Our group is the Modulation/Demodulation/Mixer group. The purpose of this project is to modulate and demodulate an AM signals. There are different ways of modulating and demodulating AM signals. This project is done with the simplest design available while maintaining the desired high frequency signal. TAM MODULATION/DEMODULATIONWritten For:Prof. Ray Kwok EE 172Written By:Chad SchraderBenjamin DuboisOmar CastilloRyan ClarkeEfrem HabteFarial MahbubIntroduction: (Written by Efrem Habte) Our group is the Modulation/Demodulation/Mixer group. The purpose of this project is to modulate and demodulate an AM signals. There are different ways of modulating and demodulating AM signals. This project is done with the simplest design available while maintaining the desired high frequency signal. TModulation(Written By Ryan Clarke)Modulation is a technique used for encoding information into a RF channel. Typically the process of modulation combines an information signal with a carrier signal to create a new composite signal that can be transmitted over a wireless link. In theory a message signal can be directly sent into space to a receiver by simply powering an antenna with the message signal. However, message signals typically don't have a high enough bandwidth to make direct propagation an efficient transmission technique. In order to efficiently transmit data, the lower frequency data must be modulated onto a higher frequency wave. The high frequency wave acts as a carrier that transmits the data through space to the receiver where the composite wave is demodulated and the data is recovered. There are a few general types of modulation; Frequency Modulation (FM), Phase Modulation (PM), and Amplitude modulation (AM). Frequency modulation encodes data by performing shifting of frequency, phase modulation performs shifts in phase, and amplitude modulation controls the envelope of the carrier wave. AM is usuallythe simplest to implement and is thus the scheme we chose for our modulator.In an AM radio system a high frequency sinusoidal wave is amplitude modulated by a lower frequency message signal.This can be expressed byVam(t) = [(A + Vm(t))] Cos(2πfct)where Cos(2πfct) is the carrier frequency and Vm(t) is the modulating signal. In our application fc = 915 MHz and Vm(t) = Audio Signal. We can take Vm(t) = VmCos(2πfmt) where fm = highest frequency component in the message signal. For transmitting audio, fm = 20 kHz. The constant "A" is chosen such that Vam(t) never becomes negative. Thus Vam(t) can be modified to Vam(t) = A[(1 + mCos(2πfmt))] Cos(2πfct).In this expression "m" is known as the modulation index. After performing the multiplication of the modulated signal the spectral output can be determined.Vam(t) = A[(Cos(2πfct) + mCos(2πfmt) Cos(2πfct))] Vam(t) = A[(Cos(2πfct) + m/2[Cos(2π(fc-fm)t) + Cos(2π(fc+fm)t))]]From the multiplication it is evident that the resulting spectrum consists of the center frequency fc and two side band frequencies (fc - fm) and (fc + fm). What has occurred is that the low frequency message signal has been translated to a muchhigher frequency range for greater transmission efficiency. Either of the side bands can beused to recover the message signal at the demodulator. One simply needs to filter out the unwanted side band before sending the signal to the demodulation stage. This modulationscheme is typically implemented in circuitry by a component called a Double-Side-Band mixer. The mixer physically multiplies the carrier wave, driven by an oscillator, with the message signal to produce the AM signal.Diode Double Balanced Mixer(Written by Chad Schrader)Our group implemented the modulator by using a diode double-balanced mixer design. We decided to use this design because it seemed like it would be the most straightforward. The mixer basically consists of two balanced to unbalanced transformers(baluns) and a Schottky diode quad. The Schottky quad basically is a rectifier that multiplies the local oscillator input (915 MHz) and the audio input (20 kHz). We implemented the Schottky quad by putting four Schottky diodes in a ring formation. The two baluns are used to match the circuit to the external circuits that are connected to the mixer. We made our own baluns by buying some toroidal ferrite cores and #32 wire and wrapping the cores according to a schematic for a 1:1 balanced to unbalanced transformerfound in the ARRL Handbook. The cores were wound using three wires twisted together. There were 7 winds on the toroid in total. This circuit was constructed on a protoboard, taking care to try to keep the leads between components as short as possible. SMA connectors were used for interfacing with the other circuits in the transmitter.Envelope DetectorThe demodulator was implemented using an envelope detector circuit. This consists of a diode and a low pass filter circuit. The low pass filter circuit is simply a capacitor and a resistor in parallel. The values for the resistor and the capacitor were calculated using the following equation:211])[(mcRCwhere c is the local oscillator frequency (915 MHz) and m is the frequency of the audio signal (20 kHz). Once we found a value for RC (71034.2), we assumed a value of 1000 for R. This gave us a value of 23 pF for C. In our implementation of thecircuit, we used a 24 pF capacitor because that was the closest standard value. The circuit was constructed on a protoboard, again trying to keep the leads between components as short as possible. We also used SMA connectors for the input and output of this circuit.Demodulator and Envelope Detector(Written by Farial Mahbub) Amplitude Modulation (AM) refers to the method of adjusting an electromagnetic carrier frequency by varying its amplitude in accordance to the analogue signal to be transmitted.There are two essential methods that are used to demodulate AM signals and in this portion of the report we will discuss both. The figure below represents the circuit used as the Demodulator in this project. The first method of demodulation is using the envelope detector. The envelope detector is essentially made up of a rectifier and a low pass filter (see figure below). In this project a diode was used as the rectifier to pass current in one direction only. In orderto calculate the value of the RC time constant to be used, the following equation is used: 2πfC >> 1/(RC) > 2πfmVr = Vp (1 –e -1/fcRC)where fC and fm are the