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Above 108MHz with FM radio (or other)?

A slight nit to pick here, John; there's "mixing" in the sense that you use it here, and then there's modulation based on multiplication, which may be something else entirely. A "mixer", meaning the specific sort of circuit that this term is generally applied to, does preserve the original frequencies. However, a "balanced mixer" does not, and the difference is that the latter actually DOES perform an operation that is solely a multiplication. In mathematical terms:

sin(A) * sin(B) = 1/2[cos(A-B) - cos(A+B)]

which clearly doesn't have the original frequencies in the output. So pure multiplication results in DSBSC, which is then often converted to SSB by filtering out one of the sidebands. To get the original carrier back, a constant term has to be introduced, i.e.,

sin(A) * [K + sin(B)]

Here, the K term corresponds to the DC offset which exists when , for instance, the modulating signal (here, sin(B)) is applied to the carrier (sin (A)) via a modulation transformer (classically, in the "plate" circuit of a class C output stage), as is the case very commonly in commercial AM installations. The result is conventional "AM", with a "full" carrier.

Just to clarify that for those who may be trying to follow the terminology, since this IS sci.electronics.basics after all. I know YOU know this stuff already...

Bob M.

Reply to
Bob Myers
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whkatever.. i'm so happy for you.. it still doesn't alther the fact that the majoritiy of his remarks I found on Google were simply disparaging one-liners often made without the slightest provokation.

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Fat, sugar, salt, beer: the four essentials for a healthy diet.
Reply to
Steve Evans

sorry, roger, but I think Bob Myers is the only person here to have got it absolutely right.

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Fat, sugar, salt, beer: the four essentials for a healthy diet.
Reply to
Steve Evans

Even a double balanced mixer cannot completely remove the two frequencies we start with, only lower their level a lot.

In this case there is no need for that level of detail. The OP just wanted a simple way to tune in the air band on a common FM radio.

If he had asked for a very high quality communications receiver class solution there would have been a need to talk about double balanced mixers.

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Roger J.
Reply to
Roger Johansson

It's the "disparaging" and "without the slightest provocation" parts that I'd probably not see eye to eye with you, about. When I first wrote my question (and you can check google) on these phone indicators that I wanted to operate without a battery, his reply wasn't exactly sugar and honey -- but it was exactly the kind of information I didn't have and needed to hear about:

He made the job really hard for me to properly achieve. But then, learning to get the job done well and right, even for a hobbyist, isn't a bad thing. One grows from it and learns better why the standards *are* standards, too. In the end, I developed a circuit that presents 20M continuous (it doesn't present different momentary loads) to the phone line and still provides me with an LED indicator. Don's challenge to me was from someone who knew the material and I worked to meet it.

I think Don is brusk when people start asking about pirate broadcasting or else receiving broadcasts on bands where they aren't supposed to (for experimental purposes when you are licensed it is one thing, but for general tapping in it is another), when it looks even something like that's what the question is about. He pushes those considering such ideas to find another avenue, just as I'm certain he was pressing me to not improperly hook up equipment to my phone lines that was ignorant of the intended use and might otherwise interfere with it.

I can respect that.

In any case, he's helped at least one person I know of. Me.

Jon

Reply to
Jonathan Kirwan

thats fair enouugh, Johathan. You gotta speak as you find.

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Fat, sugar, salt, beer: the four essentials for a healthy diet.
Reply to
Steve Evans

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