SPRREAD SPECTRUM TECHNIQUES The use of frequency hopping in modern power electronics
Ever wondered if there could be an integration of communication systems and power electronics? Well, talkative power is a phenomenon to allow data modulation into a group of power converters in parallel. The system provides maximum efficiency using the Frequency Hopping technique. Here is a guide to frequency hopping and its role in power electronics!
Brief History of Frequency Hopping
Many important figures throughout the globe researched Frequency Hopping and developed similar devices. During the late nineteenth and early twentieth centuries, people like Italian electrical engineer Guglielmo Giovanni Maria Marconi, Nikola Tesla, German physicist, and engineer Jonathan Zenneck, etc, documented paths to Frequency Hopping. The research was done to help their respective countries during the world war times in maintaining secure communication without any sensitive information leakage to the enemies. But the major credit goes to actress Hedy Lamarr and her friend George Antheil for developing the technique and obtaining the patent. Earlier the authorities rejected the idea but in 1962, the US Navy utilized the frequency hopping technique during a war crisis.
What is Frequency Hopping?
Frequency hopping is a spread spectrum technique that increases the bandwidth of the baseband signal (information signal) by repeatedly, randomly, and rapidly changing the transmitter signal frequency. A frequency hopping spread spectrum (FHSS) divides the available channel bandwidth into consecutive frequency slots on which the transmitter signal hops randomly.
The Frequency hopping system contains the standard components of the spread spectrum like an encoder, PN sequence generator, modulator at the transmitting end and decoder, PN sequence generator, and demodulator at the receiving side. The Pseudo-Random Noise generator provides a binary-valued sequence using an shift register with linear feedback. The PN sequence achieves synchronization between the transmitting signal and the received signal. It also helps the FHSS system to choose frequencies for the transmitting signal. The PN sequence is called random but it is deterministic and periodic in nature. The frequencies are selected “randomly” with the help of this code. Hence, the PN sequence is named Pseudo-random.
The modulation technique used in Frequency Hopping is either M-ary FSK (M-ary Frequency Shift Keying) or PSK (Phase Shift Keying). But in most cases, M-ary FSK is used because phase coherence is difficult to maintain with random frequency hops. As it is a “Frequency-Hopping” spread spectrum technique, there is a frequency synthesizer at the transmitting and receiving end. It generates stable frequency slots with minimum phase noise to enhance the bandwidth of the baseband signal and allow the transmitter signal hopping. At the mixer, the FSK-modulated signal is mixed with frequencies from the synthesizer.
The resultant signal is transmitted over the channel toward the receiving end. At the receiver, there is a mixer where the frequency synthesizer extracts the modulated FSK signal. It is further demodulated and sent to the decoder to obtain the original information signal.
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Frequency Hopping in Power Electronics
Frequency Hopping is majorly used in wireless communication, military, and various civilian applications. The most popular use of frequency hopping is in CDMA (Code division multiple access) and 802.11 (WiFi) standards along with other modulation techniques like DSSS, and OFDM. When it comes to power electronics, there are striking similarities with communication systems. Although power electronics and communication are branches of Electrical and Electronics Engineering, both operate in the same manner. “Talkative Power” is termed the integration of power electronics and communication systems through power-line communication with the help of Data modulation. In the communications system, an information signal is modulated and sent from the transmitting side to the receiving side. The signal is further demodulated and the end-user extracts the information. Similarly, power systems amplify the voltage and generate either a stepped-up or stepped-down voltage.
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A controllable power converter transforms either DC-to-DC, DC-to-AC, AC-to-DC, or AC-to-AC. The desired amplitude of output voltage is achievable by varying the dc supply voltage or duty cycle of the gating signal. The dc supply voltage can be controlled through a chopper or rectifier to obtain the desired output voltage. But increasing the number of stages in a power electronics system increases the size of the system, cost, and harmonics. If the power converter is a DC-to-DC converter, the output voltage would contain high ripples (AC components). The best option is to use a modulation technique in the switching of the power converter to control the output voltage directly. Since the duty cycle of the gating signal is a contributing factor to the amplitude of output voltage, PWM (Pulse Width Modulation) was chosen to be the suitable modulation technique. This is because the duty cycle is the ratio of “ON Time” and the total time of a single pulse. PWM technique moves into ON-state and OFF-state compared to other modulation techniques.
Considering a buck or boost DC-DC converter, the input dc supply voltage signal is modulated at the input side, and the resultant PWM signal is amplified and sent to the receiving output side for demodulation. In communication systems, the double modulation technique is preferred to control the duty cycle of the switching signal. The double modulation technique enhances the power system’s performance by data modulation. The data signal is modulated on the amplified PWM signal without changing the duty cycle of the gating signal. It allows only angular modulation techniques like FSK and PSK to participate in double modulation. The double modulation technique with FSK and PSK holds well in a single power converter.
In the case of distributed power systems with parallel converters, there is interference in the transmission and communication. The white noise superimposes the transmitting signals at the same switching frequency. It showcases that there is more interference than the original signal as it drastically decreases the signal-to-noise ratio (SNR). To avoid interference, the frequency hopping technique is chosen. A frequency hopping technique with DPSK modulation (Differential Phase Shift Keying) enables the transmission over random frequency slots. Differential Phase Shift Keying changes the phase of the carrier frequency. Once new channels are generated through a PN sequence generator, the transmitting signal hops between different frequency slots and travels simultaneously to the receiving end along with the process of power conversion. At the output of the converter, we get the desired DC power with some ripples in the output voltage.
The ripples, AC components in a DC voltage, deterministically represent the information signal. In this way, Frequency Hopping enables a power converter to achieve great talkative power!