USRP Environmental Setup 4~6

Introduction

-Basic End-to-End Simulation:
Important to go beyond basic simulations and gain experience in actual implementation
Utilization of Real Hardware Devices:
Implementations using real hardware devices are necessary
-Started with Everyday Use Cases:
AM/FM radios are commonly used in daily life
Focus on FM Radio with Optional Addition of AM:
Prioritize FM radio and consider adding AM as needed
-One of the Necessary Functions for Receiving such Radios is Filter
Learning and discussing concepts related to filters is essential
From simple filters to matched filters, learning and implementing various types of filters are necessary for receiving radios effectively

AM/FM Radio Configuration

-AM/FM Radio Implementation

Basic structure of an AM radio
Antenna:
Antenna symbol at the very left, which is used for capturing RF signals from the air
RF Front End:
Process RF(Radio Frequency) signals coming in from the antenna before being converted into audio, video, or data signals that the rest of the system can use and understand
This is typically an amplifier and a filter
It amplifies the signal received by the antenna and filters out frequencies outside the desired range, reducing noise and improving the quality of the signal for the next stages. Frequency Down Conversion:
This stage mixes the received RF signal with a local oscillator frequency(Another, slightly different high frequency that the radio itself creates) to “down convert” the signal to a lower intermediate frequency (IF)
The IF is chosen to make filtering easier and is standard in most receivers to allow for more specialized components that can be produced at scale
Baseband Filter:
After down conversion, the signal is passed through a filter that further selects the desired signal and eliminates others
This filter is designed to pass the IF while blocking frequencies outside of the IF band
-The Baseband Filter is like your brain at the party. It “listens” for the specific story (radio station) you want and ignores all the others. It filters out any other noises or stations that are not the one you’re tuned into. This way, you get a clear “conversation” from the radio station you chose without any “background noise” from the other stations
Envelope Detector:
The envelope detector extracts the audio information from the IF
In AM signals, the information (such as voice or music) is contained within variations of the amplitude of the wave
The envelope detector demodulates the signal by detecting these variations
-In an AM radio signal, the sound information (like someone talking or music) is represented by how high the waves are—this is called the amplitude
The envelope detector’s job is to look at these waves and figure out how high they are
Measures the height of our ‘sound waves’, which in AM radio, is where our actual information like music or voice is stored
Audio Output:
Finally, the demodulated audio signal is often passed through an audio amplifier and then sent to a speaker or headphones, where it’s converted into sound waves that can be heard
All this information is turned back into sound—music or voice—that we can listen to with our ears

-Carrier Wave:
Steady, rapid wave that you see going up and down very quickly
It’s like a fast, steady hum that carries the actual sounds you want to hear
Serves as the vehicle that carries the message
-Message Wave Envelope:
This is the smooth curve that outlines the top and bottom peaks of the carrier wave
It represents the actual information—like music or voice
Notice how the height of the carrier wave changes?
This changing height (or amplitude) is what carries the sound information
This is the cargo (passengers, goods) that the train (carrier wave) is carrying
-The basic AM/FM signal formulas are as follows
-The difference is whether the message is transmitted by signal power(AM) or by frequency(FM)
-Amplitude Modulation

$m(t)$ is the AM signal that’s transmitted
$A(t)$ represents the amplitude of the signal, which changes over time
It’s modulated by the information you want to transmit, like voice or music
$cos(ωt + ϕ(t))$ is the carrier wave — a cosine wave with a constant frequency represented by $ω$(omega), which is 2π times the frequency of the carrier wave. $ϕ(t)$ is the phase of the signal at any time
-Frequency Modulation

• $y(t)$ is the FM signal that’s transmitted
  
• $A(t)$ is the amplitude of the carrier wave, which remains constant in FM (it does not change like it does in AM).

Modulation

Process:
Impose an input signal on to a carrier wave
Change some property, such as amplitude, or frequency
(1)Input signal(Base Signal)
-Data signal wave we want to transmit
-Height varies according to the loudness of our voice
(2)Carrier Wave
-Pure wave of contant frequency
-Very much like a sine wave
-By itself, it doesn’t carry much information of what we can relate to
-Needed to deliver our data signal
-Frequency of the carrier wave is generally higher than data signal wave
Why do we need modulation during a transmission?
Purpose
It is like throwing a paper far away and if we wrap a rock with it, we are able to throw it farther away
Data signal wave is like the piece of paper and a rock is the carrier wave
Modulation is the process of wrapping the rock with a piece of paper to throw the paper to a direction where we want
-Data signal wave(Input signal) is a lower frequency wave
Lower frequency wave is more susceptible to interference than higher frequency wave
Modulation makes the data signal to a higher frequency wave
-Higher frequency wave would have a more bandwidth, thus carrying more data
-Lower frequency signals would require a much larger size antenna at the receiving end
Modulation reduces the size of the antenna with the increase of wave frequency
-Security is another reason
Data signal is hidden inside the carrier wave
Reference:
https://www.youtube.com/watch?app=desktop&v=dvGcCk1vbjk
https://www.taitradioacademy.com/topic/how-does-modulation-work-1-1/

What is AM?

AM(Amplitude Modulation)

Process: Add this input signal to the pure carrier wave
Result: Carrier’s amplitude(height) will change corresponding to the input signal that is fed into it(base signal)
(Only the amplitude is changed. The result wave’s frequency remains the same as the carrier wave)
-From a trough to a peak, modulated wave becomes stronger and stronger
At the highest point, or peak, of data signal wave, the amplitude of the carrier wave is the strongest
At the lowest point, or trough, of data signal wave, the amplitude of the carrier wave is the weakest
Example:
Get radio to play back the signal as actual music or dialog by tuning it to the whatever frequency the station looking for broadcast sets
Makes the antenna resignate at the freqency and ignore everything else in the air
FM(Frequency Modulation)

Process: Add input signal to the pure carrier wave
Result: The frequency of the carrier wave will be changed
(The result wave’s amplitude remains the same, only frequency is changed)
-From peak to trough, the frequency of result wave becomes lower and lower and from a trough to a peak, the result frequency becomes higher and higher
At a peak of the data signal wave, the frequency of the of carrier wave becomes the highest
While at the lowest point(trough) of the data signal wave, the frequency of the carrier wave is reaching the lowest
Example: Tuned 99.5 FM in your dial, FM works by using a very small changes in frequency to carry signal
Tuned to signals just above and below 99.5MGHZ, making transformers send voltage to speakers
Changes of frequency to carry our speech information
AM vs. FM
AM
-AM frequency range is much lower than FM
Thus AM covers much larger areas than FM
This is the reason why most news stations use AM
-AM is more susceptible to noises, because noise affects amplitude
This is why we hear static from AM radio channels
-Although, process makes FM more complicated to engineer than AM, you get a clearer higher quality signal
-Less acceptable to interference as well, because interference manifests as amplitude spikes and AM will see spikes as sound to produce, and come out of their radios
-FM cares more about variations in frequency rather than amplitude, FM can ignore that type of interference

Reference:
https://www.taitradioacademy.com/topic/how-does-modulation-work-1-1/

Freq Down Conversion(Frequency Downconversion):
Converts the received RF signal to a lower intermediate frequency (IF) or directly to baseband (the frequency range starting from near 0 Hz up to a higher cut-off frequency)
Downconversion is achieved using a mixer and a local oscillator
The purpose of this is to make the signal easier to process and filter at the next stages
Baseband Filter:
After downconversion, the signal is passed through a baseband filter
This filter is designed to remove any remaining undesired frequencies and noise, leaving only the desired signal, which is now at a much lower frequency than the original RF signal
Envelope Detector:
Extracts the audio signal from the modulated carrier wave
For amplitude modulation (AM) signals, this can be as simple as a diode and a capacitor
The diode rectifies the signal (allowing only one polarity of the signal to pass through), and the capacitor smooths out the variations to extract the modulation, which is the audio signal

Audio Output: Finally, the audio signal is output from the envelope detector and can be sent to a speaker or headphones for listening.