V.Vidyalal, V. Sivanand and K. Rajasree,

Floyds Inc., "KAILAS"

EDAKKAD P.O, CALICUT,

INDIA - 673005

INTRODUCTION

Eventhough there are a large number of FM transmitters in this country, the range of satisfactory reception is limited to around 40 Km due to the inherent disadvantages of VHF transmissions. Therefore reception at fringe areas are poor and noisy. At the receiving site, one possible obvious solution to overcome this problem is to use a good quality pre-amplifier with a dipole as the antenna. The following circuit was designed for such areas where the transmission is weak and noisy. This circuit was tested at approx. 180 Kms from a standard FM commercial transmitter. This preamplifier amplifies weak signal embedded in noise to sufficient levels and therefore the standard sensitivity of an FM radio (5 m V) is increased upto 1m V. Ideally suited for mobile radios where the receiving antenna is a 3m whip, this circuit can be used for portable radios as well. It has been designed with easily available components and due to the virtue of its design, substitutions are possible for the transistor used in the circuit.

CIRCUIT DESCRIPTION

The heart of the circuit is a high frequency NPN transistor (C2570 or C1730) configured to operate in the common base mode with a low collector current. This ensures a good signal to noise ratio. L1 and L2 along with C1 and C2 ensures HF operation for the entire FM band (88-108 MHz). The signal to be amplified is coupled to the emitter by C 1. After amplification by T1 the collector is tuned by L2 and C5 and therefore the entire FM band is amplified. For optimum low noise performance, use the transistor specified. However any low noise HF NPN transistor with an Ft of 250 MHz and hfe of 30min may be used as an equivalent. A gain of approx. 28dB @112 MHz was obtained with this circuit, with a noise figure of 1.3dB. The power supply feeding the circuit can be between 3.6V to 9V and the circuit draws only approx. 1.5 mA when powered by a 6V battery.

All the components used are commercially and easily available, except inductors L1 and L2. These are home made coils with an air core, and is to be wound as follows : - For L1 , use 20SWG enamelled copper wire and close wind 5 turns on a smooth rod of 5mm diameter. Slip the coil out of the rod. For L2 , use 24SWG enamelled copper wire and space wind 6 turns on a rod of 5mm diameter. The length of the coil should be approximately 8mm in diameter. Strictly adhere to the above mentioned winding details. Use of a glass epoxy PCB is recommended. All component leads should be cut as short as possible and the PCB after assembling should be cleaned well with petrol or toluene. After checking for correct assembly, connect the output to the FM receiver antenna terminal. Tune the receiver to a weak station. Apply 6V DC power to the circuit. Tune the trimmer capacitor C6 with a plastic alignment tool for resonance marked by an improvement in reception. Measure the voltages at the points as shown in the figure to check for faults or wrong connections if any.

This circuit was tested with radios or two-in-ones models such as Sony CFSW 365S, Sony FH10W, Pioneer Car Stereo model KEH-3250QR and the recent Philips 8 band portable transistor radio model D L 841. It performed exceptionally well and weak stations buried in noise could be easily pulled in.

SEMICONDUCTOR

T1 = C2570 or C1730. (The circuit works with any low noise HF NPN transistor with an Ft of 250MHz and hfe of 30min. Check pin configuration before soldering.

CAPACITORS

C1 = 1nF Ceramic, C2 = 27pF Ceramic, C3 = 22nF Ceramic, C4 = 100nF Ceramic, C5 = 22pF trimmer C6 = 1nF Ceramic, C7= 100 m F/12V electrolytic, C8 = 100nF Ceramic.

RESISTORS (All metal film, 1/4 W).

R1 = 1k ohms, R2 = 1M, R3= 100 ohms.

INDUCTORS :- Winding details of L1 and L2 as given in text.

1. WE the authors are aware that many improvements and refinements can be made, but the idea is to keep it simple as possible, and executed by anyone who can make a circuit board and has the patience !!!!.

Mains-powered equipment contain high AC voltages. These can be potentially lethal. If you do not know how to safely handle the inside of equipment containing high voltages, please do not attempt to open the equipment. Before carrying out any work inside equipment containing high Voltages, ensure that you disconnect the Unit from the Mains. (2) The information included herein is provided as-is, with no warranties expressed or implied. No responsibility on the part of the author is assumed for the technical accuracy of the information given herein or the use or mis-use of said information.

The equipment described in this article was designed, fabricated, and tested spending my own personal time and using my own personal resources.

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