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Wed, Sep 29, 8:08 AM (2 days ago)
Isotron 20
 
 
The best way to operate HF
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AM OPERATION
Last month was about using the correct mode to check SWR. One of the modes discussed was AM Modulation. What is this modulation?
 
AM stands for Amplitude Modulation. It is the most basic and oldest voice modulation we have. How is it produced in the transmitted?
 
You start with a carrier. This is a constant radio frequency signal. Let's use 7.200 Mhz's. You have an oscillating circuit that produces this constant frequency. Sending this out on the air, you will detect a signal from your radio, but if you are listening on the AM mode you will hear nothing. Even the noise will go away if the signal is strong enough.
However, we want human speech to be sent on this signal. We can do this, and this is why we call it a "carrier". This is the media that delivers the voice from your radio to the next.
 
In the radio, you mix the voice signal from a basic audio amplifier with the Carrier. The circuit for this is called a "mixer". Both of these signals are fed into one amplifying device. What you get out is an Amplitude Modulated signal. It is fairly simple, but what does the AM signal consist of?
 
You put 2 signals together, RF carrier and Audio, you will get 3 signals out.
 
You still have the RF carrier (7.200 Mhz), however when it was mixed with audio, it developed 2 other signals. These are called Side Bands.
 
Your Audio signal, for example has a frequency of 1,000 Hz. This will vary with your voice, but let's keep it constant for now. We can hear this tone. When it is mixed with the RF Carrier, the result is another signal that is the sum of the 2 frequencies. 7.200 Mhz + 1,000 Hz. This is called an Upper Side Band, USB.
 
If you raise the audio frequency the Upper Side Band will increase in frequency and vice versa.
 
The other signal is the difference or subtraction of the signal. Or 7.200 Mhz - 1,000 Hz. This is called the Lower Side Band. This signal is interesting. It works backwards. If you raise the audio signal, the Radio Frequency gets lower and vice versa. The Lower Side Band has been used for encryption in the past. Therefore, when your voice frequency goes up, the signal frequency goes down.
 
Enough for now. Next article will address another characteristic of AM and how we receive it.
 
73,
Ralph WD0EJA
Oct 2021
 
Oct 2014
 
 
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