Spectrum Audio Analizer

Circuit Design Project

Page 1

PART I: INTRODUCE BLOCK DIAGRAM AND OPERATION THEORY
I. Basic block diagram of audio spectrum analyzer: 1. Block diagram:
Input Filter 1 VU-LED Circuit

Filter 2

Display

(n circuits) Filter n

Basic block diagram of audio spectrum analyzer 2. Operation theory: Input signal comes into each filters. Filters are band pass filters. They only passsignals belonged to fixed frequency range and remove others. We can use transistor and discrete capacitors and resistors to make active filters or we can use OP-AMP and other passive device. The more filters, the higher frequency resolution of spectrum analyzer, shown as the higher quality of the circuit. VU-LED Circuit employed to display signal level behind filters. This kind of display circuit can bediscrete devices or specific ICs. The more outputs, the higher resolution of amplitude. In the above diagram, we know that each filter needs and VU-LED circuit for display, the more filters (the higher frequency resolution of the circuit), the more VU-LED circuits. This matter will make the circuit complex for wiring. To overcome this weak point, we will consider following improvement in thoseblock diagrams:

Circuit Design Project

Page 2

II. Block diagram of audio spectrum analyser uses multiplex display: 1. Block diagram:
Input Filter 1 Switching circuit VU-LED Circuit

Filter 2

Display matrix
.

Filter n

Oscillator

Counter& decoder

Driver

Block diagram of audio spectrum analyser uses multiplex display

2. Operation theory: The advantage of this diagramis only one VU-LED circuit employed, regardless the number of filters, the number of wiring to display circuit is also more simple because display matrix is the combination of rows & columns. The number of columns is adapted to the number of filters, and the number of rows is the number of VU-LED outputs. This diagram also includes n filters, outputs of the filters are connected to switchingcircuit, at one time, switch passes signal from only one filter. Switch is controlled by counter & decoder, clocking signal from oscillator comes to conter & decoder, the number of outputs of counter & decoder is n. Ouputs of counter &decoder are also connected to driving circuit to scan columns. If the frequency of clock is fast enogh then our eyes are cheated to have the feeling that all columns arebright simultaneously but actually, at one time, only one column is bright.

Circuit Design Project

Page 3

PART II: FILTERS
I. Introduce filters: Generally, in electronic equipment, if they need to reject or pass any frequencies they often use freqency filters. At first, it’s mainly contructed by inductor L and capacitor C. Nowaday, OP AMP with small dimension, many characteristic, lowcost, simple in design calculation so it’s prefered to make active filter RC. There are many kinds of filters such as Butterworth and Chebyshev. Before consider filters, we must consider filter orders. The order of a filter idenfifies its cuttoff slope. The higher the order number, the steeper the cutoff slope is. Filter orders increase in steps of 6dB/octave. The simplest filter is first-orderfilter has cutoff slope of 6db/oct. Other high-grade active filters may have higher orders, for example, second order filter has cutoff slope of 12dB/oct.The following part, as project requirement, mainly introduce about bandpass filters. Following is introdution about filters:

1. Low pass filters: A Butterworth filter is designed to have a very flat frequency response within its bandpass and asmooth, uniform roll-off characteristic. Below figure shows a frequency response graph for a typical first-order Butterworth filter.
Fc (Cutoff frequency) Amplitude

Frequency

Another common used is Chebyshev filter. Frequency response graph of a Chebyshev lowpass filter was show as below figure.
Fc

Circuit Design Project

Page 4

Notice that the frequency response is not flat...