Adjustable AM transmitter

Hi Everyone,

I am trying to come up with an AM audio transmitter design. I've already got a few ideas but I'd like to be able to adjust the frequency anywhere from 100kHz to 2Mhz.

I thought about getting a 2MHz crystal oscillator and playing with the count to decrease the freq at discrete intervals but then I'd be limiting myself to only those frequencies. I would like to be able to "dial in" anywhere between the two limits.

Just as side notes, I plan on using a 9V battery for power and using a Pot to attenuate the power to appropriate levels.

Thanks for any help you guys can provide!

At those frequencies a simple LC oscillator is probably best and would let you tune to any frequency you wanted. Buffer the output then amplitude modulate the output stage.

If you want to do it digitally there are two options, directly synthesize the frequency using DDS or use a PLL with a programmable divider. You still need to buffer and apply modulation later in the circuit though. Applying modulation directly to the oscillator will work but is likely to give unsatisfactory results.

Brian.

As the wanted frequency range is .1-2 MHz, i.e. 1 :20, you would need a LC oscillator with either the C or L varied by 1:400 ! For instance if your minimum circuit capacity was 10 pF then you would need the maximum capacity of 4000 pF. This sort of thing can be done using the miller effect, but it is complicated as you have to keep the amplitude relative constant so you do not get any harmonics produced because the second harmonic of 100KHz is 200 KHz which is within the pass band of your driver circuits, unless you also have a tunable filter, that tunes as the oscillator does. One technique could be to use a 3.1 -5 MHz oscillator and mix it with a 3MHz oscillator and select the low frequency component with a LPF. 3.1 -5 is 1 : 1.66 so the capacity change would be ~ 1:3, 10pF circuit capacity, so a 30 - 150pF variable should work.
Frank

yeah namely an FM transmitter rather than an AM transmitter hahaha :)

Dave

If I don't care about the modulation, would it be easier to implement this as an FM transmitter?

You say you don't care about the modulation, have you got a medium wave FM radio then?
Frank

I have a spectrum analyzer with an AM and FM demod builtin.