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A light weight metallised diaphragm forms one plate of a capacitor and the other plate is fixed, the capacitance thus varies in sympathy with the acoustic signal. The capacitance acquires a fixed charge, via a high value resistor (input impedance of FET) and since the voltage across a capacitor is equal to its charge divided by its capacitance, it will have a voltage output which is proportional to the incoming audio (baseband).
The fixed plate at the back is known as Electret which holds an electrostatic charge (dielectric) that is built in during manufacture and can be held for about 100 years. The IGFET (needs to be powered by a 1.5 volt battery via a 1KW resistor) output is then coupled to the output by an electrolytic capacitor.
Improving the signal to noise ratio in FM can be achieved by filtering, but no amount of filtering will remove the noise from RF circuits. But noise control is achieved in the low frequency (audio) amplifiers through the use of a high pass filter at the transmitter (pre-emphasis) and a low pass filter in receiver (de-emphasis) The measurable noise in low- frequency electronic amplifiers is most pronounced over the frequency range 1 to 2KHz. At the transmitter, the audio circuits are tailored to provide a higher level, the greater the signal voltage yield, a better signal to noise ratio. At the receiver, when the upper audio frequencies signals are attenuated t form a flat frequency response, the associated noise level is also attenuated.
The carrier oscillator is used to generate a stable sine-wave at the carrier frequency, when no modulating signal is applied to it . When fully modulated it must change frequency linearly like a voltage controlled oscillator. At frequencies higher than 1MHz a Colpitts (split capacitor configuration) or Hartley oscillator (split inductor configuration) may be deployed.
A parallel LC circuit is at the heart of the oscillator with an amplifier and a feedback network (positive feedback). The Barkhausen criteria of oscillation
requires that the loop gain be unity and that the total phase shift through the system is 360o. I that way an impulse or noise applied to the LC circuit is fed back
and is amplified (due to the fact that in practice the loop gain is slightly greater than unity) and sustains a ripple through the network at a resonant frequency of
Hz.
The Barkhausen criteria for sine-wave oscillation maybe deduced from the following block diagram
Condition for oscillation
xo+ yo = 0o or 360o
Condition for Sine-wave generation
A1 * A2 = 1
The above circuit diagram is an example of a colpitts oscillator, an LC (L1, C1 &C2) tank is shown here which is aided by a common emitter amplifier
and a feedback capacitor (C_fb) which sustains oscillation. From the small signal analysis in order for oscillation to Kick off and be sustained
the frequency of the oscillator is found to be , where C* is
.
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CD: Instantaneous capacitance about the Diode's terminals CO: is the capacitance at zero Reverse bias voltage |
Applying this to an LC tank : as the capacitance decreases the frequency increases. So placing a fixed reverse bias on the varactor will yield a fixed capacitance which can be placed in parallel capacitor and inductor. A bypass capacitor can be used to feed the baseband voltage to the varactor diode, the sine-wave baseband voltage has the effect of varying the capacitance of the varactor up and down from the level set by the fixed reverse voltage bias. As the baseband peaks the varactor's capacitance is at a minimum and the overall frequency will increase, applying this logic to when the baseband troughs the frequency will decrease. Looking at the three cases for the varactor diode, Maximum capacitance, Nominal capacitance set by V_bias (no modulation) and Minimum capacitance and observing the frequency will show that by modulating the reactance of the tank circuit will bring about Frequency Modulation.
 with peak negative baseband influence. |  with peak positive baseband influence. |
The diagram below show's a proposed modulation scheme, with the amplifier and phase network discussed earlier in the oscillator section.
Looking at the Buffer amplifier as an electronic block circuit, it may resemble a common emitter with low voltage gain or simply an emitter follower transistor configuration.
Frequency multipliers are tuned input, tuned output RF amplifiers, where the output resonant tank frequency is a multiple of the input frequency. The diagram of the simple multiplier below shows the output resonant parallel LC tank which is a multiple of the input frequency.
The circuit above is good for low multiplying factors (i.e. *2 ), for triplers and especially quadruplers, current idlers are used to improve efficiency. These series resonant LC's help in the output filtering of the input, but more importantly they aid in the circulation of harmonic currents to enhance the transistor's non-linearity. The idlers can be tuned to fi, 2fi , N-1(fi), the final output tank is tuned to fo = N(fi).
Other devices can be used instead of the transistor, one of which is called a Step Recovery Diode (SRD) or snap diode : it accumulates part of the input cycle and then releases it with a snap. The circuit efficiency or power loss is proportional to 1/N as opposed to 1/N2 for a good transistor multiplier. Of course the transistors current gain will make up for some of the loss provided by the transistor multiplier circuit.
So for high efficiency transistor power amplifiers, it is important to realise that most of the non-linearity is provided in the collector-base junction (varactor diode behaviour) and not the base-emitter, in order to maintain a high current gain.
The above multiplier circuit is a quadrupler and is used in very complex transmitter systems, because of its size and relative complexity it will not be included in the final design for the project, but it is worth noticing how it increases efficiency compared with the first simpler Class-C operation multiplier circuit. The series resonant circuits (current "idlers") help with the output filtering problem, but more importantly they improve circulation of harmonic currents which enhances non-linearity .
The final stage of any transmitter is the Antenna, this is where the electronic FM signal is converted to electromagnetic waves, which are radiated into the atmosphere. Antennas can be Vertically or Horizontally polarised, which is determined by their relative position with the earth's surface (i.e. antenna parallel with the ground is Horizontally polarised). A transmitting antenna that is horizontally polarised transmits better to a receiving antenna that is also horizontally polarised, this is also true for vertically polarised antennas. One of the intended uses for the transmitter is as a tour guiding aid, where a walkman shall be used as the receiver, for a walkman the receiving antenna is the co-axial shielding around the earphone wire. The earphone wire is normally left vertical, therefore a vertically polarised whip antenna will be the chosen antenna for this particular application.
Taking a centre fed dipole with a length of approximately half a wavelength, due to a capacitive effect at the ends of the antenna the overall length in practice is shorter (95% of the theoretical length). For dl half the wavelength, n is found to be 3.2. Rr = 789.5 * ( 0.5 * .95)3.2 = 72.9 » 73W.
For an end fed half wavelength making a few elementary changes to the above equation, i.e. making the length 97.5% and halving and then negating the exponent to give n = -1.6 which results in the radiation resistance equal to 789.5 * (0.5 * .975)-1.6 = 2492 » 2.5KW
Maximum power transfer between the antenna and the electronics circuitry will have to be looked at in order to produce an antenna that will be efficient in transmitting an audio signal to a receiver. Taking the case of the receiver with an antenna of impedance Zin connected with the input terminal, which is terminated with a resistor Rg. The maximum power transfer theorem shows that with a voltage induced in the antenna the current flowing into the receiver will be I = V / (Zin + Rg). The power transferred will be I2.Rg, differentiating the power with respect to Rg and letting the derivative equal to Zero for max. power transfer, it is shown that Zin + Rg = 2Rg, which means that Rg will be equal to Zin.
Now that a qualitative view of some of the characteristics of an antenna have been looked at, it is now time to look at some of the basic types of antenna that can be considered for this project.
Placing the + and - terminals in the middle of the antenna and ensuring that the impedance at the terminals is high (typically 2500W) and the impedance at the open ends is low ( 73W ). This will ensure that the voltage will be at a minimum at the terminal and at a maximum at the ends, which will efficiently accept electrical energy and radiate it into space as electromagnetic waves. The gap at the centre of the antenna is negligible for frequencies above 14Mhz.
When considering an antenna type and size for this project 2 things have to be taken into account, the frequency of transmission and the portability of the antenna.
Transmitting in a frequency range of 88 to 108 MHz, the mean frequency is (88 * 108)½ = 97.5MHZ. Rounding this off to 100MHz, calculating the wavelength gives (3*108 / 100*106 ) yields a wavelength of approximately 3 metres. l/2 = 1.5 m ; l/4 = .75m ;l/10 = 30cm
The above analysis concludes that the use of an adjustable end fed whip antenna with an affective length of 30 to 75 cm could be used with considerable affect.
An impedance matching system maybe merely a special wide-band transformer which is used for broadband matching (i.e. between 88 & 108Mhz), which maybe a two pole LC band-pass or low pass resonant circuits to minimise noise and spurious signal harmonics. The purpose of the impedance matching network is to transform a load impedance to an impedance appropriate for optimum circuit operation. Detailed analysis and calculations will be used latter on when evaluating the final design of the system.
Here are a few equations that determine the inductance and capacitor values from the above figure, when RL (Rantenna) and Ro (the output impedance of the amplifier) are known.
 | Quality factor : determines the bandwidth |
 | The impedance of the inductor @ the designed frequency |
 | The impedance of the capacitor @ the designed frequency |
The use of this matching network is predicted on the fact that Ro < RL according to the equation for calculating the inductance XL. This method of matching is similar to the so called quarter wave transformer for transmission lines.
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