AM Radio Superhetrodryne Receiver Breadboard Implementation

You can check the working circuit I had implemented on bread board here Click here to see working circuit video
For Matlab Am signal extraction Click here to see Matlab code video

1. Objective:

To design,simulate and implement AM receiver on the bread board.

2. Problem Statement:

Implement the knowledge we have gained in the class about the Amplitude modulation and Amplitude Demodulation by building AM receiver.

3. System Design:

This system design consist of a Superhetrodryne Receiver. However, for this project we have simplified the Superheterodryne Receiver into simple receiver. The schematic of Superhetrodryne Receiver is shown below.

But for the project we have simplified this circuit into simple circuit. Merging RF and IF filter into simple circuit called Tank Circuit. The simplified circuit for the AM Receiver is shown below

When an AM, DSB-LC modulated signal is received from Antenna it receives a lot of signals. In order to hear a particular station the unwanted signals. For example these unwanted signals could be noise and other undesired station. So, all these had to be filtered in order to be herd in the speakers. For this project we have two filters responsible for filtering signals. They are Antenna, Tank circuit and Low Pass Filter which are described down below

• Antenna:
The size of the Antenna was picked using equation 1.

𝑓 =(𝑉/𝛌)---1
Where f is the frequency of the AM station and v is the speed of the light in the. Space and λ is the wavelength. Solving this equation 1 gives the velocity of with the signal is propagation in space. As we know effective antenna have a length equal to half of the wavelength of the RF signal. Thus, higher the frequency would have small antenna. Due to this factor I designed for 1450 MHz AM station


• Tank Circuit:
𝑓₀=1/(2π√𝐿𝐶) ---2
Where f₀ is the frequency we want to pass though the band pass filter i.e 1450 MHz. For my designed I picked the capacitor value 100 pF. Plugging the value of capacitor and frequency in equation 2 the solved value for Inductor value was solved for 120 uH.
The tank circuit also takes care of the image station is the frequency where the same station can be heard.

F_Image = f_c+2IF ----3

Where Fc is the carrier frequency at which the signal is modulated. IF is the image frequency; IF =455 kHz
Also, in another hand the received signal from the Antenna consists of a lot of AM signal. For example the equation 4 below best describes it in Mathematical way for the received signal by the antenna

Φ_AM=[A₀+m₀(t)] cosw_0Ct + [A₁+m₁(t)] cosw_1Ct + … [A_n+m_n(t)] cosw_nCt ----4
Equation 4 shows the received signal by antenna for different carrier frequencies. But we can only list one station at a time. So, undesired neighboring station can be filtered with the help of tank circuit. After signal is passed through a tank circuit only one signal will come out. For example,

Φ_AM=[A₀+m₀(t)] cosw_0Ct ----5
Equation 5 shows output of tank circuit. Basically tank circuit is a band pass filter which allows to pass a particular frequency cutting off undesired frequency (neighboring station and image station). This, equation gives the frequency we want to hear with DC offset and modulated frequency which needs to be taken care of in-order to hear the desired signal which is m₀(t) transmitted by the AM station.


• Envelop Detector
Envelop detector: This block is made up of a diode and a RC circuit in parallel which acts a low pass filter. This phase is also called Demodulation phase. The output of the Tank circuit is taken to diode. The reason we use diode because diode gets rid of the negative amplitude. And during the negative cycle the charged capacitor is discharged through the resistor making a ripple effect, creating an envelope.
Also, during this envelop detection process high frequency component is taken off by low pass filter. For our case cosw_0Ct is the higher frequency component shown in the equation 5. The higher frequency is the frequency in which the signal is modulated in order to propagate the signal. Because of this, AM has long wave characteristics. The Low pass filter consists of a resistor and a capacitor in parallel. The right choice of these passive components will give us the desired cutoff frequency. For our case I have designed my cutoff frequency at 4 KHz. Since, the human voice has the maximum frequency range of 4 KHz. If the frequency is known then the resistance

𝑤 = 1/(𝑅𝐶)---6
I picked the standard capacitor value first. The value picked for Capacitor was 20nF. And the value of omega(w)

w = 2*π*f---7.
Solving equation 6 the value of resistor came out to be 1.99 K Ohm

Amplifier:

The second last part of the circuit consist of the Amplifier. The use of the amplifier is to amplify the received signal. The amplifier use was non inverting amplifier. The gain of the circuit is given by the following equation

𝐺𝑎𝑖𝑛 = 1 +(𝑅2/𝑅1) ---7

For my simulation I had gain of 10. Thus the value of R2 was picked which was 10 K Ohm. Solving the equation 7 the R1 value came about to be 1 K Ohm

Audio jack/Speaker:

The final part of the circuit consists of 2 components. They are speaker and audio jack. The output of the amplifier was conned to Audio jack and then connected to speaker where we could hear the AM desired station.

4. Simulation:

The simulation consists of a three parts. All the block were individually simulated and then put together as block. The three parts of the simulated signal are as follows.

• Tank Circuit

In figure 3 in the tank circuit shows the AC current source acts as an Antenna. Simulation including antenna for the tank circuit. The value picked for the capacitor and inductor comes from the equation 2.

Form the figure 4 we can see that the center frequency for the band pass is at 1.450 MHz Also the figure 5 below shows that the neighbouing station is filtered which is at 1.390MHZ which is separated more than 10 KHz which is that standard distance between carrier waves which is shown in figure 5.

Envelop Detector

The figure 7 we can see that the charging and discharging the capacitor making a ripple effect creating the envelop during positive half and negative half cycle decoding the signal m(t).

>Low pass filter :

The figure 8 shows the simulation of low pass filter. The simulated value of R and C came from equation 6. The calculated cutoff frequency was 4 KHz. As we can see in the simulated wave form that the -3dB also the corner frequency was at around to 3.94 KHz as shown in figure 9. This show that the higher frequency were cut off in this circuit block and letting only lower frequency less than 4 KHz to pass which is a voice signal

Final circuit(Merging the components together in single schematics):

Figure 10 shows two AC current source. The smaller frequency acts a small signal to be modulated and 1.45 MHz is the carrier signal. As we know modulation index is given by

𝜇 = 𝑚_𝑝/𝐴---- 8

Where µ is the modulating index of the signal , A is the amplitude and mp is the DC offset. For the simulation I used the value of m_p =A= 0.02 which makes the modulating index of the signal which is shown below in the figure 11.

For the figure 12 we can see the input signal was amplified 10 times so the signal could be heard by human ear. In your case if you don't hear the signal you can amplify more.

5 Hardware Implementation on Breadboard:

The hardware implementation were divided into two part. The tank circuit and the remaining circuit.

• The tank circuit:
As mentioned above the tank circuit consist of an inductor and a capacitor. The tank circuit was implemented on a bread boar with a load of 1 M Ohm. Then white nose was of amplitude 1V was given. The output of the tank circuit was plugging into the spectrum analyzer where we could see frequency spectrum. The output of the came out to be a band pass filtered. Then the capacitor was tuned in such a desire frequency is inside the band pass filter. We tuned capacitor the band pass filter because the inductor given to us constant. The figure 12 below shows the schematic of a Tank circuit.



The figure 13 shows that the filter was tuned to pass the 1.45 MHz signal
After the tank circuit was tuned to the desired carrier frequency then the rest of the components were placed together. The diode followed with a low pass filter acted as a envelop detector. And the LT1001 Amplifier was used to amplify the signal. The simulated amplification did not work so I amplified it by 100 times. However, it amplifier cannot amplify to 100 times, here Vcc = Vout since it on saturation mode. Then the output of the amplifier was plugin in to device and then to speakers


The figure 14 show the received signal form antenna and captured by Oscilloscope. The eye diagram can be seen clearly in this figure. This signal is the mixture of all the received signal in time domain. The captured frequency by oscilloscope is 22.682 KHz.


The figure 15 shows the output from the envelop detector which is the recovered signal m(t) . We can see there that the received signal is less than 4 KHz which mean that the low pass filter is working and the signal is ready for the next process which is the amplification process.


The figure 16 shows the captured image of the AM signal after the amplification in time domain.


Figure 17 shows the implemented hardware on the breadboard hooked up with antenna which is green colored wire


6 Discussion

I divided the circuits into 4 parts. They are tank circuit, envelop Detector, low pass filter and the whole circuit merged together. Beside this I also had some issue during implementing the hardware on the breadboard. The issue was the signal I was getting on my circuit was very weak signal. After playing with the antenna I learned that the signal are not wide spread in equally. So, I lowered the antenna down instead it holding it high the conventional way, I could get stronger signal. I also, learned that tuning capacitor changes the AM station where as tuning the potentio meter (resistor) changes the volume. This was interesting fact I learned during the hardware implementation. Moreover I also learned that The RF and IF filter can be also implemented with a signal circuit which is called tank Circuit. I gained a lot of experience demodulating AM signal. I could see that a simple wire could also work as Antenna. I would suggest you people to to understand the circuit well before jumping in implementation, so they can fix error right away instead of wondering around

7. Conclusion

Conclusion: Overall, the AM receiver was successfully designed and implemented on breadboard. This project gave me over all idea how AM signal I demodulated.