The best and simple circuit diagram for a radio receiver to receive radio waves in Citizen’s Band


I am building a two way radio system using the citizen's band in India.

The transmitter is using a 555 timer IC clocked at a frequency between 26.957-27.383MHz to transmit AM waves.

Can you please suggest a possible circuit diagram (with components mentioned) for an AM receiver which will be able to receive requencies between 26.957-27.383MHz ?

The circuit diagram i am following so far is :

Shortwave AM receiver

Best Answer

The ZN414 was designed for AM broadcast frequencies, and is only good up to around 3MHz. However it works wells as an Intermediate Frequency amplifier and detector. You would need a Local Oscillator and mixer to convert the 27MHz signal down to below 1MHz, where the ZN414 is most sensitive. The most commonly used IF frequency is 455kHz, which allows you to use a standard CB radio crystal to select the channel.

The circuit below uses a BC547 in the LO, and a BF244B as the mixer (which also provides some amplification). The crystal must be cut for a frequency that is 455kHz below (or above) the channel frequency, eg. 26.67MHz for receiving 27.125MHz. When this local frequency is mixed in with the incoming signal through a nonlinear circuit (the BF244) sum and difference frequencies are produced. The difference frequency of 455KHz is selected by the IF transformer (which is tuned to this frequency).

27MHz Superhet AM Receiver

However this not the simplest design. For simplicity with high sensitivity the Superregenerative receiver cannot be beat. It uses a single transistor with positive feedback, which causes the gain to increase exponentially until it breaks into oscillation. Then a quenching circuit is used to continuously reset it at a supersonic frequency. The result is very high gain with minimal parts count. This circuit is commonly used in cheap 'walky-talkies', r/c toys, and 433MHz remote controls.

Here is an example. T1 provides all the RF amplification and AM detection. T2 and T3 just amplify the audio signal.

27MHz Superregenerative RX