ANALYSIS AND DESIGN
This chapter deals with the process of obtaining the design specifications of the wireless Frequency Modulated Transmitter with a frequency of 100MHz in a range of 100m under accurate conditions. From the block diagram shown in FIG. 2.1, various circuit components are now chosen and appropriate approximate values are now chosen to match its properties as well as perform duties of the stages. The entire system could be grouped into two major stages
The Q1 stage consisting the oscillator, the audio amplifier and the modulator
The Q2 stage which is made up majorly of the RF amplifier and the antenna 3.2
One transistor is used as the audio frequency (AF) amplifier. The Colpitt’s oscillator is used. It consist a transistor which amplifiers the AF and the tank circuit for the generation of the carrier wave. The model of the oscillator used is shown in FIG. 3.1.
FIG 3.1: CIRCUIT REPRESENTATION OF THE COLPITTS OSCILLATOR WITH AF INPUT. 3.2.1
DESIGN SPECIFICATION FOR THE COLPITTS OSCILLATOR
For a transistor to function as an amplifier it must be biased in the forward active region. Theraja & Theraja, (2002). i.e.
Base emitter junction must be forward biased (VBE= 0.7v)
A BC108 transistor with the stated configuration was used. A 9V DC supply was used to power the entire system. Direct and alternating current flow in this circuit. The direct current is brought about by the 9V DC battery while the alternating current generator is the carbon microphone. Salgat (2007) stated that capacitors do not allow direct current through them. Hence the DC equivalent of FIG. 3.1 is as shown in FIG 3.2 and FIG 3.3.
Now for the forward active mode,
VBE=0.7V, choosing Ic = 5.5mA, Bf =200 Ic = Bf IBIB = 5.5mA/200 =0.0275mA. Applying Kirchhoff’s Voltage Law (KVL) to the collector-emitter loop 9 – VCE – R2(Ic + IB) = 0
9 – 3.5 – 5.5275R2 = 0
R2 = 9 – 3.5 = 0.995KΩ
Applying KVL to the base-emitter loop
9 – R1IB – 0.7 – R2(IC + IB) = 0
R1 = 9 – 0.7 – 5.5275 = 99.83KΩ
Value of R1 used in design = 100 KΩ
Component CC in FIG 3.1 is a coupling capacitor.
In practical application, the carbon microphone (the alternating current generation in FIG 3.1) accepts a large range of frequency. This must be carefully screened to allow only the audio range (20Hz – 20KHz) to be amplified and transmitted. Theraja & Theraja (2002) stated that by using various combinations of resistors, inductors and capacitors, circuits can be made that have the property or rejecting either low or high frequency or band of frequency. These frequency selective networks are called filters. Filters are broadly of two types
Active filters- which use transistors and op-amps in combination with resistor, inductor and capacitor elements.
Passive filters- which consist only series-parallel combinations of resistor, inductor and capacitor elements. Theraja & Theraja (2002) further stated that there are four types of passive filters- high-pass filter, low-pass filter, band-pass filter and band-stop filter. Only the high-pass and low-pass filter are used in this project work.
High-Pass Filter (HPF)- allows signals with higher frequency to pass through while rejecting lower frequencies. The minimum frequency which it allows to pass through is called cut-off frequency. There are RC and RL high-pass filters. Only the RC high pass filter is used in this project work.
Low-Pass Filter (LPF)- allows only low frequency to pass through but attenuates (to a higher or lesser extent) all higher frequencies. The cut off...
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