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
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,
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.
Define "handle."
John
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
On 8 Sep 2006 15:27:42 -0700, in message , "Radium" scribed:
Perhaps you should have read just a wee bit further, in particular, just as far as the very next sentence:
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.
Radium wrote:
Isn't Nyquist Criteria for digital data only? AM radio is analog.
Can aliasing occur in analog AM radio?
On Sat, 09 Sep 2006 02:20:44 GMT, in message , "G. Schindler" scribed:
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?
On Sat, 09 Sep 2006 02:20:44 GMT, in message , "G. Schindler" scribed:
An interesting article:
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.
Bye. Jasen
PAM [Pulse Amplitude Modulation] is used for sampling. AM seems to be the "smooth" equivalent of PAM.
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?
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
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.
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
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.
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.
-- Errr.......... Since when is 150kHz audio?
-- The wagon wheels seem to turn backwards in the movies.
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.
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