Design Your Circuit. Part VII – Single IC FM Transmitter


The Frequency Modulation (FM) system conveys information over a carrier wave by varying its frequency. The broadcast band of FM falls within the VHF part of the Radio spectrum. Usually 87.5 – 108.0 MHz is used to transmit and receive the FM signals. See the important aspects of FM transmission and the Design of an FM transmitter using IC UPA 1651.


It is necessary to consider a number of factors during the design of an FM transmitter. The performance of an FM transmitter depends on two important aspects.

1. Tuning of the FM transmitter to the desired frequency. Even a slight change in the Coil specification or slight change in the Variable capacitor value can shift the harmonic frequency instead of the 88-108 MHz FM band.

2. Length of the Antenna used to transmit the frequency.

These are the important parameters for the optimum performance of an FM transmitter.
1. Transmitter frequency, output power and range of transmission.

2. Antenna length.

3. Coil diameter, length, number of turns and gauge of the wire used for coil winding.

Tank Circuit

The most important part of an FM transmitter is the Tank Circuit. The FM transmitter needs an oscillator to generate the radio Frequency (RF) carrier waves. The name ‘Tank’ circuit comes from the ability of the LC circuit to store energy for oscillations. The purely reactive elements, the C (Capacitor) and the L (Inductor Coil) simply stores energy to be returned to the system.

Inductor

Inductor for FM transmitter can be made from enameled copper wire with gauge from 22 – 26 SWG (Standard Wire Gauge). Even the Hook up wire can be experimented. The length, inner diameter, number of turns etc are the important parameters to be considered while making the inductor. Then only the inductor resonates in the 88-108 band FM frequency. Coil can be wound around a Screw driver (with the same diameter as that of the coil). Refill of Dot pen, Gel pen etc can also be used to wind the coil.

After assembling the circuit, it is necessary to adjust the spacing between the coils by pulling the windings gently. If the coil is air core, placing a sponge inside the coil will make it intact. The inductance of the coil can be calculated using the formula.

L = n2 r2 / 9r + 10 x

Where r is the inner radius of the coil, x is the length of the coil and n, number of turns. The resulting value is in Micro Henry.

Trimmer capacitor

Trimmer is the variable capacitor capable of changing its capacitance. So by adjusting the Slug (moving shaft) the tank circuit can be tuned exactly.

A small button type variable capacitor with a value of 22 pF can be used to adjust the resonant frequency of the tank circuit. The variable capacitor and the inductor coil form the Tank circuit (LC circuit) that resonates in the 88-108 MHz. In the tank circuit, the capacitor stores electrical energy between its plates while the inductor stores magnetic energy induced by the windings of the coil. The resonant frequency can be calculated using the formula

f = 1 / 2 X √LC = Hz

Where f is the frequency in hertz, x is the coil length, C is the capacitance of trimmer in Farads, and L is the inductance of coil in Henry.

Antenna

Unlike AM transmitters, Antenna length is an important factor for Good FM transmission. A plastic wire or Telescopic aerial can be used as antenna. The length of the antenna is very important to transmit the signals in the suitable range. As a rule, the length of the antenna should be ¼ of the FM wave length. To determine the length of antenna, use the following equation.

By multiplying the Wave frequency and wave length will give the speed of light.

Speed of Light = Frequency of Oscillation x Wavelength = in Kms/ Sec

Wave length = Speed of light / Frequency = in Meters

Antenna length = 0.25 x Wavelength = in Meters

By using this formula it is easy to select the antenna length. Generally 30-50 cm long antenna is sufficient for low power FM transmitters.
FM transmitter using IC UPA 1651.


UPA 1651 is a small Wide band UHF Silicon MMIC Amplifier IC with 4 radiating pins. It’s important features are :
1. Broad frequency response of 1200 MHz typical at 3db down
2. Operates at 5V. Maximum Supply voltage is 6V DC. Above 6 volts, IC will be destroyed.
3. Power dissipation 250mW
4. High power gain 19 db at 500 MHz
5. Covers HF band through UHF

Pin connection of UPC 1651

IC UPC 1651 has a button shaped body with 4 radiating legs. 3 legs are almost same sized while the 4th one is slightly longer. It is the output pin. Place the IC with the longest leg facing downwards and identify the pins as shown.

Pin 1 – Vcc
Pin 2 – Input – Short pin
Pin 3 – Ground
Pin 4 – Output – Longest pin

It is easy to construct the FM transmitter using the IC UPA 1651.

1. Assemble the circuit on a small piece of Perf board

2. Components should be soldered as close as possible with minimum lead length. This is essential for preventing unnecessary oscillations.

3. Place the IC on the Perf board and mark its leg positions. Then using the trimmed leads of resistors, connect the legs with the circuit. Do not use wires to connect the components.

4. Coil is not Air core but it has a plastic core. Coil consists of 5 turns of 26 SWG enameled copper wire. Wind the coil on an empty Refill of Gel pen (it has same diameter) and the same can be used as the plastic core.

5. Solder the MIC directly to the PCB. Polarity of MIC should be confirmed before soldering. Enclosing the MIC in a plastic tube can increase its sensitivity.

6. A 30 cm plastic wire or Telescopic antenna can be used are Areal.

7. After assembling, power it with two 3 volts Lithium cells or 6 V mini battery.

8. Use a good quality FM Radio as receiver. Pocket FM radios, Mobile phone radios etc have more reception than FM radio in the Tape recorder. Keep the circuit close to the radio and tune it from 88 MHz to 108 MHz band. At a particular point, a whistling sound will be heard. Gently tap on the MIC. The sound will be heard. Adjust, trimmer, coil etc to get clear sound. Orient the areal to different directions to find out the correct position.

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