From: Don Pearce
Subject: Re: How does a mixer work?
Date: Fri, 18 Oct 2002 08:33:33 +0430
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On 17 Oct 2002 20:53:57 -0700, firstname.lastname@example.org (Michael Black) wrote:
>Don Pearce wrote in message news:...
>> On Wed, 16 Oct 2002 21:22:52 -0700, "Alfred Lorona"
>> >If you apply a 10 hertz and a 15 hertz signal to a resistor all you can
>> >expect as output is a 10 hertz signal and a 15 hertz signal. In a mixer, the
>> >idea is to obtain the difference between the two frequencies which is 5
>> >hertz and the sum of the two frequencies which is 25 hertz at the output.
>> >The output will also contain the two original frequencies of 10 and 15
>> Only as a spurious result of less than perfect balance. This is not
>> intrinsic to the process. I am talking about "real" double balanced
>> mixers, of course. For a single diode acting as a non-linear element,
>> you are right.
>No, you're basing the explanation on a balanced mixer, when in reality
>a mixer is merely a non-linear device that gives the sum and difference
>of the input signals. A double balanced mixer is one type of mixer,
>but it doesn't hold that all mixers are that kind.
>There are good reasons for using a double balanced mixer, but there are
>plenty of applications where a simple diode or transistor (or tube)
>works fine. The concept of mixing dates at least to the regen receiver,
>circa 1915, and I don't believe people were talking about a mixer stage
>at that point, merely the concept of mixing. And plenty of radios have
>been built over the years with mixers that aren't double balanced, right
>up to today.
>The topic is how does a mixer work, not what's the best mixer. To base
>an answer on a specific type of mixer can't supply a full answer, because
>then it gets bogged down in the details of that mixer.
>You have to explain the general, before you can explain the specific.
>> [stuff deleted]
>> >No, mixers do not work with DC input signals.
>> Oh yes they do. DC on the LO port works just fine. If the mixer is
>> transformer coupled, then obviously you can't use DC on the RF and IF
>Again, this is only true of certain mixer schemes. Apply a DC voltage
>to some types, and you'll be messing with the biasing of the circuit.
>Even the concept of "LO port" is specific to a type of mixer, and it
>sure seems specifically like a double balanced mixer using 4 diodes.
The reason I did this was because conceptually and mathematically the
double balanced mixer is by far the easiest to understand. It is, if
you like, the "perfect" mixer. Sure there are plenty of lesser mixer
topologies that work, but if you want to understand mixing, far better
not to have to deal with all the side effects just at the beginning.
If you wanted to explain how two and two makes four, would you use an
example that introduced three, five, eighteen? Keep it simple.