FM Transmitter Noah Walker 3/5/2021 1. Introduction The goal of this project was to design and build a FM Modulator that works to transmit a voice signal in the FM Band of 88MHz to 107MHz. The device should be made of low cost, discrete components, battery powered and hand - held. All designs are expected to use discrete transistors. The radios should be able to effectively communication over a minimum of 10m but no more than 61m (see FCC Part 15 regulations). 2. System Design 2.1 Alternatives Alternatively, to the design chosen, was a one stage FM transmitter. I decided against this type of transmitter due to the lower voltages sent via the signal, making it harder for the transmitter to pick up a voice signal. While the two stage boosted the signal pre modulation , yielding a larger voltage output from the antenna 2.2 Schematic Figure 1: FM Transmitter Schematic 2.2.1 S chematic Explanation From the 9VB ATTERY in Figure 1 , R6 creates a load for the Condenser Mic to be powered on. From there C4 removes the DC voltage from the voice signal. The signal is then trans ferred into Q1, which in combination with R1, R 3, and R4 is a com mon collector amplifier, w hich uses R3 to feed the signal back and consistently boost it. The signal then goes through C3 which removes any excess DC voltage s till left over After that, the signal heads into Q2, C5, L1, C7, and R5. This is the FM modulation compo nent, C5 and L1 work in unison to form a tank oscillator , that oscillates at 101 MHz , this value is found us ing the equation in section 2.4 Carrier Frequency. The transistor is then ke pt on by using C 7 to constantly send power back into the signal and keep it going. The signal is then modulated through this process and outputted through the a ntenna 2.3 Simulation Results All the following simulations are done using an input from a Function generator with a sine wave, wi th frequency 5kHz, and amplitude 0.1 Vp. Figure 2: Microphone Simulation 2.3.1 Microphone Measuring This measurement was taken after C 4 , found in Figure 1 , but before the junction with Q1 and R3. Figure 3: Simulated Post Common Collector Amplifier 2.3.2 Common Collector A mplifier Measuring This measurement w as taken at the node connecting R3, R1, Q1, and C3 as found in Figure 1. Figure 4: Simulated Antenna Output 2.3.3 Antenna Measuring This measurement was taken at the node w ith L1, C 7, and the Antenna as found in Figure 1. 2.4 Theoretical/Calculations Carrier Frequency: 𝑓 𝑐 = 1 2 𝜋 √ 𝐿𝐶 = = 1 2 𝜋 √ ( 0 328 ∗ 10 − 6 ) ( 7 5 ∗ 10 − 12 ) L = Impedance value of tank oscillator C = Capacitance value of tank oscillator Inductance Value (In uH): 𝐿 = 𝑑 2 𝑛 2 18 𝑑 + 40 𝑙 = 𝐿 = 0 75 2 4 2 18 ∗ 0 75 + 40 ∗ 0 25 d = Diameter in inches n = Number of turns in the coil l = Length in inches 3. Prototyp ing Figure 5: PCB Prototype of FM Transmitter 4. Testing 4.1 Testing Verification To verify the prototype worked, I measured the output from the antenna of the oscilloscope and found the frequency it was modulated to 9 8.8 MHz I then used the FM receiver provided and tuned it to the found frequency from the oscilloscope and spoke into the cond enser microphone attached to the PCB. When I had it properly tuned to the right channel, I was able to hear my distorted voice when speaking into the microphone through the FM receiver. 4.2 Testing Outputs Figure 6: PCB Post DC Coupling from Microphone Figure 7: PCB Post Common Collector Amplifier Figure 8: PCB Antenna Output 5. Conclusion The PCB prototype for the FM transmitter worked, with all stages working as shown in Figure 6 Where you can see the voice signal going through in. Figure 7 which shows the voltage increase from Figure 6 , and Figure 8 in which you see the modulated signal that has been modulated up to 9 8.8 MHz . The FM receiver additionally worked on receiving the signal. The overall transmission range was found to be 11.2 meters from the FM transmitter to consistently get a signal.