Circuit Project Description
The circuit was designed to create a portable and sensitive regenerative receiver that will be suitable for shortwave band listening.
TerminologyShortwave (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 KHzReceiver – an electronic device that converts incoming signals, in the form of microwave, electrical or radio, to a form that is perceivable by humans such as pictures, sound, light, navigational positions, digital data, measurement, and othersColpitts Oscillator – an electronic oscillator circuit comprising of two capacitors and an inductor, forming the LC circuit Circuit Explanation
From the receiving side of the circuit, the antenna was made of random wire coupled with the inductor L1 having a single turn while inductor L2 has 4 turns wound around a 35 mm film container. Both hook-up wires are of american wire gauge of 22 with a 5 mm distance from each other. This tuning circuit has a variable or vernier capacitor C1 placed in parallel for the purpose providing finer adjustment after setting the frequency range on the inductors. The capacitor C2 should be of Negative-Positive-Zero (NPO) type so it does not vary with the temperature and will provide timing and tuning applications because these types of capacitors have dielectric with enough temperature, low K, high Q, and stable electrical properties under fluctuating temperature, frequency, voltage and time.
Transformer T1 is an audio interstage transformer used to couple the signal and isolate the DC with minimal loss. It has turns ratio of 1:1.5. It functions as complex impedance to AC signal but short circuit to DC. For adjusting the volume of the circuit, a 20K ohm potentiometer R3 is used while the reaction is controlled by the 50K ohm potentiometer R9. The transistors utilized in this design are of 2N3904 because of its low collector saturation voltage, collector output capacitance, and high speed switching. The whole receiver circuit can be tuned from 22 meters to 11 meters of distance and having a capacitive grounding by connecting a wire from the receiver to any metal thing attached on the floor.
The process of regeneration is performed by transistor Q1 that operates as common collector Colpitts oscillator. In this type of oscillator, the voltage divider made by the two capacitors produces the feedback necessary for oscillation. Although it does not oscillate in this circuit, it functions as a regenerative amplifier with a control on the reaction by resistor R9. The transistor’s input capacitance has a great effect in the operation of regenerative amplification. There is more than enough volume that will be produced by transistors Q2 and Q3 by forming high gain audio amplification. In this stage is when the resistor R3 comes into the picture as the volume control. The oscillation mode is utilized when reproducing a carrier wave (CW) or single side band (SSB).
To achieve the maximum selectivity and sensitivity, the oscillation stage should be near the threshold oscillation. In the output side of the circuit, a high impedance crystal earphone is utilized because its sensitivity has the ability to receive the weak radio signals.
There are several types of receivers that are used in various industries such as measuring receivers which are used in broadcast station to measure its signal strength; as communications receivers which is a part of a radio communication network; as specialized receiving scanners that can scan two or more discrete frequencies automatically; as satellite television scanners used in communication satellites for receiving television channels; as audio receivers used in home theater systems and home stereos; as telemetry receivers for reporting and remote measurement; and as crystal radio receivers powered by radio waves.
Regenerative receivers have placed a vital role in the field of radio communication like in radio repair where the broadcast radio signals are monitored, as interference source identifier, in adjustment of oscillators, and in amateur radios.