In the analog signal Amplitude, Frequency and Phase are few considerable parameters. There are modulation and demodulation schemes available to achieve transmission and reception of the message between source and destination. There are ASK, FSK and PSK modulation technique available to choose. Each scheme will have it's own pros and cons.
We will try to build a FSK type modulator and demodulator.
Basically we need to change the frequency of logic symbols '1' and '0' so that the receiver will understand what the transmitter is talking about. There are many way to achive the task. In the receiver end we need to identify the logic level from the incoming frequency. Mostly we adopt PLL based receiver.
Lets forget about the theory and we will do it in simple and practical way! Let's start!!
We will operate transmitter and receiver with 3 V DC power supply. So we need two power supply. One for Tx and another one for receiver.
We will design a simple linear power supply using LM317 adjustable regulator. The AC input from transformer can be fed into X1 and the 3V output available at X2. We will be using U1 (LM317) in TO-92 case. Resistor R1 and R2 are used to program the output voltage. We will be using 470 ohms and 330 ohms.
Resistor R3 and D2 are used for power ON indication.
Vout = 1.25(1+(470/330)
Adjustable voltage regulator LM317 in TO-92 case.
The input and output bulk capacitors for the regulator.
We will be using any RF generator with very limited output power so that our transmitter and receiver will work for say around 10 meter distance only.
We can use any RF oscillator capable of generating 76~108MHz. The advantage is easy for testing our transmitter with a standard FM radio receiver with frequency dial or digital frequency display.
We will go will a simple colpitts oscillator for our FM transmitter.
Now our transmitter is assembled and ready for testing.
We are using BF494 transistor with TO-92 case. This transistor is capable of operating beyond our band and having enough hFE gain.
The LC tank circuit can be easily calculated. But due to parastic capacitance and inductance along with printed circuit board material, layout, components placement changes our effective LC tank values and oscillator center frequency.
The LC tank circuit and Q-factor of the circuit can tuned to our requirement by trial and error method.
Connector X3 is provided for adding external antenna. We can add few centimeters of wire into the connector X3 for effective radiation.
After assembly just apply 3V power to the transmitter. Keep a standard FM receiver ON. Depends on the country and region the FM may change. We can also inject a test signal into the connector X2. If our transmitter is within the FM band frequency then we will receive the test sound signal on the standard FM radio.
Once we are able to hear the test sound on any nearby FM radio then we can confirm that our transmitter is OK and we can proceed to construct our own FM receiver.
We can change the inductor diameter, SWG size, number of turns or gap between the winding to adjust our oscillator frequency to fall in the standard FM band.
This simplifies our effort if we have access to very limited test equipments. Since the transmitter is MONO we can construct a simple MONO FM receiver instead of STEREO receiver. Even with a STEREO FM receiver there is an option to force the receiver to operate in MONO mode.
Since we need a simple MONO fm receiver we will look for low cost FM only MONO receiver to reduce our design complexity and reduce BOM cost and board size.
Depends on availablity, cost, DIP-16 package and less BOM cost we will settle down...Read more »