High Level and Low Level Modulation

High Level and Low Level Modulation

You know for wireless communication is suitable for long distance communication. In wireless, for data transmission modulation become essential to avoid interference and to reduce antenna size significantly. Especially, in modulation process, we translate the low frequency baseband signal to higher frequency by modulating with high frequency carrier signal. For a typical communication system we generate the high frequency (carrier) signal by using local oscillator. Source signal or message signal is modulated with local oscillator. Then modulated signal is transmitted thru antenna. 

Low Level Modulation

In low level modulation, message signal is modulated with local oscillator that produces high frequency. Then the frequency of message signal is translated to much higher frequency. Then the modulated signal passes thru wideband amplifier.

High Level Modulation

In high level modulation, source or message signal is passed thru wideband amplifier, then the signal is modulated with local oscillator.

What is Wideband Amplifier  

For often, signal is amplified in communication process so that signal can reach at receiver with acceptable energy level. For the name 'wideband amplifier' we can primarily guess that it can pass signal with large bandwidth. In case of modulation process, i.e., for amplitude modulation (AM), we know after modulation it contains upper side band and lower side band at both side of carrier frequency.  That posses large amount of bandwidth. So, it is essential to pass thru wideband amplifier so that all available frequencies can pass thru amplifier and get amplified.

 

MATLAB Code for Comparison of High Level and Low Level Modulation

% The code is written by SalimWireless.Com clc; clear; close all; % Parameters Fs = 100e3; % Sampling frequency (100 kHz) Fc = 10e3; % Carrier frequency (10 kHz) t = 0:1/Fs:0.01; % Time vector for 10 ms A_carrier = 1; % Carrier amplitude A_message = 0.5; % Message amplitude A_high = 10; % Amplifier gain for high-level modulation % Message Signal (audio) f_msg = 500; % Message signal frequency (500 Hz) message = A_message * cos(2*pi*f_msg*t); % Message signal (cosine wave) % Carrier Signal carrier = A_carrier * cos(2*pi*Fc*t); % Low-Level AM Modulation low_level_am = (1 + message) .* carrier; % High-Level AM Modulation amplified_message = A_high * message; % Amplify message for high-level modulation high_level_am = (1 + amplified_message) .* carrier; % Plotting figure; % Message Signal subplot(4, 1, 1); plot(t, message); title('Message Signal'); xlabel('Time (s)'); ylabel('Amplitude'); % Carrier Signal subplot(4, 1, 2); plot(t, carrier); title('Carrier Signal'); xlabel('Time (s)'); ylabel('Amplitude'); % Low-Level AM Signal subplot(4, 1, 3); plot(t, low_level_am); title('Low-Level AM Modulated Signal'); xlabel('Time (s)'); ylabel('Amplitude'); % High-Level AM Signal subplot(4, 1, 4); plot(t, high_level_am); title('High-Level AM Modulated Signal'); xlabel('Time (s)'); ylabel('Amplitude'); % Enhancing visibility sgtitle('Comparison of High-Level and Low-Level Modulation');  

 

Output

Frequency modulation of a voice signal

The voice ranges from 330Hz to 3.3kHz. We typically band restrict it to 4 kHz, or Fmax, for simplicity in mathematics.

According to the Nyquist Sampling Theorem, Sampling Frequency (Fs) must be greater than or equal to 2 Fmax (8 kHz) in order to prevent Aliasing.

Sampling frequency and carrier frequency are not the same.

By modulating, the baseband signal (also known as the original or unmodified signal) is moved to a higher frequency where it can be transmitted with a smaller antenna.

Sampling is a method for acquiring the baseband signal so that it can be reconstructed (if Fs >= 2Fmax) with the least amount of loss to the signal fidelity. It is impossible to restore a signal to its original form if it has been under-sampled.

The local oscillator on your SDR, which governs the frequency it covers, is controlled by the channel frequency. As a result, you are picking up signals between 16 KHz below and 16 KHz above 107.5 MHz. (Let assume the sampling frequency at FM radio station was 32 KHz).

You wish to search up a concept called heterodyning. When you modulate (multiply) a signal by a sine wave, the result is two copies of the signal that are separated in frequency space by the sine wave's frequency. Therefore, in your situation, you've instructed the local oscillator to run at 107.5MHz, which shifts the radio tower's 107.5MHz carrier to 0Hz and 215MHz even though the high frequency version is silenced by your radio's low pass filter. read more ...