Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: How does a mixer work?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 17 Oct 2002 08:07:33 +0100
NNTP-Posting-Date: Thu, 17 Oct 2002 08:07:42 BST
"Asa Cannell" wrote in message
> How does a mixer work? I don't understand why it needs a non-linear
> element. I think examples of why a linear element (resistor) wont work
> would help illustrate why a non-linear element (diode) will. Also,
> what does 'mutiply' mean in terms of mixing? What if two DC signals
> are applied to a mixer, like 2V and 4V? Will the output be 8V?
The basic reason is based on a standard trigonometric result.
Sin(A)*Sin(B)= 0.5*(Cos(A-B) + cos(A+B))
This means that if you multiple two frequncies, A and B, you get other
frequncies at (A-B) and (A+B)
If you have a non-linear device, an output from it can be expressed as
Vo = aVi + bVi^2 + cVi^3 etc...
i.e a power series in Vi, the input voltage.
If we take say, the squared term, and *sum* in two signals for Vi, i.e.
Vi=V1.Sin(A) + V2.Sin(B))^:
Second term = b(V1.Sin(A) + V2.Sin(B))^2
If you expand this function there will be a term of
Which is the bit we are after, sum and difference frequencies are
Hence, a non-linear amplifier, has bits in it that multiply.
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