Sunday, May 9, 2010

Simple AM Transmitter and Receiver

Update: I actually made a math error as I did not know
what I was doing at the time.

I actually inadvertently made a 7 MHz circuit instead of a
50 MHz circuit so I have changed the title but kept the component
values and fixed the equations so that they are now correct.

Sorry if I threw anyone off. I am still learning myself.

My fist adventure in radio! I started off by building a Hartley oscillator with one transistor and then amplitude modulated a signal with a second transistor. I then built an AM receiver schematic I found and calculated the tank circuit for 7 MHz.

Understand that this transmitter is extremely simple and haphazardly thrown together, it does not transmit any further than across the room but it is useful for understanding the basic concept of a transmitter.

AM Transmitter Schematic
(Q1 makes the Hartley oscillator, Q2 amplitude modula
tes the signal by attenuating the signal rather than multiplying it with the carrier (this makes it a very weak transmitter!). It will only broadcast across a room. To increase power you would need to add some amplification and use a better modulation method. You may find this helpful for calculating the resistance values needed)

AM Receiver Schematic

(I recommend replacing the 120k
regenerative feedback resistor with a variable resistor. I used 2N3904 transistors in my build)

The Tank Circuit

The operating frequency of the Hartley oscillator and the frequency tuned in by the receiver is determined by the inductor (L) and the capacitance (C) values in the tank circuit.

Explanation of a "tank circuit".

In my circuit the variable
capacitor's max capacitance is at 290 pF and the coils are roughly 1.746 uH (Micro Henrys). Turning the variable capacitor lowers it's capacitance and thus increases the resonant frequency.

I made the air-core inductor out of a .25 inch diameter soda straw.

Diameter: .25 inches
Length: .75 inches

Turns: 31

This equation can be used for calculating the dimensions of an air-core inductor,

  • L is inductance in uH
  • d is coil diameter in inches
  • l is coil length in inches
  • n is number of turns.

I found this air-core inductor calculator to be a very handy tool for designing coils.

Calculating Inductance Needed

To calculate the inductance L (in mico Henrys) needed you will need to know 2 things.
What frequency you want to operate in and your maximum variable capacitance
(290 pF according to my multimeter).

The formula for resonant frequency is,

Which can be rearranged to find the proper inductance value needed,

The easy way to calculate this is using realistic component values is,

So using my values as an example we take the minimum
frequency we want to tune and the maximum capacitance
my variable capacitor can reach,

Which comes out to,

Because of the previous errors I made (explained above) the actual
inductance I used was 1.746 uH which is still in the ball park for 7 MHz.