Digital Block For A Fucntion Generator Essay

, Research Paper

Digital Block Design

Design introduction

The Digital Block is the heart of this digitally controlled function generator. Symmetry and frequency variation is direct result of the design of this block. In this design the emphasis was on simplicity and some target specifications at the top of the frequency variation were compromised in order to achieve greater simplicity. This design achieves 28 frequencies in the range of 100Hz to 100kHz. Whilst the 17 frequencies from the range 100Hz to 10kHz are symmetry variable in five steps from 0.1 to 0.5. The last eight frequencies only managed to achieve symmetry variations 0.5 and 0.25. This is due to some limiting factors that will be discussed later. The table of achievable frequencies can be found at the end of this report.

User inputs are also digitally processed in this function generator and sent out as digital signals to other parts of this function generator namely the amplifier module. Also a filter selector circuit is built in after the digital block. The user inputs controlling frequency and symmetry are also built into this control block.

Therefore it is clear that the digital block can be divided into 4 distinct blocks, each with its own functionality. These blocks are the control module, counter module, filter control module and 8-bit D/A. The D/A chosen is the DAC0801LCN. It s specifications can be found in the appendix. The layout of this design is shown below.

The interconnections will be discussed later but the thickness of the lines indicates the number of bits in the bus lines.

As the name suggests the control module takes user inputs, processes them and sends them out to the respective modules. The functionality module generates the count, varying between 0 to 255 in 256 steps or in 64 steps. The 8-bit D/A changes this count into a wave and the filter selector module selects which filter in the filter block to pass the signal through.

While extensive testing has been done on the functionality module, the control module and filter selector module are relatively simple and have not been tested due to the inability to do so. The control module consists primarily of switches and ROM s while the filter selection module is made up of two analogue multiplexers. These modules are discussed in further detail in later sections.

Control Module

Introduction

This block essentially converts user input into digital signals. We have four user inputs coming in, and five control signals outputted, with the extra output being generated internally. This function is carried out as follows:

Input Devices

There are two basic types of inputs: One being a graduated circular switch, and the other a simple up/down switch. Their basic function is outlined below:

Up/down switch: This is basically a toggle switch which can be switched from VCC to Ground [Logic 1 or 0].

Circular Graduated switch: This is a switch much like the controls on most scopes. Switch rotation moves a contact along a potentiometer. This contact is connected to an A/D converter. The number of bits of the A/D converter is different for each input.

User Inputs

The four user inputs are as follows:

Type: This utilizes the up/down device. Up (Logic 1) corresponds to a Triangle wave and Down (Logic 0) corresponds to a Square wave. This signal is fed simultaneously to the digital and Filter block. A Logic 0 (Square) will cause the filter to be bypassed as the square wave does not require filtering.

Symmetry: This employs the circular switch. 8 different symmetries are provided. Consequently, a 3-bit ADC is used. The provided symmetries and their binary is as follows:

000: 8/16 | 001: 9/16 | 010: 10/16 | 011: 11/16 | 100: 12/16 | 101: 13/16 | 110: 14/16 | 111: 15/16 |

Amplitude: This also utilizes the circular switch. This control signal is fed straight through to a multiplexer in the amplifier block. We are providing 16 different amplitudes. Therefore, a 4-bit ADC is used. The amplitudes and their corresponding binary is as follows:

0000: 10mV |0001: 20mV |0010: 40mV |0011: 60mV |

0100: 80mV |0101: 0.1V |0110: 0.2V |0111: 0.4V |

1000: 0.6V |1001: 0.8V |1010: 1.0V |1011: 2.0V |

1100: 4.0V |1101: 6.0V |1110: 8.0V |1111: 10.0V |

Frequency: 28 different values of frequency are provided using a 5-bit ADC in the circular switch. These are fed into a 5 input multiplexer which selects the frequency from the clock divider block. The frequencies are as follows:

00000: 100Hz | 00001: 200Hz |00010: 300Hz | 00011: 400Hz | 00100: 500Hz |

00101: 600Hz | 00110: 700Hz |00111: 800Hz | 01000: 900Hz | 01001: 1000Hz |

01010: 2000Hz |01011: 3000Hz |01100: 4000Hz |01101: 5000Hz | 01110: 6000Hz |

01111: 7000Hz |10000: 8000Hz |10001: 9000Hz |10010: 10000Hz|10011: 20000Hz |

10100: 30000Hz |10101: 40000Hz|10110: 50000Hz | 10111: 60000Hz|11000: 70000Hz|

11001: 80000Hz |11010: 90000Hz|11011: 100000Hz|

Band/Frequency selector: This is the internally generated output mentioned above. It selects which filter is used corresponding to the frequency band the chosen user frequency is in. This generated by passing the 5-bit frequency value to a multiplexer which then send a 4-bit binary number

Внимание, отключите Adblock

Вы посетили наш сайт со включенным блокировщиком рекламы!
Ссылка для скачивания станет доступной сразу после отключения Adblock!

Скачать