Sunday, 13 February 2011

VFO Controlled FM Transmitter



Examining the schematic diagram shown in Figure 6-1 shows that the transmitter consists of three stages: an audio amplifier, a voltage-controlled oscillator (VCO), and a buffer amplifier. The audio stage takes the audio input in the 20–15000 Hz range and amplifies it by a factor of about five times, and increases in gain at 6 dB/octave above 2.1 kHz to add the needed preemphasis to the audio. This audio is fed to a VCO consisting of a Colpitts oscillator with a varactor diode acting as an FM modulator, and the VCO feeds a buffer stage to bring the output up to about 0.5 mW, which will produce a usable signal up to about 200 feet with a short (6-inch) whip antenna. A zener diode provides a fixed 6 volts to the VCO for improved stability with declining battery voltage. Nine to twelve volts is recommended, although the transmitter will operate down to six volts, but the zener regulator will not function properly at this voltage, and more drift may be expected.

Referring to the circuit diagram, audio input is applied at coupling capacitor C1 and ground. Audio is fed through R1 to audio stage Q1, whose gain is determined by the ratio of R4 and R2 to R1. C2 provides bypassing above 2 kHz, reducing the feed-back and increasing the gain as frequency increases; this provides preemphasis. R5 and R3 are bias resistors. Audio appears at the collector of Q1, amplified about five times. This audio is fed to gain control R6 through C3, and potentiometer R6 sets the deviation or modulation level. Audio from the wiper of R6 is fed through C4 and isolation resistor R7 to the varactor-modulator diode D2. D2 is reverse-biased through R8 with about 6.8 volts reverse bias. This sets the capacitance of D1 to about 15 pf.

D1 is a 6.8-volt zener diode, with C6 and C7 acting as bypass capacitors to reduce noise and provide an RF ground. The VCO consists of Q2 with associated bias resistors R10, R11, and R12. The VCO is a Colpitts oscillator configuration. This setup has the advantage of reducing the loading on the oscillator frequency-determining circuit by the indefinite and variable transistor parameters, reducing drift caused by the transistor. It is an excellent oscillator when low drift is desired because the transistor input impedance is swamped out by relatively large capacitors C10 and C11.

The series combination of C11, C12, L1, and trimmer C9 in parallel with C8 form the main oscillator “tank” circuit. L1 is adjustable with a slug to set the coarse frequency, and C9 is used for fine adjustment.The varactor diode appears in series with C5, and the total capacitance of about 3.5 pf appears in parallel with C8. Adding this 3.5 pf to the 22 pf of C8 and thenominal 6 pf capacitance of C9 (variable from 2–10 pf ) totals about 31.5 pf, which appears in series with the series combination of C10 and C11 (41 pf ). This provides a total effective capacitance of about 18 pf, and L1 tunes with this capacitance. It can be shown that the variation of C10 and C11 caused by the transistor capacitances has only a small effect on the tuned frequency. This contributes to frequency stability.

Audio on D2 varies its capacitance, which varies the capacitance of the tuned circuit, causing frequency modulation of the oscillator, with very little AM component. This will be much smaller than that which would result from modulating the VCO transistor Q2 directly in order to vary its collector capacitance. Oscillator output from a low impedance point (across C11) is fed through C22 to buffer stage Q3. R13, R14, and R16 provide bias for Q3, while R15 suppresses possible UHF self-oscillation in Q3.

C13 is an RF bypass, and the output is filtered by tank circuit L2, C15, and C16. RF output is taken from the junction of C15 and C16 and is about 0.5 mW into 50 ohms.

Coil data is given in Figure for the construction of L1 and L2. They are not critical, and the coils may have to be adjusted later by adding or removing a turn to obtain operation on your chosen frequency. You can find suitable slugs in the IF and video coils used in older TV receivers and junked CB radios. Data shown is for the low end of the FM band (88–92 MHz), where there is apt to be less competition from higher-power commercial stations. Do not operate near and never, never above 108 MHz because you could cause interference with aeronautical navigation systems.

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