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Shortwave Radio Frequency Preamplifier
The circuit was designed to increase or amplify the short wave reception of radio frequency having a range of 5 MHz to 20 MHz.
- Shortwave (SW) – a term used to describe the radiant energy in the visible, near ultraviolet, and near infrared wavelengths and it operates in the frequency range of 3000 KHz to 30,000 KHz
- Radio Frequency (RF) – operates within the range of 3 Hz to 300 GHz
- Preamplifier (preamp) – a device normally used together with the sound equipment to be of assistance in improving the overall quality of sound by detecting and strengthening weak signals for subsequent and more powerful amplification stages
- Field Effect Transistor (FET) – a type of transistor that controls the current and the shape of an electric field where the flow of current or the conductivity of material is only through a single type of semiconductor material
- 2N3819 – an all purpose JFET which offers good performance at mid to high frequencies and a low cost type; has low noise and leakage, very high system sensitivity, high speed switching capability, wideband high gain at 100 MHz, high low-level signal amplification and high quality of amplification
In order to prevent overloading on the receiver, the circuit utilizes a resistive attenuator by reducing the power or amplitude of the incoming signal without deforming the waveform. From the receiving side of the circuit, the transformer T1 has its primary winding connected to the aerial antenna, and is wound 2 turns around a 1 inch diameter using a standard wire gauge of 22. The secondary winding has the same type of wire but wound 4 turns in particular spacing so as to occupy near half of the circumference of the coil so that the inductance would fall in the value around 20 uH.
The performance of capacitors in series is almost the same performance with resistors in parallel wherein the lowest and highest capacitance values are less the smallest and largest capacitor in series, respectively. Since C2 has a value of 220 pF and variable capacitor has a value of 5 pF, the effective capacitance of both capacitors in series is 143 pF and 4.8 pF. These capacitors should be tuned at about 3 pF to 200 pF to reach the 5 MHz to 20 MHz frequency. Higher values of capacitor may also be used in series with the capacitor C2.
Maintaining the specified values of the tuned circuit is necessary to avoid amplifying the gain of the FET outside the tuned frequency range. The output response of 2N3819 FET is maintained constant by R1 as is connects in series to the base while the FET functions at common source. At higher frequencies, R1 will increase the internal gate source. Buffering in the emitter follower mode by BC108C is required to maintain constant output impedance on the FET since it produces high output impedance.
For flattening the frequency response of the circuit, a drain circuit is constructed in series with resistor R3. It consists of a choke rated at 2.5 mH that prevents electric signals to be transmitted across the unwanted channels of the circuit by producing high reactance for a particular frequency. A higher value of choke coil may also be used. From a 9 V battery source, the current drain would be around 3 mA.
During the amplification of weak radio signals, a problem arises as the noise is also amplified which may originate from static of outside interference. A casing made of aluminum and metal would be preferred. These problems may be eliminated if the circuit will be constructed as tunable RF preamplifier which is useful in eliminating intermodulation distortion or cross modulation drawbacks. However, clear signals can still be achieve favorably if minimal RF signals were amplified.
Some major uses of shortwave radio band include domestic broadcasting, international broadcasting, utility stations, clandestine stations, numbers stations, amateur radio operators, time signal stations, over the horizon radar, and specialty political, religious, and conspiracy theory radio networks. Special features of the circuit include low supply voltage operation making it particularly suited to small hand-held radios.
The RF preamplifiers were utilized in the reception path of a nuclear magnetic resonance imaging and spectroscopy system and as an impedance matching device to cables under low temperature in NMR tests, to enhance the signal to noise ratio. It may also be used for conducted emission and radiated testing. This circuit is suitable in application involving wide band application of low level signals.