RadioBanter - View Single Post

March 14th 05, 11:05 PM

wrote:

thanks! also, i was just reading about mixers... and getting confused.
i've run some matlab programs to figure out how you can take something
at frequency x and frequency y, and end up with an output of frequency
(x-y). one transmitter's mixer i saw took a 160mhz and 14mhz input to
get a 146mhz output. adding or multiplying them together certainly does
not yeild 146mhz. so how does that work?

That is a fair question, and one which is frequently asked.

Look up the trig identity for (sin a)(sin b). Let a and b each be in
the form wt [w stands for greek lower case omega], so a = 2(pi)f1(t) and
b = 2(pi)f2(t) where f1 and f2 are your two frequencies in Hz. Do the
substitution & algebra on the identity, and then I think you'll see
where the signals at the sum and difference of frequencies f1 and f2
come from. :-)

The name "mixer" can be a little misleading.....it doesn't necessarily
just sum the two inputs in a linear fashion (by just adding them, like a
kitchen mixer combines the ingredients for a cake) but there is also an
element of non-linearity in the electrical network.

Depending on the nature of the non-linearity, mixing can also generate
other higher-order products, like signals at 2f1+f2, 2f1-f2, etc.
Whether this is good or bad depends on your particular application. For
example, do some reading on "intermod".