2M RECEIVER PRE AMPLIFIER ELECTORNIC PROJECT

HOW TO MAKE 2M RECEIVER PRE AMPLIFIER?

Designed specifically to complement the modified air-band portable (also described in this series), this can be used with some success on many receivers suffering from ‘deafness’ on VHF.

The circuit
An RF preamplifier is a device which improves the input signal to an existing receiver, enabling it to work more effectively. Because of the noise which is added to the signal by the preamplifier, very weak signals may not be usefully enhanced, but stronger signals will be improved considerably.

This little circuit (shown in Figure 1) uses a GEC/Plessey integrated circuit type SL560C. With the addition of four capacitors, it is used between your aerial and the aerial input of the radio’s PCB.



Putting it together
1. Use a small piece of prototype (matrix) board about 25 mm square. Use an 8-pin DIL socket for the integrated circuit (don’t risk soldering the chip – it is seldom a risk worth taking). Figure 1 shows the connections to the socket, looking from underneath.

2. Make special note of the pin numbers, so that you know how to put the chip into the socket when you have finished. The positive and negative connections to the circuit are taken from the main PCB after the ON/ OFF switch – so that the switch operates the preamplifier, too.

3. Unsolder the lead to the radio’s telescopic aerial and connect it to the free end of C4, as shown. Then solder a short lead between the telescopic aerial and C2.

4. Sometimes it is possible to cajole your little preamplifier into the radio’s plastic case, provided there is room and that you make sure that none of the soldered joints on your little board touch any of the metal inside the case when you replace the back and screw it on again.

5. If there is not enough room inside, then put the preamplifier into its own box, with battery and switch, and its own aerial. Keep the lead from the preamplifier to the aerial connection of the radio as short as possible –
perhaps using screened cable. Because your preamplifier is untuned, you will find not only that it helps with reception on 2 metres, but also that reception on the FM broadcast band is improved!


Parts list
Integrated circuit
IC1 GEC/Plessey SL560C
Capacitors
C1 1 nanofarad (nF) ceramic
C2, C4 10 picofarads (pF) ceramic
C3 10 nanofarads (nF) ceramic
Additional item
Prototype broad approx. 25 × 25mm

ASCII CODE TUTORIALS

WHAT IS THE ASCII CODE?

The ASCII (American Standard Code for Information Interchange), pronounced ‘ask-ee’, is strictly a seven-bit code based on the English alphabet. ASCII codes are used to represent alphanumeric data in computers, communications equipment and other related devices. The code was first published as a standard in 1967.

It was subsequently updated and published as ANSI X3.4-1968, then as ANSI X3.4-1977 and finally as ANSI X3.4-1986. Since it is a seven-bit code, it can at the most represent 128 characters.

It currently defines 95 printable characters including 26 upper-case letters (A to Z), 26 lower-case letters (a to z), 10 numerals (0 to 9) and 33 special characters including mathematical symbols, punctuation marks and space character.

In addition, it defines codes for 33 nonprinting, mostly obsolete control characters that affect how text is processed. With the exception of ‘carriage return’ and/or ‘line feed’, all other characters have been rendered obsolete by modern mark-up languages and communication protocols, the shift from text-based devices to graphical devices and the elimination of teleprinters, punch cards and paper tapes.

An eight-bit version of the ASCII code, known as US ASCII-8 or ASCII-8, has also been developed. The eight-bit version can represent a maximum of 256 characters.

Table 2.6 lists the ASCII codes for all 128 characters. When the ASCII code was introduced, many computers dealt with eight-bit groups (or bytes) as the smallest unit of information.

The eighth bit was commonly used as a parity bit for error detection on communication lines and other device-specific functions. Machines that did not use the parity bit typically set the eighth bit to ‘0’.

ASCII CODE TABLE

Looking at the structural features of the code as reflected in Table 2.6, we can see that the digits 0 to 9 are represented with their binary values prefixed with 0011. That is, numerals 0 to 9 are represented by binary sequences from 0011 0000 to 0011 1001 respectively.

Also, lower-case and upper-case letters differ in bit pattern by a single bit. While upper-case letters ‘A’ to ‘O’ are represented by 0100 0001 to 0100 1111, lower-case letters ‘a’ to ‘o’ are represented by 0110 0001 to 0110 1111.

Similarly, while upper-case letters ‘P’ to ‘Z’ are represented by 0101 0000 to 0101 1010, lower-case letters ‘p’ to ‘z’ are represented by 0111 0000 to 0111 1010. With widespread use of computer technology, many variants of the ASCII code have evolved over the years to facilitate the expression of non-English languages that use a Roman-based alphabet.

In some of these variants, all ASCII printable characters are identical to their seven-bit ASCII code representations. For example, the eight-bit standard ISO/IEC 8859 was developed as a true extension of ASCII, leaving the original character mapping intact in the process of inclusion of additional values.

This made possible representation of a broader range of languages. In spite of the standard suffering from incompatibilities and limitations, ISO-8859-1, its variant Windows-1252 and the original seven-bit ASCII continue to be the most common character encodings in use today.

PACKED AND UNPACKED BINARY CODED DECIMAL (BCD) NUMBERS

WHAT ARE PACKED AND UNPACKED BINARY CODED DECIMAL (BCD) NUMBERS

In the case of unpacked BCD numbers, each four-bit BCD group corresponding to a decimal digit is stored in a separate register inside the machine. In such a case, if the registers are eight bits or wider, the register space is wasted.

In the case of packed BCD numbers, two BCD digits are stored in a single eight-bit register. The process of combining two BCD digits so that they are stored in one eight-bit register involves shifting the number in the upper register to the left 4 times and then adding the numbers in the upper and lower registers.

The process is illustrated by showing the storage of decimal digits ‘5’ and ‘7’:

• Decimal digit 5 is initially stored in the eight-bit register as: 0000 0101.

• Decimal digit 7 is initially stored in the eight-bit register as: 0000 0111.

• After shifting to the left 4 times, the digit 5 register reads: 0101 0000.

• The addition of the contents of the digit 5 and digit 7 registers now reads: 0101 0111.

Example 
How many bits would be required to encode decimal numbers 0 to 9999 in straight binary and BCD codes? What would be the BCD equivalent of decimal 27 in 16-bit representation?

Solution

• Total number of decimals to be represented=10 000=104 =213 29.

• Therefore, the number of bits required for straight binary encoding=14.

• The number of bits required for BCD encoding=16.

• The BCD equivalent of 27 in 16-bit representation=0000000000100111.

HEXADECIMAL NUMBER SYSTEM TUTORIALS

WHAT IS THE HEXADECIMAL NUMBER SYSTEM?


The hexadecimal number system is a radix-16 number system and its 16 basic digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. The place values or weights of different digits in a mixed hexadecimal number are 160, 161, 162 and so on (for the integer part) and 16−1, 16−2, 16−3 and so on (for the fractional part).
 The decimal equivalent of A, B, C, D, E and F are 10, 11, 12, 13, 14 and 15 respectively, for obvious reasons.

The hexadecimal number system provides a condensed way of representing large binary numbers stored and processed inside the computer. One such example is in representing addresses of different memory locations. Let us assume that a machine has 64K of memory.

Such a memory has 64K (= 216 = 65 536) memory locations and needs 65 536 different addresses. These addresses can be designated as 0 to 65 535 in the decimal number system and 00000000 00000000 to 11111111 11111111 in the binary number system.

The decimal number system is not used in computers and the binary notation here appears too cumbersome and inconvenient to handle. In the hexadecimal number system, 65 536 different addresses can be expressed with four digits from 0000 to FFFF.

Similarly, the contents of the memory when represented in hexadecimal form are very convenient to handle.

LCD (LIQUID CRYSTAL DISPLAY) NATIVE RESOLUTIONS BASICS AND TUTORIALS

WHAT ARE LCD (LIQUID CRYSTAL DISPLAY) NATIVE RESOLUTIONS?

The physical structure of some types of displays, including LCD Monitors/TVs and plasma panels, defines how many pixels can be displayed at once.

The display produces the sharpest picture when used at its so-called native resolution. This is the physically number of horizontal and vertical pixels that make up the LCD matrix of the display.


Setting a computer display to a resolution lower than this resolution will either cause the monitor to use a reduced visible area of the screen or it will have to do extrapolation.

This extrapolation attempts to blend multiple pixels together to produce a similar image to what you would see if the monitor were to display it at the given resolution but it can result in fuzzy images.

Below are some of the common native resolutions found in LCD monitors:

14-15": 1024x768 (XGA)

17-19": 1280x1024 (SXGA)

20"+: 1600x1200 (UXGA)

19” (Widescreen): 1440x900 (WXGA+)

20” (Widescreen): 1680x1050 (WSXGA+)

24” (Widescreen): 1920x1200 (WUXGA)

30” (Widescreen): 2560x1600