Radio and Aliasing

[Radium]

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?


Thanks,

Radium

 

[Bob Myers]

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

In theory, you could get to 150 kHz, and no higher.
This gets to your second question,

In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?

Amplitude modulation can actually be viewed as very
analogous to sampling, but the simple answer is that when
you perform this sort of modulation, you wind up with
"sidebands" which extend above and below the carrier
frequency by the frequency of the modulating signal. In
other words, if you modulate a carrier with audio in the
0-10 kHz range, the sidebands will extend to 10 kHz
above and below the carrier frequency. The complete
AM signal, then, occupies a bandwidth twice that of the
modulating signal.

Bob M.

 

[John Larkin]

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?


Thanks,

Radium

Define "handle."

John

 

[Radium]

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

 

[Eeyore]

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

Errr.......... 150kHz maybe ? Maybe SSB can do better ?

Graham

 

[Alan B]

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

Perhaps you should have read just a wee bit further, in particular, just as
far as the very next sentence:

 

[G. Schindler]

Someone seems to be misreading the question (conceivably, it could be me).

Because of the reference to sampling I believe the question is meant ask
the highest modulation frequency that can be transmitted over a 150 kHz
AM broadcast. The answer here is 75 kHz due to Nyquist Criteria.
Nyquist Criteria is the reason for limiting the Digital Audio data to
one half the sampling rate.

One of the implications of this is that the highest frequency component
(including distortion and/or harmonics) of the "data" must be less than
one half the carrier frequency (or modulation frequency) to avoid
aliasing effects.

As someone said, however, there are legal limits that should keep one
from approaching this.

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

In theory, you could get to 150 kHz, and no higher.
This gets to your second question,



Amplitude modulation can actually be viewed as very
analogous to sampling, but the simple answer is that when
you perform this sort of modulation, you wind up with
"sidebands" which extend above and below the carrier
frequency by the frequency of the modulating signal. In
other words, if you modulate a carrier with audio in the
0-10 kHz range, the sidebands will extend to 10 kHz
above and below the carrier frequency. The complete
AM signal, then, occupies a bandwidth twice that of the
modulating signal.

Bob M.

 

[Radium]

Someone seems to be misreading the question (conceivably, it could be me).

Because of the reference to sampling I believe the question is meant ask
the highest modulation frequency that can be transmitted over a 150 kHz
AM broadcast. The answer here is 75 kHz due to Nyquist Criteria.
Nyquist Criteria is the reason for limiting the Digital Audio data to
one half the sampling rate.

Isn't Nyquist Criteria for digital data only? AM radio is analog.

One of the implications of this is that the highest frequency component
(including distortion and/or harmonics) of the "data" must be less than
one half the carrier frequency (or modulation frequency) to avoid
aliasing effects.

Can aliasing occur in analog AM radio?

As someone said, however, there are legal limits that should keep one
from approaching this.

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

In theory, you could get to 150 kHz, and no higher.
This gets to your second question,


In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?

Amplitude modulation can actually be viewed as very
analogous to sampling, but the simple answer is that when
you perform this sort of modulation, you wind up with
"sidebands" which extend above and below the carrier
frequency by the frequency of the modulating signal. In
other words, if you modulate a carrier with audio in the
0-10 kHz range, the sidebands will extend to 10 kHz
above and below the carrier frequency. The complete
AM signal, then, occupies a bandwidth twice that of the
modulating signal.

Bob M.

 

[Alan B]

Because of the reference to sampling I believe the question is meant ask
the highest modulation frequency that can be transmitted over a 150 kHz
AM broadcast. The answer here is 75 kHz due to Nyquist Criteria.
Nyquist Criteria is the reason for limiting the Digital Audio data to
one half the sampling rate.

The fist part of the question was: "Lets say there is an AM station with a
carrier frequency of 150 KHz. What is the highest frequency of modulation
that it can handle?"

With an theoretically allotted bandwidth of 300kHz (lower and upper
sidebands), I can modulate a tone of 150kHz. Now, this has no relationship
to Nyquist, but simply to how high a tone can be transmitted inside an
allotted bandwidth. Even so, the answer is still half the bandwidth, since
the modulated tone will appear both above and below the carrier. Then
eliminate the carrier and the upper sideband and we run into some physical
constraints, such as: a 150kHz carrier with a lower sideband modulation of
150kHz will be theoretically be broadcasting at zero Hz.

This brings to mind a question: can upper-sideband modulation exceed the
lower boundary of lower-sideband modulation?

The second part of the question was: "In digital audio, the sample rate
must be at least 2x the highest frequency. What is the equivalent in analog
AM radio?" My answer, I don't think there is an equivalent. Nyquist is
used for decoding information from an analog signal by taking samples at
some fixed rate, and the upper bound of intelligible information is set at
one-half the sampling rate. I don't believe this has anything to do with
the capability of a broadcast transmitter to transmit a tone of some
maximum frequency.

Comments?

 

[Alan B]

Someone seems to be misreading the question (conceivably, it could be me).

An interesting article:

http://broadcastengineering.com/mag/broadcasting_revisiting_nyquist/index.html

 

[jasen]

Isn't Nyquist Criteria for digital data only? AM radio is analog.

no it's for sampling. and each peak (or valley) of the carrier is a
sample of the transmitted signal. It has other analogue uses too.
like switched capacitor filters.

Can aliasing occur in analog AM radio?

modulate a 150Khz signal at 200 Khz an there'll be a 50Khz alias generated.

Bye.
Jasen

 

[Radium]

no it's for sampling. and each peak (or valley) of the carrier is a
sample of the transmitted signal. It has other analogue uses too.
like switched capacitor filters.

PAM [Pulse Amplitude Modulation] is used for sampling. AM seems to be
the "smooth" equivalent of PAM.

modulate a 150Khz signal at 200 Khz an there'll be a 50Khz alias generated.

So I take that the the highest pitch that can be transmitted on an
analog AM 150 Khz station is 150 KHz. Or is it 150/2 khz?

Does Nyquist apply to AM as well or just PAM?

 

[Don Bowey]

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

Errr.......... 150kHz maybe ? Maybe SSB can do better ?

Graham

Uh.. Nope.

Regulations aside, the highest modulating frequency you should use with a
150 kHz carrier is 60 kHz. Anything higher will be garbage.

Don

 

[Don Bowey]

Someone seems to be misreading the question (conceivably, it could be me).

Because of the reference to sampling I believe the question is meant ask
the highest modulation frequency that can be transmitted over a 150 kHz
AM broadcast. The answer here is 75 kHz due to Nyquist Criteria.
Nyquist Criteria is the reason for limiting the Digital Audio data to
one half the sampling rate.

In "AM radio" the modulation is not synchronized to the carrier, so 2:1
isn't adequate. I would say 2.5:1 for carrier to modulation frequency will
deliver a good output signal.

One of the implications of this is that the highest frequency component
(including distortion and/or harmonics) of the "data" must be less than
one half the carrier frequency (or modulation frequency) to avoid
aliasing effects.

As someone said, however, there are legal limits that should keep one
from approaching this.

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

In theory, you could get to 150 kHz, and no higher.
This gets to your second question,


In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?

Amplitude modulation can actually be viewed as very
analogous to sampling, but the simple answer is that when
you perform this sort of modulation, you wind up with
"sidebands" which extend above and below the carrier
frequency by the frequency of the modulating signal. In
other words, if you modulate a carrier with audio in the
0-10 kHz range, the sidebands will extend to 10 kHz
above and below the carrier frequency. The complete
AM signal, then, occupies a bandwidth twice that of the
modulating signal.

Bob M.

 

[Don Bowey]

The fist part of the question was: "Lets say there is an AM station with a
carrier frequency of 150 KHz. What is the highest frequency of modulation
that it can handle?"

With an theoretically allotted bandwidth of 300kHz (lower and upper
sidebands), I can modulate a tone of 150kHz. Now, this has no relationship
to Nyquist, but simply to how high a tone can be transmitted inside an
allotted bandwidth. Even so, the answer is still half the bandwidth, since
the modulated tone will appear both above and below the carrier. Then
eliminate the carrier and the upper sideband and we run into some physical
constraints, such as: a 150kHz carrier with a lower sideband modulation of
150kHz will be theoretically be broadcasting at zero Hz.

This brings to mind a question: can upper-sideband modulation exceed the
lower boundary of lower-sideband modulation?

The second part of the question was: "In digital audio, the sample rate
must be at least 2x the highest frequency. What is the equivalent in analog
AM radio?" My answer, I don't think there is an equivalent. Nyquist is
used for decoding information from an analog signal by taking samples at
some fixed rate, and the upper bound of intelligible information is set at
one-half the sampling rate. I don't believe this has anything to do with
the capability of a broadcast transmitter to transmit a tone of some
maximum frequency.

Comments?

Yes.

Nyquist does apply to AM modulation, and AM is considered a linear system
where each sideband (when demodulated) is identical to the modulation
signal.

Consider a 100 kHz carrier and a 500 kHz modulation signal.... Between
successive samples (the carrier frequency), five cycles of the modulation
signal will be available to be sampled. Since four will be unsampled, one
cannot say the process is linear, failing the basic requirement for AM. Use
40 kHz for the modulation, and the result will be linear.

Don

 

[G. Schindler]

no it's for sampling. and each peak (or valley) of the carrier is a
sample of the transmitted signal. It has other analogue uses too.
like switched capacitor filters.




modulate a 150Khz signal at 200 Khz an there'll be a 50Khz alias generated.

Right ... Aliasing will happen anytime the audio signal exceeds 1/2 the
carrier frequency so it would also happen for 100 kHz audio on a 150 kHz
carrier.

 

[G. Schindler]

no it's for sampling. and each peak (or valley) of the carrier is a
sample of the transmitted signal. It has other analogue uses too.
like switched capacitor filters.

PAM [Pulse Amplitude Modulation] is used for sampling. AM seems to be
the "smooth" equivalent of PAM.

modulate a 150Khz signal at 200 Khz an there'll be a 50Khz alias generated.

So I take that the the highest pitch that can be transmitted on an
analog AM 150 Khz station is 150 KHz. Or is it 150/2 khz?

Does Nyquist apply to AM as well or just PAM?

Nyquist applies here too and you could visualize this by trying it on
paper. Draw out a sine wave and try to modulate it with a signal of
it's own frequency. Remember that AM impresses the information on the
"envelope" of the carrier peaks.

These two links do a pretty good job of helping you visualize the
process but unfortunately neither allows you to visualize what happens
when you violate Nyquist Criteria. I'll keep looking.

http://www.williamson-labs.com/480_am.htm

http://cnyack.homestead.com/files/modulation/modam.htm

 

[John Fields]

Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?

Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).

Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.

Errr.......... 150kHz maybe ? Maybe SSB can do better ?

 

[John Fields]

These two links do a pretty good job of helping you visualize the
process but unfortunately neither allows you to visualize what happens
when you violate Nyquist Criteria.

 

[G. Schindler]

In "AM radio" the modulation is not synchronized to the carrier, so 2:1
isn't adequate. I would say 2.5:1 for carrier to modulation frequency will
deliver a good output signal.

Yes, and no (I think) .... If we were sending a single frequency tone,
anything greater than 2:1 should be adequate. The real rub comes when
trying to send something like music or voice with multiple time varying
frequencies. These signals are full of harmonics by their vary nature.

We normally place a low pass filter in front of our modulator/sampling
circuit circuit to eliminate alias causing frequencies. Unfortunately,
in the real world filters are not perfect and we have to select our
filter so that any alias causing signal is reduced to insignificance.
Because of this we will have to choose a filter frequency that is
substantially lower than the Nyquist frequency (Nyquist Frequency =
Fc/2). But really, all of this is really a compromise of practicality
to keep the real ratio above 2:1.

I don't think synchronization in itself will have any effect except when
it is indicative of higher frequency components in the modulation signal.

One of the implications of this is that the highest frequency component
(including distortion and/or harmonics) of the "data" must be less than
one half the carrier frequency (or modulation frequency) to avoid
aliasing effects.

As someone said, however, there are legal limits that should keep one
from approaching this.


Radium wrote:

Bob Myers wrote:




Hi:

Lets say there is an AM station with a carrier frequency of 150 KHz.
What is the highest frequency of modulation that it can handle?


Legally, 5 kHz (well, OK, I can't say that for a 150 kHz
transmitter; I'm talking about the legal limits on radio
stations in the AM broadcast band, 535 - 1705 kHz).


Sorry. I meant to ask what is the highest audio frequency that can
[physically] be broadcasted through a 150 Khz carrier.



In theory, you could get to 150 kHz, and no higher.
This gets to your second question,



In digital audio, the sample rate must be at least 2x the highest
frequency. What is the equivalent in analog AM radio?

Amplitude modulation can actually be viewed as very
analogous to sampling, but the simple answer is that when
you perform this sort of modulation, you wind up with
"sidebands" which extend above and below the carrier
frequency by the frequency of the modulating signal. In
other words, if you modulate a carrier with audio in the
0-10 kHz range, the sidebands will extend to 10 kHz
above and below the carrier frequency. The complete
AM signal, then, occupies a bandwidth twice that of the
modulating signal.

Bob M.