Why can't I mix two signals of the same frequency?

[namtey]

Hi

I am trying to mix two seperate 250MHz signals to get a 500MHz output using a Mini-circuits ADE-1H+, but the output signal has severe amplitude variations. It works fine when the two input signals are of different frequencies, the amplitude fluctuations are only a problem when both signals are the same fequency.

Could somebody explain why this is the case, and whether it is possible to stop this?

 

[audioguru]

The signals add and make double the amplitude of one. Then the phase changes and the signals subtract and cancel each other. So the output fluctuates in level.

 

[MikeMl]

You are "mixing" them, but one is slightly a different frequency than the other, and what you are seeing is the "beat". The mixer generates the product Asin(ω1t) X Bsin(ω2t). Look up the trig identity to see what the product looks like. Now make ω1 and ω2 almost equal.

 

[BrownOut]

Stop it by using a single signal and a frequency doubler. You don't actually need two signals to get the double frequency.

 

[namtey]

That makes sense! I just realised that in the application the signals will be locked together so there would be no phase offset and hence no fluctuation in amplitude.

So for my test setup I will lock the 2 sig gens to a 10Mhz ref.

Thanks guys.

 

[audioguru]

You need a frequency doubler circuit, not a mixer.

 

[mvs sarma]

Hi

I am trying to mix two seperate 250MHz signals to get a 500MHz output using a Mini-circuits ADE-1H+, but the output signal has severe amplitude variations. It works fine when the two input signals are of different frequencies, the amplitude fluctuations are only a problem when both signals are the same fequency.

Could somebody explain why this is the case, and whether it is possible to stop this?

No two things are identical. you can perhaps feed the same signal in both ports.
as already clarified by AudioGURU, you get twice the frequency.

 

[BrownOut]

A frequency doubler requrires only a single port.

 

[MikeMl]

No two things are identical. you can perhaps feed the same signal in both ports.
as already clarified by AudioGURU, you get twice the frequency.

..., riding on top of a voltage level which is proportional to the phase difference between the voltages applied to the input signal ports of the mixer.