Simple Medium Wave Modulator

If you insist on using a valve radio and listening to medium-wave stations, you have a problem: the existing broadcasters have only a limited number of records. Here there’s only one remedy, which is to build your own medium-wave transmitter. After that, you can play your own CDs via the radio.

The transmitter frequency is stabilised using a 976-kHz ceramic resonator taken from a TV remote control unit. Fine tuning is provided by the trimmer capacitor. If there’s another station in the background, which will probably be weak, you can tune it to a heterodyne null, such as 981 kHz. As an operator of a medium-wave transmitter, that’s your obligation with respect to the frequency allocations. And that’s despite the fact that the range of the transmitter is quite modest. The small ferrite coil in the transmitter couples directly into the ferrite rod antenna in the radio.

Medium-Wave Modulator Circuit Diagram

The modulator is designed as an emitter follower that modulates the supply voltage of the output amplifier. As the medium-wave band is still mono, the two input channels are merged. The potentiometer can be adjusted to obtain the least distortion and the best sound. The RF amplifier stage has intentionally been kept modest to prevent any undesired radiation. The quality of the output signal can also be checked using an oscilloscope. Clean amplitude modulation should be clearly visible. 

The medium-wave modulator can simply be placed on top of the radio. A signal from a CD player or other source can be fed in via a cable. Now you have a new, strong station on the radio in the medium-wave band, which is distinguished by good sound quality and the fact that it always plays what you want to hear.

Author: Burkhard Kainka - Copyright: Elektor

Short Wave Superregenerative Receiver

Short-Wave Super regenerative Receiver Circuit diagram. Super regenerative receivers are characterized by their high sensitivity. The purpose of this experiment is to deter-mine whether they are also suitable for short-wave radio. Super regenerative receivers are relatively easy to build. You start by building a RF oscillator for the desired frequency. The only difference between a super regenerative receiver and an oscillator is in the base schema. Instead of using a voltage divider, here we use a single, relatively high-resistance base resistor (100 kΩ to 1MΩ).

Super regenerative oscillation occurs when the amplitude of the oscillation is sufficient to cause a strong negative charge to be applied repeatedly to the base. If the regeneration frequency is audible, adjust the values of the resistors and capacitors until it lies somewhere above 20 kHz. The optimum setting is when you hear a strong hissing sound. The subsequent audio amplifier should have a low upper cutoff frequency to strongly attenuate the regeneration signal at its output while allowing signals in the audio band to pass through. This experimental schema uses two transistors. A Walkman headphone with two 32-Ω earphones forms a suitable output device. 

Short-Wave Super regenerative Receiver Circuit diagram :

Short-Wave Super regenerative Receiver Circuit Diagram

The component values shown in the schematic diagram have proven to be suitable for the 10–20 MHz region. The coil consists of 27 turns wound on an AA battery serving as a winding form. The schema produces a strong hissing sound, which diminishes when a station is received. The radio is so sensitive that it does not require any antenna to be connected. The tuned schema by itself is enough to receive a large number of European stations. The schema is usable with a supply voltage of 3 V or more, although the audio volume is greater at 9 V. 

One of the major advantages of a super regenerative receiver is that weak and strong stations generate the same audio level, with the only difference being in the signal to noise ratio. That makes a volume control entirely unnecessary. However, there is also a specific drawback in the short-wave bands: interference occurs fairly often if there is an adjacent station separated from the desired station by some-thing close to the regeneration frequency. The sound quality is often worse than with a simple regenerative receiver. However, this is offset by the absence of the need for manual feedback adjustment, which can be difficult. 

Author :Burkhard Kainka  - Copyright : Elektor