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The above transmitter block gives a conceptual feel for this type of PLL implementation. LC tuned VCO's have good deviation sensitivity, but poor stability with respect to frequency drifts due to the ageing affects and non-zero temperature coefficients of the inductor and capacitor. This is where the feedback stability of a PLL comes into play, by dividing the output carrier frequency by a factor which will make it equal to a reference frequency such as a crystal oscillator. The divide by N network also plays a part in minimising interference from the crystal oscillator (XO). The low pass filter prevents feedback of modulated frequencies and eliminates the possibility of the loop locking to a side band.
The overall system is quite stabile, which is good, but the circuitry involved is quite large even if the devices were all integrated circuits it still would be quite bulky and rather complicated. The complication would be in making the system multi-channelled. For multi-channel capability it would mean changing the divide by N factor and including another block to change the reference frequency XO to equal that of the feedback network in order for the tracking of phase to be possible when the VCO drifts from its centre frequency.
Note : the 7805 is a 5 volt regulator, which enables the MC1648 to be powered from a 9 Volt battery.
This was a design that built and tested for the feasibility of using a VCO on it's own for wide-band frequency modulation of an audio input. The MC1648 (Appendix C) is the voltage-controlled oscillator used at the heart of this modulation scheme. The frequency of the tank is controlled by the resonant frequency of L1, D1 and D2. D1 and D2 (MV1404 was used) are both varactor diodes, which as seen before in section 3.6 (Reactance modulator), will have a nominal value of capacitance when a certain reverse dc bias is applied to it, the 10K (variable) / 5K potentiometer takes care of this bias voltage. D1 and D2 are effectively in parallel and their effective capacitance is added together. To change the output carrier frequency the 10 variable resistor is varied. A signal generator was used to simulate the audio baseband, its voltage was varied from .5 to 1.5 volts and its frequency was varied from 200Hz to 10Khz. A 1.2K resistor was used in conjunction with a probe, which was connected into a spectrum analyser. The results were as expected (from the data-sheet), for a 5.5 volt bias applied, 100MHz was seen on the analysers, screen and side-bands were also seen as a result of the voltage generator, the side-bands increased as the baseband voltage and frequency was increased, which shows Carson's Rule in practice. A Walkman radio receiver was set to 100MHz and the voltage generator's signal could be successfully demodulated. As the voltage was increased at the signal generator, the sound in the receiver's earphone became louder and as the generators frequency was increased, the sound increased in pitch, proving that modulation and demodulation had taken place.
This is a rather interesting design, but it has to be considered as only a functional block and not a complete transmitter. To make it into a transmitter an audio amplifier section needs to be inserted in order to interface with a microphone for audio modulation and possibly a Class-C output amplifier terminated with an impedance matched network before going into an antenna. If this were done, the transmitter would take up quite a considerable area and this is only considering single-channel transmission. Making the transmitter multi-channel would add on extra circuitry due to the fact that a more stable method than just tweaking the variable resistor will have to be found and also the Class-Cs output tank resonant frequency would also have to be changed. For this reason alone, it cannot be considered as part of a final working design.
This simple FM transmitter is built around two amplifiers, Q1 is a common emitter with a dc gain of 1 and an Ac gain that can be set by the potentiometer R4, this will amplify the signal from the Electret and pass it on to the next stage by coupling capacitor C3.
Q2 is at the heart of the RF section, because of C4 (which ac grounds the base) and the feedback cap C7 (that splits the capacitance C5 and C6) the RF section is a colpitts oscillator in the common-base mode. The inductance of L_var and the effective capacitance of C5 and C6 work out the centre frequency. The base-collector junction capacitance (which acts like a varactor diode) is varied as the amplified base-band signal changes it's reverse-bias voltage, this capacitance will inevitably be part of the over all tuning capacitance of the resonant tank. The Antenna, (very short end fed wire) can be resistively matched by an ordinary low-value resistor.
This is quite an effective little transmitter that can be easily made and has a range of about 60 feet indoors.
The transmitter above makes use of the old 'reflex' technique dating from the time when active devices were expensive and were sometimes made to perform two functions at the same time.In this case the transistor Q1 is acting as an audio amplifier for the signal from the Electret microphone. The amplified signal appears at the collector, R2 being the collector load resistor. R1 provides bias and DC feedback to set the collector to about 3.4V producing a simple common emitter amplifier.
At the same time the transistor is operating as a common base oscillator at VHF, the base being grounded to RF by C3. RF feedback to the emitter through TC1 sustains oscillation. The frequency is determined by L1, TC1, stray capacity and the collector base capacity of Q1. Now the collector base junction is reverse biased and looks like a variable capacitance diode. Since the amplified audio appears across this diode its capacity, and hence the VHF frequency, will swing in sympathy with the audio input. Output is taken direct from the collector using a short aerial. The frequency of oscillation is set with TC1. The RF power INPUT to the transistor is only about 8mW from a total power to the bug of about 25mW.
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