AM mixer

[Powzoom]

I'm designing a simple A.M. radio for class. I have an oscillator and audio input. How do I mix these together to get Amplitude Modulation? I attached my best guess design but it looks like only the top half of an AM wave. Please comment.

Also, does the oscillator and signal have to be at the same voltage amplitude?

With the diode design, does the combination of the osc. and signal have to be greater than 0.7V because of the diode? B/C the audio signal I have outputs at around 300mV

Thanks

 

[Roff]

Are you making a transmitter? A simple AM radio receiver will not have audio and an RF carrier that need to be mixed.

 

[RadioRon]

I'm designing a simple A.M. radio for class. I have an oscillator and audio input. How do I mix these together to get Amplitude Modulation? I attached my best guess design but it looks like only the top half of an AM wave. Please comment.

Also, does the oscillator and signal have to be at the same voltage amplitude?

With the diode design, does the combination of the osc. and signal have to be greater than 0.7V because of the diode? B/C the audio signal I have outputs at around 300mV

Thanks

You are close, but don't forget that with a very simple mixer setup like this, you can expect to see the sum and difference frequencies at the output, plus the original input frequencies. The plot appears to contain all of these. I think it will look a lot more like what you expect if you were to put the output through a high pass filter to exclude the 10KHz signal from getting to the scope.

With a diode mixer, you usually use one of the two signals to drive the diode on and off and the other one is adjusted to give you the amount of output that you want. In your case, you have declared your sources to be 0.3 Volts, and I suspect this is Vrms. In this case your input is varying around 0 volts to a peak value of 0.42 volts. You should probably increase one of the two signals (doesn't matter which one) to a higher level so that you are pushing 0.7 Vpeak into the diode. One other problem is that the right hand side of the diode has no DC load, and it seems to me that it needs one to bleed off the DC component that will result from its rectifying the input signal. Maybe you need a 10K ohm to ground, but you can experiment with the value as I'm not sure.

 

[audioguru]

Because the diode does not have a DC load, then its first few milliseconds will look good on a sim program but a second or two later it won't mix anymore.

 

[mvs sarma]

I'm designing a simple A.M. radio for class. I have an oscillator and audio input. How do I mix these together to get Amplitude Modulation? I attached my best guess design but it looks like only the top half of an AM wave. Please comment.

Also, does the oscillator and signal have to be at the same voltage amplitude?

With the diode design, does the combination of the osc. and signal have to be greater than 0.7V because of the diode? B/C the audio signal I have outputs at around 300mV

Thanks

if possible define it as germanium diode. you may in reality use OA79, OA81, OA85 if these animals still available.

otherwise schottky diodes could also be tried.

 

[Nigel Goodwin]

You appear to be confused as to what you want? - a mixer doesn't give you modulation, you need a modulator instead.

 

[stevez]

Amateur radio publications might be one of many sources of information on the subject.

 

[mvs sarma]

Amateur radio publications might be one of many sources of information on the subject.

In fact any non linear device should cause modulation. as pointed out by audio guru, the circuit needs termination and the levels of carrier and audio signal are to be optimized.300mV may not be proper if silicon diodes are involved. Carrier levels of +10dbm are not uncommon for passive modulators.
audio can be adjusted to get optimum.