5 Final Design , Construction and Assembly

5.1 Introduction

This chapter will discuss the final design in detail and give instructions on how it can be built and implemented.

5.2 Final Circuit Design

After considering 4 basic designs, it was concluded that a mixture of the 3rdand 4thdesign's be implemented for transmitting human speech across the commercial bandwidth (88 MHz to 108MHz). The design had to be portable, low powered and be able to have the capability of transmitting to more than 1 channel.

Although the variable and shunt capacitors C4 and C5 are set up to transmit from 88 to 108Mhz, the transmitter only has an effective tuning range 6 MHz (30 out of the 100 channels) this is due in part to the feedback capacitor C6 being at the right impedance for positive feedback to occur.

5.3 Oscillator analysis

When analysing circuitry with transistor a few theoretical assumptions will have to be made. The main assumption made for the analysis would be the small signal resistance r'e = VT/IC » 20W at start of oscillation and rising to about 28W after initial oscillations through the system. When the power to the circuit is turned on, unit step is applied to the tank circuit, The capacitor charges up and then releases its charge into the inductor, when the inductor finishes absorbing the charge it's magnetic field will break down and releases the charge back into capacitor and the cycle happens all over again at the resonant frequency of the tank.

According to the Barkhausen criteria for sine wave oscillation (section 3.5 The Oscillator) of the tank the amplifier (common base, section 2.10.3 Common Base) and feedback must has a loop gain of unity. Taking 100MHz as the resonant frequency of the tank
L = 0.1mH C4 + C5 = 25.33pF (theoretical value),

The second Barkhausen Criterion states that in order for sustained oscillation to happen the phase shift through the network at the resonant frequency will have to be zero, L1, C4 and C5 will yield 0o at the resonant frequency. There is a – 90o phase shift through C6. The emitter–collector channel has an interesting property when it comes to current and voltage, the current entering the emitter leads the voltage across the collector, hence a + 90o phase shift. Putting the capacitor and transistor together there will be a 0o between the input and output nodes.

5.4 Components List



note: if you do not have the font symbol installed on your computer, the resistor values will be followed by a W instead of the Greek Omega, to denote Ohms

5.4.1 Resistors

R1 10KW Carbon Film Bias for the Electret microphone
R2 1MW Carbon Film DC bias for the Base of Q1
R3 100KW Carbon Film DC bias for the Base of Q1
R4 150W Carbon Film Sets the DC & AC gain of Q1
R5 10KW Carbon Film Sets the DC & AC gain of Q1
R6 10KW Carbon Film Forms a HPF with C2 (pre-emphasis)
R7 1KW Carbon Film Sets the gain for the oscillator

5.4.2 Capacitors

C1 0.1mF Non-polarised tantalum Audio coupling capacitor
C2 0.1mF Non-polarised tantalum Forms HPF with R6
C3 10nF Ceramic AC grounds the base of Q2
C4 1 - 12pF Silver Mica Tuning cap for Multichannel
C5 20pF Ceramic Shunt capacitor for tuning
C6 5.6pF Ceramic Feedback for oscillation

5.4.3 Inductor

L1 0.1mH Toko

5.4.4 Transistors

Q1 & Q2 2N3904 T092

5.4.5 Microphone

5.4.6 Input - Out connections

Input is provided for an external tie clip Electret microphone, which will disconnect the fixed small Electret microphone. In order to achieve this a 3.5 mm jack is used which has a multiplex feature for switching between different references, which is dependant on the socket being empty or filled by the 3.5mm jack.

5.5 Construction and assembly

One of the most versatile properties of this design is this design is that the parts are very easily obtained, the circuit can even be built on ordinary vero-board. One thing that had to be observed was to keep the leads of the devices small and compact the circuitry. With a simple carbon film 70W resistor incorporated into the straight wire antenna.

The PCB design sank 8.37mA at 9v dc battery and yielded an output power of 8mW. Type of power source used was a Duracell Alkaline Manganese Dioxide 9volt PP3 battery. The duration for effective transmission is 14 Hours of continuous use.

5.5.1 Pcb Layout

The PCB design above was designed using easy-pc a dos based PCB design package. the above figures show the under side of the double sided board, most of the underside is devoted to a ground plane, the topside (component side) is totally devoted to a ground plane. A foil isn't needed for the component side, all that needs to be done is drill the under side and use a track cutter on the component side to enlarge the spacing from hole edge to the ground plane. The secnd figure above displays where the components fit in on the board.

Note: the capacitor C4 is mounted on to the PCB on it's side to allow for access to the tuning circuitry. Also the RF section (between C3 and just before the output is coated with household cling film and then wrapped with aluminium cooking foil. This will prevent any stray signal feedback from interfering with modulation.

The PCB board slots into a handheld instrumentation case 90*65*25(obtained from RS), a total of 4 holes were drilled, two on top (for the miniature electret and a 3.5mm socket for antenna), one at the side (for a switch) and one in the front (for tuning the capacitor).

5.6 Antenna Considerations

The antenna used for the project was an end fed whip antenna with a fully extended length of 75cm. From section 3.11.1 Radiation Resistance which dealt about radiation resistance of an antenna. Taking again the frequency to be about 100Mhz, which yields a wavelength of 3 metres, the radiation resistance is calculated to be about RL=4.6KW. The output impedance of the oscillator is seen as the impedance of the tank in parallel with the impedance of the feedback capacitor plus the bias resistor R7 which at 100MHz works out to be Ro = 30W. To match the output with the input impedance of the antenna a simple shunt resistor of 4.570KW maybe placed between the output and the input to the antenna. Or a more elaborate scheme maybe employed by using an LC bandpass filter to make the antenna look like it has the same impedance as the output of the transmitter (see section 3.12 Impedance matching).

Q can be calculated as 12, XL as 370 and XC as 373. At a frequency of 100MHz, XL is equal to 0.589mH and XC is equal to 4.2pF. The network's bandwidth can be calculated as break frequency over quality factor, which is calculated as 8.3Mhz. This is quite a sophisticated way of matching the load, but it does have it's downside, especially when the transmitter is multi-channelled, in that the frequency is not a constant. Taking the frequency as 106MHz, the inductance will have to be 0.55mH and the capacitance as 4.02pF. So choosing an inductance of 0.6mH and a capacitance of 4.1pF could possibly match the transmitter with the antenna.

The method chosen for matching the impedance of the antenna was the resistor placed between the output node and the antenna. This yielded a respectable range of 80 feet in a household environment and about 50 feet inside a lab.

An extension to the antenna was discussed and implemented using a thin coaxial cable with it's outer conductor grounded to the board, which tended to change the centre frequency of the transmitter.

5.7 Overall frequency of the transmitter

The frequency stability of due to ageing effects and the non-zero temperature coefficients of the components (see section 2.7 Temperature stability of the Tank) tends to vary the frequency of the transmitter, so mild adjust of the receiver is called for every so often, this will be ok for analogue FM receivers, but for digital receivers (which are slowly becoming popular) this can be quite tedious as was found during testing.