# RF mixers

#### powzoom

Joined Jan 18, 2009
41
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

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#### bertus

Joined Apr 5, 2008
20,189

#### powzoom

Joined Jan 18, 2009
41
I've looked through those links and found LOTS of info on the theory of AM, mixing, vectors, and calculus but I did NOT see any info on how to actually implement these theories with circuits. I'm very lost when it comes to modulation.

#### Wendy

Joined Mar 24, 2008
21,914
The way you're doing is not pretty, but it works. Try adding a LC circuit to get rid of the low frequenies.

One way to modulate AM is to vary the gain of the RF Amp with the audio. Think in terms of components being variable resistors, and putting this variable part as part of a RF attenuator.

#### powzoom

Joined Jan 18, 2009
41
I tried adding an LC circuit that resonates at the carrier. f=1/(2pi(LC)^.5) = 750kHz. But all I get at the output is basically the carrier, which I guess makes sense because I eliminated everything else. But it doesn't look like AM.

Which frequencies should I be blocking?

What better methods are there for doing this?

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#### flat5

Joined Nov 13, 2008
403
To Amplitude Modulate a carrier you change the output power of the radio frequency signal at the audio rate.
Does that make sense to you?

#### KL7AJ

Joined Nov 4, 2008
2,224
An ideal mixer is actually a MULTIPLIER...the output amplitude.will be the PRODUCT of the two different signals at any time.

Depending on the actual application, the two mixer signals may be pretty close to the same level (as in high level A.M. transmitters) or vastly different. (The downconverter of a receiver, for instance, might have a local oscillator an order of magnitude greater than the incoming R.F. signal...or more.

The best primer on this is the ARRL Handbook

eric