This is helpful because the mixer has a lot of frequency components that can cause noise in the in the output and waste power amplifying the unwanted frequencies. The BPF can prevent those components from ever reaching the amplifier.                         Since the BPF used for this lab is a resonance circuit, it has a "resonator Q" term which describes the quality of the resonance circuit. Q can be measured experimentally by dividing the resonance frequency by the bandwidth. It is proportional to resistance at a given frequency with given inductance and capacitance values and inversely proportional to bandwidth.

    The mixer circuit can be constructed with a diode or transistor. A transistor-based mixer, which we are using in this lab, can amplify the signal. The mixer multiplies 2 frequencies together and outputs several different frequency components. For this lab AM, the difference frequency between RF signal and the oscillator input. I was curious about the other frequencies, so I did a quick search on the output of a mixer circuit. I found only what the lab had already mentioned; that two particular frequencies of interest are the sum and difference frequencies and nothing about all of the other frequencies that you can see on the spectrums in the simulations and measurements. After my brief research, I also pondered whether or not you could transmit a hidden message over different frequencies so that it could only be deciphered by combing the signals into one with a mixer. I think it would be plausible, but I'm not sure how secretive the message would be.

    The oscillator was the most interesting to me because I always hear about converting electricity from AC to DC, but never the other way around with the exception of a brief mention in my Power Systems Analysis class in a discussion on solar energy conversion. The oscillator could be used to generate the DC current generated by a solar panel in to AC power. It would be useful if the oscillator could split the output of a solar panel into a three phase power so that it could be sent to the power grid. 

    In lab, the version of the Hartley Oscillator circuit used in the prelab had an oscillatory output that was not even close to a sine wave. We tried the second version of the Hartley Oscillator to no avail. After a bit of frustration from getting no output at all, we rebuilt the first version. We finally got a sine wave and were excited until we realized the frequency was only about 100 kHz. This wasn't acceptable because the frequency needed to be on the order of mega-Hertz to get a difference frequency around 200 kHz when mixed with the RF frequency picked up by the antenna. We found that the 47pF and 22 pF capacitors used to realize the 69pF capacitor had been connected in series instead of parallel. After correcting the mistake, we were able to achieve frequencies between 1.1 MHz and 1.7 MHz by replacing the two capacitors with a trimming capacitor. That sufficiently covers the range of the radio frequencies of the RF frequencies that are available to be received. After finally getting the oscillator circuit to work, we attemted to test it with our radio. We did not put get an output. I suspect that the mixer was not correctly set up or that a few of the radio components may have come loose.

    Looking foward, I think these three circuits may help with our project. We are attempting to transmit an audio signal to be picked up by our radio. The oscillator circuit would provide the carrier frequency and the mixer would modulate it with the audio signal. The bandpass filter could be used to block everything but our desired transmission frequeny band width from being amplified and sent t the antenna.