Dave Platt wrote:
In article .com,
wrote:

yes, that is what i meant. i didn't even know amplitude shift keying
existed.

am i correct about afsk being audio tones transmitted over FM, while
fsk is just alternating between two non-modulating frequencies?

That's pretty close, but not exactly on target.

AFSK can be used on top of either AM or FM. In amateur systems it's
most commonly used with FM.

FSK shifts the carrier frequency... a process which counts as a
modulation. The amount of frequency shift (in Hz) and the rate at
which you shift it (also in Hz) vary a lot from one usage to another.
There are both narrow-band and wide-band FSK modulations in use.

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?

thanks for the help!
jason

In article . com,
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?

You need a nonlinear function in order to get mixing behavior, and
create the sum and difference frequencies.

If you just add (e.g.) sin 160x and sin 14x, you end up with a simple
sum of sins, and no other frequencies show up - ordinary superposition.

Run the resulting sum through a nonlinear function (e.g. square it, or
take the square root, or clip it whenever its absolute value exceeds
1.0, or something like that), and take a look at the frequency content
of what you get as a result.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
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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".