High Level and Low Level Modulation

• 📘 Overview
• 📚 Low Level Modulation
• 📚 High Level Modulation
• 📚 Simulator for High and Low Level Modulations
• 📚 MATLAB Code
• 📚 Further Reading

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, the message signal is modulated with the carrier signal (from the local oscillator) at low power levels. After modulation, the signal is then passed through a wideband power amplifier to increase its strength before transmission. This method is energy-efficient and maintains signal fidelity but requires a linear amplifier, which can be more complex or expensive.

High Level Modulation

In high-level modulation, the carrier signal is first amplified to a high power level. The message signal is then used to modulate this high-power carrier. The modulation typically occurs in the final stage of the power amplifier. This method is more suitable for high-power transmissions like AM broadcasting, but it may introduce nonlinear distortion if not designed carefully.

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 for low-level A_carrier_high = 10; % Carrier amplitude for high-level (amplified) mod_index = 0.5; % Modulation index (m < 1 for no overmodulation) % Message Signal (audio) f_msg = 500; % Message frequency (500 Hz) message = mod_index * cos(2*pi*f_msg*t); % Message signal % Carrier Signals carrier = A_carrier * cos(2*pi*Fc*t); % Low-level carrier carrier_high = A_carrier_high * cos(2*pi*Fc*t); % High-level carrier % Low-Level AM Modulation low_level_am = (1 + message) .* carrier; % High-Level AM Modulation % Message is not amplified — carrier is amplified high_level_am = (1 + message) .* carrier_high; % Plotting figure; % Message Signal subplot(4, 1, 1); plot(t, message); title('Message Signal'); xlabel('Time (s)'); ylabel('Amplitude'); grid on; % Carrier Signal subplot(4, 1, 2); plot(t, carrier); title('Carrier Signal (Low-Level)'); xlabel('Time (s)'); ylabel('Amplitude'); grid on; % Low-Level AM Signal subplot(4, 1, 3); plot(t, low_level_am); title('Low-Level AM Modulated Signal'); xlabel('Time (s)'); ylabel('Amplitude'); grid on; % High-Level AM Signal subplot(4, 1, 4); plot(t, high_level_am); title('High-Level AM Modulated Signal'); xlabel('Time (s)'); ylabel('Amplitude'); grid on; % Overall Title sgtitle('Comparison of High-Level and Low-Level AM Modulation');  

 

Output

By observing the above figure, one might think that both modulated signals are identical. However, in practical scenarios, high-level modulation tends to experience more distortion due to the nonlinearities introduced by power amplifiers operating at higher output levels.

Further Reading

1. Differences between Baseband and Passband Modulation Techniques
2. Baseband ASK, FSK, and PSK
3. Pass-band ASK, FSK, and PSK

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 ...