We pulled into the parking lot a little before 5:00 pm. The volunteers were already gathered and had made two long lines of empty boxes. Our two trucks were filled with the food that Free the Need volunteers had collected throughout the week; food that would have otherwise ended up in landfill. The food is brought to this local Klamath River center to be distributed to families in need.
The volunteers worked together like a well-oiled machine. Vegetables, fruit, canned foods, bread, cheese, and milk, along with flowers, were put into the boxes. After checking them to make sure the food looked evenly distributed, everyone lined up to receive boxes for their families and neighbors. The whole operation was joyful, friendly and took less than an hour.
In addition to Klamath River, Free the Need delivers food to families in San Francisco. The organization is all volunteer, so any cash donations received go 100% to food distribution. Free the Need believes in an abundant universe. Its volunteers are breaking the chain of poverty by distributing surplus food that would otherwise go to waste. If a family is freed of the need for food, they can put more energy into creating a better life. Free the Need believes that there is enough of what people need to go around, and that once people feel filled up, they have enough surplus to give to others.
I also believe in an abundant universe, and am honored to be a member of Free the Need’s board of directors. I attended a planning meeting for 2018, and was impressed by the care with which plans and procedures for the new year were discussed. Free the Need is launching a fund-drive to raise money to purchase an additional truck needed for food transportation. They are also looking into a storage facility in San Francisco to support their growing work. For more information, or if you would like to volunteer or make a gift of support, please visit their website at www.freetheneed.org/.
Now share a summary of the difficult problems of carefully organized PCB design techniques 1 How to choose PCB board? The PCB sheet must be selected to meet the design requirements and mass production and cost balance. The design requirements consist of two parts: electrical and mechanical. This material problem is usually more important when designing very high-speed PCB boards(frequencies greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at several GHz frequencies will have a great influence on signal attenuation and may not be useful. For electrical purposes, it is important to note whether the dielectric constant and the dielectric loss are compatible with the designed frequency. 2 How to avoid high frequency interference? The basic idea of avoiding high-frequency interference is to minimize the interference of high-frequency signal electromagnetic fields, the so-called crosstalk. You can use the distance between the large high speed signal and the analog signal, or add group/shot tracks next to the analog signal. Attention should also be paid to the digital noise interference to the simulated ground. 3 In high-speed design, how to solve the problem of signal integrity? Signal integrity is basically a problem of impedance matching. The factors that affect impedance matching include the architecture and output impedance of the signal source, the characteristic impedance of the alignment, the characteristics of the load end, and the topology architecture of the alignment. The solution is to rely on the topology of the terminal and the alignment. How is the 4 difference distribution line method realized? The difference pair wiring has two points to note. One is to keep the length of the two lines as long as possible, and the other is the distance between the two lines(this distance is determined by the difference impedance) to remain unchanged, that is, to maintain parallel. There are two ways to parallel, one is to walk on the same ease-by-side, and the other is to walk on the over-under two adjacent layers. Generally speaking, there are more ways to implement the former side-by-side. 5 For a clock signal line with only one output, how to achieve a difference distribution line? To use the difference distribution line, it must be that the signal source and the receiving end are also differential signals. Therefore, it is impossible to use a difference distribution line for a clock signal with only one output http://www.pcbindex.com/ 6 Can a matching resistor be added between the difference lines at the receiving end? The matching resistance between the difference lines at the receiving end is usually added, and its value should be equal to the value of the difference impedance. This will make the signal better. 7 Why do differential pairs of wiring have to be close and parallel? The wiring of differential pairs should be properly close and parallel. The so-called appropriate proximity is because this spacing affects the value of the differential impedance, which is an important parameter for designing differential pairs. The need for parallelism is also due to maintaining the consistency of differential impedance. If the two lines are suddenly and suddenly close, the differential impedance will be inconsistent, which will affect signal integrity and time delay. How to deal with some theoretical conflicts in actual wiring? 1. Basically, it is right to divide the Modulo / number separation. It should be noted that the signal alignment should not cross the segmented place(moat) as far as possible, and that the reflux current path of the power supply and signal should not be too large. 2. Crystalline vibration is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must satisfy the specification of loop gain and case, and the oscillation specification of this analog signal is easily disturbed, even if the addition of frame guard tracks may not be able to completely isolate the interference. And far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, it must be possible to close the distance between the crystal vibration and the chip. 3. There are indeed many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that the resistance capacitance or ferritehead added by EMI can not cause some electrical characteristics of the signal to be inconsistent with the specification. Therefore, it is best to use the technique of scheduling alignment and PCB stacking to solve or reduce EMI problems, such as high-speed signal walking layers. Finally, the resistance capacitance or ferrite bead is used to reduce damage to the signal. 9 In high-speed PCB design, the blank area of the signal layer can be copper, and how should the copper of multiple signal layers be distributed on the ground and connected power supply? In general, the vast majority of copper dressing in blank areas is grounded. Only pay attention to the distance between the copper and the signal line when laying copper next to the high-speed signal line, because the copper applied will reduce the characteristic impedance of the alignment. Care should also be taken not to affect the characteristic impedance of its layer, for example in the structure of a real layer. Is it possible to calculate the characteristic impedance of the signal lines above the power supply plane using a microstrip line model? Can the signal between the power supply and the ground plane be calculated using a banded line model? Yes, the power supply plane and the ground plane must be regarded as reference planes when calculating the characteristic impedance. For example, the four-layer board: the top layer-the power layer-the stratum-the bottom layer. At this time, the model of the top level alignment characteristic impedance is a microstrip line model with the power supply plane as the reference plane. Will the addition of test points affect the quality of high-speed signals? As for whether it will affect the quality of the signal, it depends on how fast the test method and signal are. Basically, the additional test point(without the existing online perforation(via or DIP pin) as the test point) may be added to the line or pulled out of the line. The former is equivalent to adding a small capacitor on the line, and the latter is an additional branch. Both of these conditions will have a slight effect on the high-speed signal, and the degree of influence is related to the frequency speed of the signal and the rate of change of the signal edge. The impact can be known through simulation. In principle, the smaller the test point, the better(and of course the requirements of the test apparatus) the shorter the branch, the better. 12 How should the ground lines between the boards of several PCB components be connected? When the signals or power supplies between the various PCB boards are connected to each other during the action, for example, the A board has a power supply or a signal sent to the B board, there must be an equal amount of current flowing from the formation back to the A board(this is Kirchoff board law). The current on this layer will flow back where the impedance is minimal. Therefore, at each interface, whether the power supply or the signal is connected to each other, the number of tubes assigned to the formation must not be too small to reduce the impedance, which can reduce the noise on the formation. In addition, it is also possible to analyze the entire current loop, especially the part with a large current, and adjust the connection of the stratum or the ground line to control the current movement(for example, making a low impedance somewhere, allowing most of the current to go from this place), Reduce the impact on other more sensitive signals.