Amplitude Modulation

Hi!

I am a newbie studying for the Technician Exam. And, I have questions about amplitude modulation.

Single-Tone Audio Signal

Can I say that if one mixes a carrier signal -with a fixed high frequency and a fixed amplitude- with a single-tone audio signal -with a fixed low frequency and a fixed amplitude- the result is one radio signal (RF) which has (1) a fixed high frequency -the fixed high frequency of the carrier signal- and (2) a fixed amplitude determined by the frequency of the single-tone audio signal?

Voice Signals

Can I then say that mixing a carrier signal with multiple voice signals with varying frequencies and amplitudes results in one radio signal (RF) which has (1) a fixed high frequency -the fixed high frequency of the carrier signal- and (2) varying amplitudes determined by BOTH the frequencies and the amplitudes of the voice signals?

Thank you!

Dr. T

An RF carrier amplitude modulated by a single audio tone will leave you with the original RF carrier at the original frequency, plus an upper and lower sideband spaced out from the carrier center frequency by the modulating frequency and the carrier and sideband levels will be determined by the original RF carrier level vs the depth of modulation. So, an RF carrier amplitude modulated by a single tone will leave you with three signals.

If you add more modulating frequencies you will have two addatonal sets of corresponding sidebands above and below the carrier. Depending on depth of modulation and/or other non linear aspects of the transmitter chain, you can have more resulting sidebands due to Intermodulation products generated in the transmitter chain. And that's not good.
prcguy

DoctorTony said:

Hi!

I am a newbie studying for the Technician Exam. And, I have questions about amplitude modulation.

Single-Tone Audio Signal

Can I say that if one mixes a carrier signal -with a fixed high frequency and a fixed amplitude- with a single-tone audio signal -with a fixed low frequency and a fixed amplitude- the result is one radio signal (RF) which has (1) a fixed high frequency -the fixed high frequency of the carrier signal- and (2) a fixed amplitude determined by the frequency of the single-tone audio signal?

Voice Signals

Can I then say that mixing a carrier signal with multiple voice signals with varying frequencies and amplitudes results in one radio signal (RF) which has (1) a fixed high frequency -the fixed high frequency of the carrier signal- and (2) varying amplitudes determined by BOTH the frequencies and the amplitudes of the voice signals?

Thank you!

Dr. T

Click to expand...

I am not understanding your description.

The way I explain AM to people is like this:
If you take a 1 MHz signal and mix a 1 kHz tone with it, you get three signals out. The signals are 1 MHz minus 1 KHz, 1 MHz, and 1 MHz plus 1 KHz.

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That is exactly what you get, sorry my description didn't work for you.
prcguy

krokus said:

I am not understanding your description.

The way I explain AM to people is like this:
If you take a 1 MHz signal and mix a 1 kHz tone with it, you get three signals out. The signals are 1 MHz minus 1 KHz, 1 MHz, and 1 MHz plus 1 KHz.

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Click to expand...

I think krokus meant he didn't understand the OP's description. Both prcguy and krokus descriptions were pretty clear to me.... let's hope they were for the OP.

Reply to PRCGUY

prcguy said:

An RF carrier amplitude modulated by a single audio tone will leave you with the original RF carrier at the original frequency, plus an upper and lower sideband spaced out from the carrier center frequency by the modulating frequency and the carrier and sideband levels will be determined by the original RF carrier level vs the depth of modulation. So, an RF carrier amplitude modulated by a single tone will leave you with three signals.

If you add more modulating frequencies you will have two addatonal sets of corresponding sidebands above and below the carrier. Depending on depth of modulation and/or other non linear aspects of the transmitter chain, you can have more resulting sidebands due to Intermodulation products generated in the transmitter chain. And that's not good.
prcguy

Click to expand...

Dear PRCGUY,

Thank you for reply. I agree. Aren't the multiple sidebands displayed by a frequency analyzer proof that the amplitude of the one radio signal (RF) is contingent on BOTH the frequencies and amplitudes of the voice signals?

Dr. T

Not sure exactly what you are looking for but the amplitude of an AM modulated carrier will change with the level of the modulating signal and at 100% modulation the peak power of the RF carrier will be 6dB higher or 4X the power of the unmodulated carrier. Changing the modulating frequency will change the appearance on a spectrum analyzer but if the level of the modulating frequency is held constant then the amplitude of the modulated carrier will not change when the modulating frequency is varied within the design range of transmitter.

For example, in an AM transmitter designed for voice communications, whatever the amplitude of the carrier is modulated by say a 400Hz tone, it should be the same when modulated by a 2KHz tone, etc. As you approach or exceed 3KHz audio modulating frequency in an AM transmitter designed for voice communications the level of the modulating signal will be rolled off and at some higher audio frequency range the carrier will not be modulated at all because the audio frequencies will be completely removed.
prcguy

DoctorTony said:

Dear PRCGUY,

Thank you for reply. I agree. Aren't the multiple sidebands displayed by a frequency analyzer proof that the amplitude of the one radio signal (RF) is contingent on BOTH the frequencies and amplitudes of the voice signals?

Dr. T

Click to expand...

popnokick said:

I think krokus meant he didn't understand the OP's description.

Click to expand...

That was what I meant. PRC and I must have been typing at the same time, but he submitted earlier. Either way, I didn't see that post until two minutes ago.

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prcguy said:

Not sure exactly what you are looking for but the amplitude of an AM modulated carrier will change with the level of the modulating signal and at 100% modulation the peak power of the RF carrier will be 6dB higher or 4X the power of the unmodulated carrier. Changing the modulating frequency will change the appearance on a spectrum analyzer but if the level of the modulating frequency is held constant then the amplitude of the modulated carrier will not change when the modulating frequency is varied within the design range of transmitter.

For example, in an AM transmitter designed for voice communications, whatever the amplitude of the carrier is modulated by say a 400Hz tone, it should be the same when modulated by a 2KHz tone, etc. As you approach or exceed 3KHz audio modulating frequency in an AM transmitter designed for voice communications the level of the modulating signal will be rolled off and at some higher audio frequency range the carrier will not be modulated at all because the audio frequencies will be completely removed.
prcguy

Click to expand...

Dear PRC,

Thank you for your reply!

Most explanations about amplitude modulation state that the amplitude of the modulating signal changes the amplitude of the carrier signal to generate the radio signal (RF).

I do know that, unlike single-tone signals, which feature a fixed amplitude and a fixed frequency, voice signals feature varying amplitudes and varying frequencies.

I am trying to confirm that BOTH the amplitude AND the frequency of a voice signal -which is the modulating signal- determine the amplitude of the radio signal (RF).

Furthermore, I am trying to confirm that, as shown by a spectrum analyzer, the MULTIPLE sidebands, which feature varying amplitudes and varying frequencies, associated with the radio signal (RF), are proof of the above.

Dr. T

The RF carrier is created separate from the modulating signal and there are various ways to mix or combine them in an AM transmitter. I would not say "the amplitude of the modulating signal changes the amplitude of the carrier signal to generate the radio signal (RF)". The level of an AM modulated signal is mostly dependent on the RF circuits of the transmitter and to a lesser extent in the modulation circuitry.

As I mentioned before, going from a continuous wave carrier with no modulation to 100% AM modulation will increase the peak power by 6dB or 4X. That's the total impact on the level of an AM modulated signal from the modulating signal. A properly adjusted 100W AM transmitter will produce 25W unmodulated carrier and 100W peak at 100% modulation.

You can see and measure this with a spectrum analyzer or SDR dongle receiver. Measure the unmodulated carrier level, then at 100% modulation from a single modulating tone there will a mirror image signal above and below the carrier (upper and lower sidebands), each 6dB down from the carrier level. These two extra signals, or sidebands, combine in phase and show up as additional power in the carrier.

Now if you look at Single Side Band or Double Side Band Suppressed Carrier signal, all the RF power will be driven or controlled by the modulating signal. Key the transmitter and don't talk or supply a modulating signal, no RF output. Key the transmitter and talk loud and the RF power will go from zero to full power and track the amplitude of the modulating signal. In these cases its the level of the voice or modulating signal that determines the ultimate RF level of the SSB or DSBSC transmitter and not necessarily the frequency of the modulating signal.

In your statement "I am trying to confirm that BOTH the amplitude AND the frequency of a voice signal -which is the modulating signal- determine the amplitude of the radio signal (RF)". I also mentioned before, the amplitude of the voice or modulating signal will determine within a 6dB range, the amplitude of an AM modulated signal and not necessarily the frequency of the voice or modulating signal.

There will be variables depending on the transmitter design where the modulating frequency may be outside the useable range of the transmitter and have less impact on the modulation level. There are also modifications to AM transmitters to allow more than 100% modulation on positive peaks, which will provide more than 6dB increase in RF level under full modulation.

I'm not an expert on the subject and don't believe I've overlooked anything on the mod freq vs RF amplitude question, but there is probably lots of info you can get with a Google search.
prcguy

DoctorTony said:

Dear PRC,

Thank you for your reply!

Most explanations about amplitude modulation state that the amplitude of the modulating signal changes the amplitude of the carrier signal to generate the radio signal (RF).

I do know that, unlike single-tone signals, which feature a fixed amplitude and a fixed frequency, voice signals feature varying amplitudes and varying frequencies.

I am trying to confirm that BOTH the amplitude AND the frequency of a voice signal -which is the modulating signal- determine the amplitude of the radio signal (RF).

Furthermore, I am trying to confirm that, as shown by a spectrum analyzer, the MULTIPLE sidebands, which feature varying amplitudes and varying frequencies, associated with the radio signal (RF), are proof of the above.

Dr. T

Click to expand...