AM Radio Broadcast Receiver

We have a project to make and we were asked to make an AM radio broadcast receiver that is tuned only at a single frequency (702 kHz) but I don't know how to make one. The stores sell kit of a simple AM pocket radio receiver but all the radio stations are available there. How can you change the frequency in an AM radio receiver? A variable capacitor? If that is the case, what value of capacitance do I need in able to tune our AM radio receiver to 702 kHz? I'm from the Philippines and would there be any difference in the calculation of capacitance if you were in other countries? Thanks.

The required capacitance depends upon the design of the circuit and what the variable capacitance is controlling. In a Superhet circuit it controls the mixer oscillator frequency.

But if you want a single frequency radio, then just build a low-noise amplifier of sufficient gain (probably about 100,000 or 100dB), with a 702kHz bandwidth filter with the desired bandwidth and rolloff, and detect the amplifier output signal with a diode envelope detector.

Maybe something like this?

Crystal Radio Circuits

But it tunes more than one frequency.

I do not know much about your radio kit. Many radios have a variable oscillator and an intermittent stage at 455khz. Most of the filtering & gain is done at 455khz.
Frequency desired. 702khz.
Oscillator tuned to 1157khz.
The mixer will output 702, 1157, 455 and 1859khz.
The IF stage will filter and detect only the 455khz.
This way, most of the radio is working only at 455 and only the oscillator needs to be changed for each channel.

If you removed the tuning capacitor and made the oscillator work with a crystal or ceramic resonator then it would work only at one frequency. I do not think you can find a 1.157mhz crystal or a 247khz ceramic resonator. Either will add or subtract from 702khz and make 455khz.

You could get the radio kit, tune to 702 and remove the knob.

I found 220khz, 262khz, 1mhz and 1,2288mhz crystals.

can you give a circuit for 180khz receiver?

parthshah08 said:

can you give a circuit for 180khz receiver?

Click to expand...

You can make a simple Xtal AM radio for any frequency. Just make sure that the coil and capacitor values agree with this equation:

f=1/(2*PI*SQRT(L*C)) or f=1/(2*PI*(L*C)^0.5)

where f is the frequency in Hz, L is Henries, and C is Farads.