended).

 

[Radium]

What is the bandwidth of the receiver? That is the limiting factor.

150 Khz

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

Find out the highest possible frequency of modulation that a 150 khz AM
station can transmit or receive.

What is the lowest physically-possible frequency of an AM station that
will allow me to hear a 20,000 khz tone on the receiver?

Surely a 1 hz station wouldn't work for this [DUH!!!!]

 

[Radium]

That still depends on the bandwidth of the receiver. It's a receiver design
issue.

Is this something you intend to build or buy?

I am simply looking for an scientific answer to a question that
interests me.

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz station can contain a modulation signal of 300 khz.

Which should I believe??

 

[Don Bowey]

What is the bandwidth of the receiver? That is the limiting factor.

150 Khz

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

Find out the highest possible frequency of modulation that a 150 khz AM
station can transmit or receive.

In a previous post you said the bandwidth of the receiver is 150 kHz. Now
you say the carrier frequency of the signal is 150 kHz, So we can assume the
bandwidth extends to 75 kHz above and below the carrier. Is this what you
assume for the receiver? If not then make your assumption set more
explicit.

What is the lowest physically-possible frequency of an AM station that
will allow me to hear a 20,000 khz tone on the receiver?

Are you assuming a standard double sideband signal, or a single sideband
signal?

Surely a 1 hz station wouldn't work for this [DUH!!!!]

 

[John Larkin]

Right, a cheapo envelope detector doesn't work well with
super-wideband AM, but there are several simple ways to perfectly
recover the signal, a carrier-phase-locked lo and another multiplier,
followed by a good lowpass filter, being one. I think an ideal diode
rectifier or a square-law detector, either followed by a lowpass, work
too.

OK, on reconsideration, an envelope or square-law type detector won't
work to properly demodulate super-wideband AM, because it will
generate too many spurious cross-products. The synchronous detector is
still OK.

John

 

[Don Bowey]

Well gee, AM is a multiplication. If you divide out the carrier, you get
the signal back ...period.

To quote our new "scientist" poster..... Well Duh!

AM is typically demodulated with a switch on the signal peaks, which is
indeed sampling. But that only messes with the peaks...

Actually AM, as assumed by the poster, is not typically "demodulated," it is
"detected." Not unlike charging a capacitor from PAM pulses.

Tell me, the dV/dt around zero crossing changes in proportion with the
amplitude, no? 'Tis information completely disregarded by a diode (or
sampling) detector, but it's there nonetheless, in all its subtlety. You
can't go and tell me it doesn't represent something now...

(Or if you don't like my case for dV/dt inbetween peaks, take absolutely
anything else about the overall shape of the waveform, outside of the peaks
themselves.)

Your question is totally unclear to me. To what signal do you refer?

Tim

Don

 

[Don Bowey]

Well, I'm in the Pacific Northwest. If there's some way we could all get
together, I'd by the first round.

That's two of us in the Northwest... The land of some great microbreweries.

Don

 

[Don Bowey]

Wrong. "Aliasing" in this case (AM radio) occurs when the
bandwidth of the modulating signal exceeds that of the carrier,
not 1/2 the carrier - the reason being that the effective "sample
rate" (if you're going to consider AM radio to be a sampling
system) is twice the carrier frequency.

You imply here that the carrier has bandwidth; it doesn't. I assume you
meant to say "...modulating frequency exceeds the frequency of the
carrier,...."

 

[Radium]

On 9/9/06 5:28 PM, in article
[email protected], "Radium"

The responses I've received have confused me.

What is the highest frequency that can be received on a 150 khz AM
radio receiver? Is it 150 khz, 300 khz, 75 khz, or 60 khz?

What is the bandwidth of the receiver? That is the limiting factor.

150 Khz

Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

Find out the highest possible frequency of modulation that a 150 khz AM
station can transmit or receive.

In a previous post you said the bandwidth of the receiver is 150 kHz. Now
you say the carrier frequency of the signal is 150 kHz, So we can assume the
bandwidth extends to 75 kHz above and below the carrier. Is this what you
assume for the receiver? If not then make your assumption set more
explicit.

Okay. Let me make a modification. Lets make the the the bandwidth of
the receiver 150 Ghz [notice that 'G'] but keep the carrier frequency
of 150 Khz. Now, what is the maximum frequency the the modulation can
be? I guess its 750 Mhz. Do I guess correctly?

Are you assuming a standard double sideband signal, or a single sideband
signal?

Standard.

If the carrier frequency is 1 hz but the bandwidth of the reciever is
40 Khz, then could I [at least in theory] hear anything on the speaker
of the receiver?

Surely a 1 hz station wouldn't work for this [DUH!!!!]

 

[Don Bowey]

Don Bowey wrote:
On 9/9/06 5:28 PM, in article
[email protected], "Radium"

The responses I've received have confused me.

What is the highest frequency that can be received on a 150 khz AM
radio receiver? Is it 150 khz, 300 khz, 75 khz, or 60 khz?


What is the bandwidth of the receiver? That is the limiting factor.

150 Khz



Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

Find out the highest possible frequency of modulation that a 150 khz AM
station can transmit or receive.

In a previous post you said the bandwidth of the receiver is 150 kHz. Now
you say the carrier frequency of the signal is 150 kHz, So we can assume the
bandwidth extends to 75 kHz above and below the carrier. Is this what you
assume for the receiver? If not then make your assumption set more
explicit.

Okay. Let me make a modification. Lets make the the the bandwidth of
the receiver 150 Ghz [notice that 'G'] but keep the carrier frequency
of 150 Khz. Now, what is the maximum frequency the the modulation can
be? I guess its 750 Mhz. Do I guess correctly?

Are you assuming a standard double sideband signal, or a single sideband
signal?

Standard.

If the carrier frequency is 1 hz but the bandwidth of the reciever is
40 Khz, then could I [at least in theory] hear anything on the speaker
of the receiver?

Surely a 1 hz station wouldn't work for this [DUH!!!!]

You are establishing unrealistic conditions.

I suggest you use Google to do some homework. Search "amplitude
modulation," which is a multiplication process having application, not only
in transmitter modulation per se', but also in designs of transmitters and
receivers where it is desirable to move a single frequency or band of
frequencies to a different frequency range (intermediate frequency).
Particularly look a the modulation product algebra.

 

[Radium]

On 9/9/06 9:42 PM, in article
[email protected], "Radium"


Don Bowey wrote:
On 9/9/06 5:28 PM, in article
[email protected], "Radium"

The responses I've received have confused me.

What is the highest frequency that can be received on a 150 khz AM
radio receiver? Is it 150 khz, 300 khz, 75 khz, or 60 khz?


What is the bandwidth of the receiver? That is the limiting factor.

150 Khz



Some of the responses have told me that Nyquist theorem means that the
frequency of the station must be at least 2x [and due to physical
limitations, at least 2.5x] that of the highest frequency of the
modulation [audio] signal. Other responses have said different. Some
have said 150 khz can contain a modulation signal of 300 khz.

Which should I believe??

What are you attempting to do?

Find out the highest possible frequency of modulation that a 150 khz AM
station can transmit or receive.

In a previous post you said the bandwidth of the receiver is 150 kHz. Now
you say the carrier frequency of the signal is 150 kHz, So we can assume the
bandwidth extends to 75 kHz above and below the carrier. Is this what you
assume for the receiver? If not then make your assumption set more
explicit.

Okay. Let me make a modification. Lets make the the the bandwidth of
the receiver 150 Ghz [notice that 'G'] but keep the carrier frequency
of 150 Khz. Now, what is the maximum frequency the the modulation can
be? I guess its 750 Mhz. Do I guess correctly?

What is the lowest physically-possible frequency of an AM station that
will allow me to hear a 20,000 khz tone on the receiver?

Are you assuming a standard double sideband signal, or a single sideband
signal?

Standard.

If the carrier frequency is 1 hz but the bandwidth of the reciever is
40 Khz, then could I [at least in theory] hear anything on the speaker
of the receiver?

Surely a 1 hz station wouldn't work for this [DUH!!!!]

You are establishing unrealistic conditions.

How is it unrealistic?

 

[Bob Myers]

What boat did I miss? Why would only the peaks of the carrier sinewave
"sample" the signal, and not the entire sinewave? And how does your
theory deal with the fact that the sine-carrier "samples" alternate in
polarity, whereas a real 2F sampler wouldn't? They are nothing like
equivalent.

Obviously, it is not only the peaks which "sample" the signal,
but you CAN use sampling theory in an analysis of AM if
you treat it from that perspective. It's hardly "my" theory.

Bob M.