Josephson Junctions, tasks

Task1.Which word means the following:

1. a regular arrangement of crystals;

2. the things that something in science regularly does;

3. to throw here and there , disperse;

4. to be greater than;

5. for this reason;

6. large enough to have an effect or be important

Task2. Answer the following questions.

1. What happens to metals at very low temperatures?

2. What makes ceramic materials different?

3. What causes superconductivity?

4. What about resistance?

5. How does J -junction work?

6. Where is it used?

7. Why are electronic devices with J-junction so good?

How does the radio propagate indoors?

It is well-known that radio wave reflect, not penetrate, when it hits a metallic surface. Anything that contains metal will reflect the radio wave, like the light fixture, file cabinet, computer, shelf, nail.....etc.. When the radio is used indoors, these metallic objects will cause a lot of reflections and each reflection will arrive at the receiver with a slight time difference owing to the path difference. For example, an 1 foot path difference will cause approximately 1 nsec delay for 900 MHz radio. The slight time difference is usually enough to cause the signal to fluctuate and smear. This problem is known as the multipath effect. As shown in the following figure, a pulse of 100 nsec becomes a lump of signals as wide as 500 nsec at the receiving end in a typical office building. This kind of smearing and fluctuating cause higher error rates for the data and the timing to jitter and will affect the performance of most radios. OCI overcomes this problem by using a correlator that always selects and locks to the strongest peak signal.

The most severe impact of the multipath effect is the complete loss of signal. When two signals with the same amplitude from different paths arrive at the same time, the resulting composite signal could be a null if the two signals are out-of-phase.

Since the wavelength of the 900 Mhz radio is just about 1 foot, the possibility of having two signals with the same amplitude arriving with ½ foot path difference (out-of-phase) is quite high. If the receiver is close to the transmitter, the direct wave is usually much stronger than the reflect wave so that the multipath effect is not evident. But as the receiver moves farther away from the transmitter, it loses the dominant direct wave and the reflect waves prevails. And the reflect waves from different paths will have little difference in amplitude as the receiver is farther away from the transmitter. As a result, nulls prevails.

You can imagine the null zones caused by the multipath effect just like blowing bubbles at the bottom of the deep sea. The bubble is very small at the beginning and grows bigger and bigger as it ascents to the surface until it merges with the air. When the receiver is near the transmitter, the null zone is not evident. But as the receiver is moved away from the transmitter, the null zone grows in size until there is no communications at all. The null zone is not static, on the contrary, it always moves randomly. People’s movement, air’s movement and fan’s vibration will all cause the null to move around.

As you can see from the figure of the null zone, the receiver will not be affected much by the null-zone if it is placed inside the inner circle (robust zone). The receiver will still work from time to time even if placed outside the outer circle (forbidden zone) but the null-zone will severely deteriorate the radio’s performance. To maintain a good quality of radio communications, it is important to identify the size of the robust zone, forbidden zone and identify the size of the robust zone, forbidden zone and the zone between them, the intermittent zone, and operate the radios only within the robust zone if possible.

To determine the size of each zone, the easiest way is to try a pair of the transmitter and receiver and see for yourself. This is because every building is different and the radio propagation is so much dependent upon the building. For example, a dry wall hardly has any effect on the radio propagation while a concrete wall attenuates the radio strength significantly. This is because the concrete wall contains more water than dry wall and water absorbs the radio wave.

To find out the size of the zones, you need to lock the transmitter on and move the receiver around the building. Most receivers have a carrier-detect light or a traffic light to indicate the presence of signal and it can be used as the indicator of the signal strength. Lay the receiver down on an open space, such as the top of the monitor or the top of the shelf and walk around it to see if it has any effect on the traffic light. You can even put your hand around the receiver (not touching the antenna). If the traffic light stays solid all the time then the receiver must be in the robust zone. Keep moving the receiver away from the transmitter until the receiver shows some flicking on the traffic light. The flickering indicates that a null is experienced.

Once a null is experienced, you can determined the size of the null by moving the receiver horizontally and vertically. Record the distance between the receiver and the transmitter, the maximum distance that you have to move the receiver to get it out of the null and the number of nulls you experienced in one cubic foot space. Then, move the receiver further away from the transmitter and repeat the above measurements again until you reach the forbidden zone. In this zone, the receiver will have a hard time to have a solid traffic light most of the time.

From the above records, you can figure out the size of robust zone as follows. The radius of the robust zone can be defined as the distance that the number of null is less than one in one cubic foot space and the size of the null zone is less than 0.5” in any direction. By this definition the null zone will occupy less than 0.01% of the space when it is in the robust zone. It also means that the failure rate caused by the null zone is less than 10-4 which is also the minimum error rate requirement for most radios.

It is most desirable to operate the radio within the robust zone. In this zone, the location of the radio is not critical if the radio is properly installed. If the radio has to be placed in the intermittent zone, some extra efforts are needed to assure a reliable radio link, such as moving the radio away from the shadow of other equipment or furniture, giving the radio as much free space around it as possible and putting the radio on top of a metallic surface. Keep in mind that the radio usually works better when it is placed farther away from the earth ground.

If you use a packet radio like LAWNII, you can reduce the packet size for the data to extend the coverage range. A small packet can survive a high error rate environment better than a large packet. The default packet size of the LAWNII is 256 bytes and it takes about 25msec to send the packet. If you use a packet size of 1, it will take less than 2.5 msec to send the packet and it can survive even at 10-3 error rate. Although a smaller packet is not an efficient way to send data due to the packet overhead (20 bytes), it is the most desirable way to send data from a low speed device such as a terminal.