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


closed as too broad by Adam Lawrence, Leon Heller, placeholder, Ricardo, Daniel Grillo Oct 31 '14 at 17:16

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Too broad - you haven't indicated (1) if you have any electronics knowledge and (2) what you have tried so far, if anything. "Design it for me" questions aren't really the focus of this site. \$\endgroup\$ – Adam Lawrence Oct 31 '14 at 16:15
  • \$\begingroup\$ Surely you have at least a basic circuit diagram already if you've done any work so far on this. If so please post it to give us something to work with. If not please do some work on your own and come back if you have problems. \$\endgroup\$ – Null Oct 31 '14 at 16:15
  • \$\begingroup\$ actually i was thinking to follow this: d3i5bpxkxvwmz.cloudfront.net/resized/images/remote/… \$\endgroup\$ – Aniruddha Sarkar Oct 31 '14 at 16:19
  • \$\begingroup\$ sorry I cannot add pics.. accessing from a mobile... \$\endgroup\$ – Aniruddha Sarkar Oct 31 '14 at 16:19
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    \$\begingroup\$ The 555 won't be stable enough for your transmitter either, even if it works at all at such a high frequency. \$\endgroup\$ – Brian Drummond Oct 31 '14 at 17:11

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


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