EE12: Laboratory Project (Part-2) AM Transmitter

Transcription

1 EE12: Laboratory Project (Part-2) AM Transmitter ECE Department, Tufts University Spring Objective This laboratory exercise is the second part of the EE12 project of building an AM transmitter in medium-wave band (550kHz-1700kHz). This part of the project involves in using the LC based oscillator (Colpitt Oscillator) already designed and design an Amplitude Modulator using voice-band signal from the line-out port of the PC to transmit voice. Vdd= 9V R 1 L 1 500µH C Vdd 10µF R E1 Q 1 2N K C 1 v rf C T 6-60pF C B 1nF R 2 R E2 470Ω C 2 Figure 1: Circuit Diagram of the Modified Colpitt Oscillator 1

2 2 The Modified Colpitt Oscillator The circuit shown in Figure-1 is the modified Colpitt oscillator. The only modification being the emitter resistor R E. It s divided into two resistors R E1 and R E2 and the LC tank is fed at the voltage divider instead of the emitter directly. The purpose is to give an extra degree of freedom to adjust the g m of the feedback by changing the ratio of R E1 to R E2. By adjusting the g m we can get an oscillation with reduced distortion. Also, it provides an low-impedance output to the next stage ie. the Amplitude Modulator. Parts List The parts remain same as the previous Laboratory exercise except for the following: L 1 = 500µH using two 1mH in parallel. The purpose of having R E1 is to vary it to the point where you get the oscillator to just get into oscillation and this will ensure an oscillation with less distortion. For PSpice you can use R E1 1.5kΩ and you should get it to oscillate. 2

3 R L Vdd Antenna C ant Q 1 v osc v audio I audio C E Figure 2: Architecture of Amplitude Modulator 3 Amplitude Modulator Architecture Figure-2 shows the architecture of the proposed Amplitude Modulator based on a simple multiplier circuit. The oscillator signal (v osc ) is fed to the base of Q 1 which is configured as an common-emitter amplifier with C E chosen such that it is very low-impedance at the oscillation frequency. The bias is provided by the current source I audio which is modulated by the input signal from the PC. Since the gain is directly proportional to the bias current, the amplitude is modulated with the input signal (v audio ). Finally, the antenna, which has an inherent inductance associated with it, is made to resonate with C ant so it carries only the RF signal and rest of the frequencies are filtered out. Circuit Design Figure-3 shows the reference design for the Amplitude Modulator. v osc is the signal from the Colpitt oscillator in Figure-1. C c1, R 6, R 1, R 2, Q 1 and C E for the common-emitter amplifier for the carrier signal (v osc ). C c2, R 7, C 2, R 3, R 4, Q 2 and R E form the modulator of the bias current for the RF amplifier. The collector current of Q 1 will not only contain the carrier signal with it s modulated sidebands, but also the baseband signal (ie. audio signal). 3

4 Vdd=9V C Vdd 10µF R 1 R 5 C 1 L 1 500µH L ant 400µΗ v c1 C ant v osc From the oscillator C C1 R 6 R 2 v b1 Q 1 2N3704 Vdd v e1 R 3 400Ω v in C C2 R 7 v b2 Q 2 2N3704 C E 2V PK R 4 R E v e2 C 2 Model of the PC lineout Figure 3: Reference Circuit for the Amplitude Modulator L 1, C 1, R 5 form the band-pass filter to filter out everything but the signal to be transmitted. With everything filtered out but the AM signal, L ant, C ant is made to resonate at the carrier frequency such that all the AM signal goes through L ant. Since the AM band frequency is so low that in order to have a resonating dipole antenna we need a very long wire (about 100m of wire), we instead use a ferrite core inductor which has a small radiating resistance. The audio signal is going to be provided from the line-out port of the PC which can be modeled as an Thevnin source with max 2V peak and a source resistance of 400Ω as shown in Figure-3. 4

5 Guidelines for Calculating Component values Choose R 1 and R 2 such that v b1 5V and the source impedance for the Thevnin equivalent is 10kΩ. Choose R 3 and R 4 such that v b2 2.8V and the source impedance for the Thevnin equivalent is 10kΩ. Choose R E such that the DC emitter current of Q 2 is approximately 1mA. Choose C 2 such that it forms a low-pass filter from v in to v b2 and choose the worst-case cut-off frequency to greater than 20KHz. Choose C E such that the magnitude of the AC impedance of C E is 3 10Ω at 500kHz. R 6 and R 7 can be chosen to be 100kΩ potentiometers. C c1 forms the high-pass filter to isolate the DC from the oscillator. Choose an appropriate value of C c1 such that the low-frequency 3dB point is well below 500KHz for example 100 KHz. Similarly choose a value for C c2 such that the low-frequency 3dB point less than 10Hz. R 5, C1, L 1 forms a band-pass filter. Choose C 1 for ω o = 750KHz and R 5 such that the bandwidth is approximately 300KHz. Hint: See Lecture-21 notes for relation between Q and bandwidth of a parallel R-L-C network. Choose C ant such that the series L ant, C ant resonate at ω o = 750KHz. Add a variable capacitor in parallel when building the circuit to tune it to the exact frequency. Guidelines for PSpice Simulation After you get the oscillation, choose R 6 such that the oscillation amplitude at the base of Q1 is small enough that it s in the small-signal domain. Provide a single tone of 5 khz at the input as your voice signal. When you provide the input signal, you can check if the modulating signal is too big or small by monitoring the collector current of Q 1. 5

6 Since the collector current will contain large portion of the input signal, it will be hard to see the amplitude modulation in time-domain. So, do a FFT on the I C1 and you should see two sidebands related to the input signal. Monitor the voltage at collector of Q 1 to make sure the signal amplitude is small enough not to distort your signal. Monitor the current in L ant. It should contain a healthy amplitude modulated signal and again you can use FFT to measure that. 6