Question

Please give brief explanations and the example of how to solve problems on buck boost LDO, modulated signals with three different frequencies.
What are the UFO VHF UHF and different types of frequency levels and what are the coding languages to take care of these.
Types of commonly used filters.
Please be brief as possible.

Answer 1

Answer:

DC-to-dc switching converters are used to change one dc voltage to another efficiently. High efficiency dc-to-dc converters come in three basic topologies: step-down (buck), step-up (boost), and step-down/step-up (buck/boost). The buck converter is used to generate a lower dc output voltage, the boost converter is used to generate a higher dc output voltage, and the buck/boost converter is used to generate an output voltage less than, greater than, or equal to the input voltage.

Figure 1: Typical low power portable system

Figure 1 shows a typical low-power system powered from a single-cell lithium-ion (Li-Ion) battery. The battery’s usable output varies from about 3.0 V when discharged to 4.2 V when fully charged. The system ICs require 1.8 V, 3.3 V, and 3.6 V for optimum operation. While the lithium-ion battery starts at 4.2 V and ends at 3.0 V, a buck/boost regulator can supply a constant 3.3 V, and a buck regulator or low-dropout regulator (LDO) could supply the 1.8 V, as the battery discharges. Conceivably a buck regulator or LDO could be used for the 3.3 V while the battery voltage is above 3.5 V, but the system would cease to operate when the battery voltage dropped below 3.5 V. Allowing the system to be turned off prematurely reduces the system’s operating time before the battery needs to be recharged.

Buck/boost regulators contain four switches, two capacitors, and an inductor, as shown in Figure 2. Today’s low-power, high-efficiency buck/boost regulators reduce losses and improve efficiency by actively operating only two of the four switches when operating in buck- or boost mode.

Figure 3:Buck mode when V_IN>V_OUT

When V_IN is greater than V_OUT, Switch C is open and Switch D is closed. Switches A and B operate as in a standard buck regulator—as shown in Figure 3.

When V_IN is less than V_OUT, Switch B is open and Switch A is closed. Switches C and D operate as in a boost regulator—as shown in Figure 4. The most difficult operating mode is when V_IN is in the range of V_OUT ± 10%, and the regulator enters the buck-boost mode. In buck-boost mode, the two operations (buck and boost) take place during a switching cycle. Care must be taken to reduce losses, optimize efficiency, and eliminate instability due to mode switching. The objective is to maintain voltage regulation with minimal current ripple in the inductor to guarantee good transient performance.

Figure 4: Boost mode when V_IN < V_OUT.

At high load currents, the buck-boost uses voltage or current-mode, fixed-frequency pulse-width-modulation (PWM) control for optimal stability and transient response. To ensure the longest battery life in portable applications, a power-save mode reduces the switching frequency under light load conditions. For wireless and other low-noise applications, where variable-frequency power-save mode may cause interference, the addition of a logic control input to force fixed-frequency PWM operation under all load conditions is included.

Buck/Boost Regulators Improve System Efficiency.

A large number of portable systems in use today are powered by a single-cell rechargeable Li-Ion battery. As mentioned above, the battery will start from a fully charged 4.2 V and slowly discharge down to 3.0 V. When the battery’s output drops below 3.0 V, the system is turned off to protect the battery from damage due to extreme discharging. When a low-dropout regulator is used to generate a 3.3-V rail, the system will shut down at

V_{IN MIN} = V_OUT + V_DROPOUT = 3.3 V + 0.2 V = 3.5 V

employing only 70% of the battery’s stored energy. However, using a buck/boost regulator, such as the ADP2503 or ADP2504, enables the system to continue operating down to minimum practical battery voltage. The ADP2503 and ADP2504 are high-efficiency, low quiescent-current 600-mA and 1000-mA, step-up/step-down (buck/boost) dc-to-dc converters that operate with input voltages greater than, less than, or equal to the regulated output voltage. The power switches are internal, minimizing the number of external components and printed-circuit-board (PCB) area. This approach allows the system to operate all the way down to 3.0 V, using most of the battery’s stored energy, increasing the system’s operating time before a battery recharge is required.

To save energy in portable systems, various subsystems—such as the microprocessor, display backlighting, and power amplifiers—when not in use, are frequently switched between full-on and sleep mode, which can induce large voltage transients on the battery supply line. These transients can cause the battery’s output voltage to briefly drop below 3.0 V and trigger the battery-low warning, causing the system to turn off before the battery is completely discharged. The buck/boost solution will tolerate voltage swings as low as 2.3 V, helping to maintain the system’s potential operating time.

UFO, VHF UHF and different kinds of frequency

In the United States, the frequencies used for these systems may be grouped into four general bands or ranges: low-band VHF (49-108 MHz), high-band VHF (169-216 MHz), low-band UHF (450-806 MHz) and high-band UHF (900-952 MHz). VHF = "Very High Frequency". UHF = "Ultra High Frequency

• Very low frequencies (VLF) range from 3 to 30 kilohertz (kHz)
• Low frequencies (LF) range from 30 to 300 kHz
• Medium frequencies (MF) range from 300 to 3000 kHz
• High frequencies (HF) - also called shortwaves - range from 3 to 30 megahertz (MHz).
• Very high frequencies (VHF) range from 30 to 300 MHz. Fixed, mobile, aeronautical and marine mobile, amateur radio, television and radio broadcasting, and radio navigation are among the users of this band.
• Ultra-high frequencies (UHF) range from 300 to 3000 MHz. Fixed, mobile, aeronautical and marine mobile, amateur radio, television, radio navigation and location, meteorological, and space communication are among the users of this band.
• Super high frequencies (SHF) range from 3 to 30 gigahertz (GHz). Fixed, mobile, radio navigation and location, and space and satellite communication are among the users of this band.
• Extremely high frequencies (EHF) range from 30 to 300 GHz. Amateur radio, satellite, and earth and space exploration are among the users of this band.
• To determine the states with the most UFO sightings, 24/7 Wall St. reviewed 2001-2015 sightings per 100,000 people with data from Cheryl Costa’s “UFO Sightings Desk Reference: United States of America 2001-2015: Unidentified Flying Objects Frequency–Distribution–Shapes.”

• Very high frequency (VHF) is the ITU designation^[1] for the range of radiofrequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as the a-high frequency (UHF).

  Common uses for radio waves in the VHF band are digital audio broadcasting (DAB) and FM radio broadcasting, television broadcasting, two-way land mobile radio systems (emergency, business, private use and military), long-range data communication up to several tens of kilometers with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR & ILS) work at distances of 100 kilometers (62 mi) or more to aircraft at cruising altitude.

  In the Americas and many other parts of the world, VHF Band I was used for the transmission of analog television. As part of the worldwide transition to digital terrestrial television, ost countries require broadcasters to air television in the VHF range using digital rather than analog format.

• Television and FM broadcasting stations use collinear arrays of specialized this frequency range could receive the audio for analog-mode programming

Types of Commonly Used Filters:

What Is a Filter?

A filter is a circuit capable of passing (or amplifying) certain frequencies while attenuating other frequencies. Thus, a filter can extract important frequencies from signals that also contain undesirable or irrelevant frequencies.

In the field of electronics, there are many practical applications for filters. Examples include:

• Radio communications: Filters enable radio receivers to only "see" the desired signal while rejecting all other signals (assuming that the other signals have different frequency content).
• DC power supplies: Filters are used to eliminate undesired high frequencies (i.e., noise) that are present on AC input lines. Additionally, filters are used on a power supply's output to reduce ripple.
• Audio electronics: A crossover network is a network of filters used to channel low-frequency audio to woofers, mid-range frequencies to midrange speakers, and high-frequency sounds to tweeters.
• Analog-to-digital conversion: Filters are placed in front of an ADC input to minimize aliasing.

Four Major Types of Filters

The four primary types of filters include

• the low-pass filter,
• the high-pass filter,
• the band-pass filter, and
• the notch filter (or the band-reject or band-stop filter).

Take note, however, that the terms "low" and "high" do not refer to any absolute values of frequency, but rather they are relative values with respect to the cutoff frequency.

Figure 1 below gives a general idea of how each of these four filters works:

Figure 1. A basic depiction of the four major filter types.

Passive and Active Filters

Filters can be placed in one of two categories: passive or active.

Passive filters include only passive components—resistors, capacitors, and inductors. In contrast, active filters use active components, such as op-amps, in addition to resistors and capacitors, but not inductors.

Passive filters are most responsive to a frequency range from roughly 100 Hz to 300 MHz. The limitation on the lower end is a result of the fact that at low frequencies the inductance or capacitance would have to be quite large. The upper-frequency limit is due to the effect of parasitic capacitances and inductances. Careful design practices can extend the use of passive circuits well into the gigahertz range.

Active filters are capable of dealing with very low frequencies (approaching 0 Hz), and they can provide voltage gain (passive filters cannot). Active filters can be used to design high-order filters without the use of inductors; this is important because inductors are problematic in the context of integrated-circuit manufacturing techniques. However, active filters are less suitable for very-high-frequency applications because of amplifier bandwidth limitations. Radio-frequency circuits must often utilize passive filters.

Filters are essential building blocks in many systems, particularly in communication and instrumentation systems. A filter passes one band of frequencies while rejecting another. Typically implemented in one of three technologies: passive RLC filters, active RC filters and switched capacitor filters. Crystal and SAW filters are normally used at very high frequencies.

Switched-capacitor filters are monolithic filters which typically offer the best performance in the term of cost. Fabricated using capacitors, switched and op-amps. Generally poorer performance compared to passive LC or active RC filters.