Final Project Blog

FM Transmitter


For our final project, we decided to create a FM transmitter with a microphone input, which would allow us to create a pirate radio station.

Our initial design was to make it compact enough to fit into altoid box.
Initial sketch:


Our schematics for the circuit:



Our first prototype in the breadboard:



our prototype has the microphone with amplifier as opposed to a regular electret microphone input that we used as our final design. Although this microphone allows us to get voice from longer distance from the mic compared to the regular electret microphone, the component itself is about 3 times the price of our circuit itself, which will increase our production cost by 400% while only increasing the range by only a little bit. We decided that the cost outweigh the benefit, and decided to use a regular electret microphone instet.

At the testing stage, we met with a couple difficulty, such as:
- trying to debug why our circuit did not work, when it was actually working but our radio was just not good enough to recieve signal (we initially used fm radio app that comes with our phone, and it will lose the signal just by orienting it differently).

-  when we used waveform to test the circuit, there were 2 frequencies where the signal could be heard. One is at the 90MHz range, which comes from the analog discovery box, and another is at the 80-90MHz range, which comes from the circuit. We spent a lot of time trying to figure out why our trimmer capacitor did not change the frequency, but it turns out it was because we were tuning to the analog discovery signal.

- We did not have a nonconductive screwdriver, which make our circuit to change frequency just as we take off our screwdriver off the trimmer capacitor, which was not great. We later tried to resolve it by plasti-dipping our screwdriver, which helps the tuning process easier because it is nonconductive(ish).


- noises in our broadcast. This happened when we used too much gain in the microphone or when our media player is at a high volume (when using a 3.5 mm audio jack input). 

We also created a prototype with a 3.5 mm audio jack input as aforementioned:



Also, using Perfboard as opposed to the breadboard make our broadcast to be clearer by quite a bit.

Our final design:

 

Our microphone input:




We added a solid wire as a structural support to our antenna because we found that by twisting the antenna differently, we will change the frequency of our broadcast, which we found to be non-ideal.

If we were to have more time, we would switcch our input to two jumper wires instead, so that we could hook it up to a mini breadboard that has different inputs ( mic or audio jack), which would allow for easily swappable input, which will make our design more versatile.

6/8/2017 Classroom Activities + "Passive RL Circuit Response" lab

6/8/2017

Today we first learn about how to calculate the omega_0 and the cutoff omegas of a parallel RLC circuit. At the cutoff omegas, the power is half the max power which occurs at omega_0.

We then were assigned to find the voltage gain of a circuit

We also learn how to calculate cutoff frequency of a circuit

"Passive RL Circuit Response" lab:




for the prelab, we were told to find the cutoff frequency of the RL circuit, which we found to be at 10^5/2pi

For RL circuits, the high pass filter occurs when we take the output of the inductor, and low pass occur when we take the output of the resistor.

Our obtained graph when it is low frequency (at omega=omega_c/10)

Graph when it is high frequency (at omega = 8* omega_c)

Note that the red(math channel) is for the input voltage, yellow is the output voltage of the resistor, and the blue is the voltage of the capacitor.

Comparing the experimental and theoretical values:

Conclusion:

From this experiment, we could see that the phase shift between the input voltage and the resistor voltage is close to 0 at low frequency, and the phase shift between input voltage and the inductor voltage is close to 0 at high frequency. There is a 90 degree shift between input voltage and resistor voltage at high frequency (resistor voltage is leading by 90), and 90 degree shift between input voltage and inductor at low frequency( inductor voltage is lagging by 90)

6/6/2017 Classroom Activities

6/6/2017

Today we learned how to construct bode plots, which consists of gain(dB) vs frequency, and phase shift vs frequency. Both of the graphs are approximation of how the graph will look like, using a semilogx graph.

We also learn a little about filter, and finding the peak frequency (omega_0) and the cutoff frequencies at which point the voltage is 1/sqrt(2) times the maximum voltage.

5/30/2017 + "Signals with Multiple Frequency Components" Lab

5/30/2017

Lecture:

Today we learned about constructing the transfer function <H(omega)>. Transfer function could be one of the 4 things: Voltage gain, current gain, impedance, and admittance. We could let omega*j to be s, which would simplify the ratio more, and finds the zeros and poles of the function. Zeros is when numerator is zero, poles is when denominator is equal to zero.

"Signals with Multiple Frequency Components" Lab

Prelab: 

We were assigned to calculate the gain as omega approaches zero and as omega approaches infinity

Our circuit:

In this lab we learned how to create a custom voltage input in waveform. 

The graph obtained from the custom input voltage vs the output voltage(green input, blue output):

We then did a frequency sweep:

Conclusion: 

From this experiment, we see that as omega approaches 0, the voltage gain of the circuit is 1/2, and as it approaches infinity, the voltage gain is 0. We could see from the graph that when it first started, the Vout (blue) is close to half the value of the Vin(green), and as omega goes higher, the ratio of Vout/Vin gets smaller and smaller.