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# Discussion Groups | Comp.DSP | frequency domain multilexing

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# frequency domain multilexing - manishp - 2012-10-13 22:56:00

```Sirs,

I have some questions on frequency domain multiplexin. Let us say that I
have to carry multipe low frequency independent signals over a high
frequency carrier.
In this case, I can take one signal and then multiply this with high
frequency carrer then would I get required signal that can be transmited?

Second, if I do the same for multiple signals with same carrier signal then
how do I eventually create a single signal that contains each of these low
frequency signals. Basically, how do I multiplex these into a single
carrier? Is is just multiplication of signals.

Thanks, Manish
```
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# Re: frequency domain multilexing - Tim Wescott - 2012-10-14 11:36:00

```On Sat, 13 Oct 2012 21:56:44 -0500, manishp wrote:

> Sirs,
>
> I have some questions on frequency domain multiplexin. Let us say that I
> have to carry multipe low frequency independent signals over a high
> frequency carrier.
> In this case, I can take one signal and then multiply this with high
> frequency carrer then would I get required signal that can be
> transmited?
>
> Second, if I do the same for multiple signals with same carrier signal
> then how do I eventually create a single signal that contains each of
> these low frequency signals. Basically, how do I multiplex these into a
> single carrier? Is is just multiplication of signals.
>
> Thanks, Manish

You multiply each signal by a _different_ carrier.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
```
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# Re: frequency domain multilexing - glen herrmannsfeldt - 2012-10-14 19:05:00

```Tim Wescott <t...@seemywebsite.please> wrote:
> On Sat, 13 Oct 2012 21:56:44 -0500, manishp wrote:

(snip)

>> I have some questions on frequency domain multiplexin. Let us say that I
>> have to carry multipe low frequency independent signals over a high
>> frequency carrier.

(snip)

> You multiply each signal by a _different_ carrier.

> station.  Then your next favorite.

I was not so long ago reading "Titanic Tragedy" which, among others,
includes a chapter (or two) on the state of wireless telegraphy
in 1912.

Among others, it was before FDM. Only one could transmit at once,
so they learned to take turns. Also, different ships (or shore
stations) had different power levels, and so different
range that they could reach.

Detection by coherer ( http://en.wikipedia.org/wiki/Coherer )
and no vacuum tubes yet for amplification.

Amazing that anyone came to help at all!

-- glen

```
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# Re: frequency domain multilexing - Tim Wescott - 2012-10-14 22:57:00

```On Sun, 14 Oct 2012 23:05:54 +0000, glen herrmannsfeldt wrote:

>> On Sat, 13 Oct 2012 21:56:44 -0500, manishp wrote:
>
> (snip)
>
>>> I have some questions on frequency domain multiplexin. Let us say that
>>> I have to carry multipe low frequency independent signals over a high
>>> frequency carrier.
>
> (snip)
>
>> You multiply each signal by a _different_ carrier.
>
>> station.  Then your next favorite.
>
> I was not so long ago reading "Titanic Tragedy" which, among others,
> includes a chapter (or two) on the state of wireless telegraphy in 1912.
>
> Among others, it was before FDM. Only one could transmit at once, so
> they learned to take turns. Also, different ships (or shore stations)
> had different power levels, and so different range that they could
> reach.
>
> Detection by coherer ( http://en.wikipedia.org/wiki/Coherer )
> and no vacuum tubes yet for amplification.
>
> Amazing that anyone came to help at all!
>
> -- glen

I'm not sure if they had it in 1912, but the next step was to have as
much frequency selectivity as you got from putting it into your antenna.

Then tank coils, then -- sometime after WW1, I think, CW transmitters and
the notion of actually transmitting and receiving on frequency.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
```
______________________________

# Re: frequency domain multilexing - Jerry Avins - 2012-10-15 00:05:00

```On 10/14/2012 7:05 PM, glen herrmannsfeldt wrote:
>> On Sat, 13 Oct 2012 21:56:44 -0500, manishp wrote:
>
> (snip)
>
>>> I have some questions on frequency domain multiplexin. Let us say that I
>>> have to carry multipe low frequency independent signals over a high
>>> frequency carrier.
>
> (snip)
>
>> You multiply each signal by a _different_ carrier.
>
>> station.  Then your next favorite.
>
> I was not so long ago reading "Titanic Tragedy" which, among others,
> includes a chapter (or two) on the state of wireless telegraphy
> in 1912.
>
> Among others, it was before FDM. Only one could transmit at once,
> so they learned to take turns. Also, different ships (or shore
> stations) had different power levels, and so different
> range that they could reach.
>
> Detection by coherer ( http://en.wikipedia.org/wiki/Coherer )
> and no vacuum tubes yet for amplification.
>
> Amazing that anyone came to help at all!

I built a coherer detector when I was a kid, using silver and nickel
filings in a glass tube. A coherer is a bit like an SCR in that once
it's triggered, it stays on until it's shaken. We fastened it to the
clapper of a buzzer, so it would go open circuit as soon as the RF
stopped. Spark-gap transmitters were illegal by then, but so what?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
```
______________________________

# Re: frequency domain multilexing - Jerry Avins - 2012-10-15 00:19:00

```On 10/14/2012 7:05 PM, glen herrmannsfeldt wrote:
>> On Sat, 13 Oct 2012 21:56:44 -0500, manishp wrote:
>
> (snip)
>
>>> I have some questions on frequency domain multiplexin. Let us say
that I
>>> have to carry multipe low frequency independent signals over a high
>>> frequency carrier.
>
> (snip)
>
>> You multiply each signal by a _different_ carrier.
>
>> station.  Then your next favorite.
>
> I was not so long ago reading "Titanic Tragedy" which, among others,
> includes a chapter (or two) on the state of wireless telegraphy
> in 1912.
>
> Among others, it was before FDM. Only one could transmit at once,
> so they learned to take turns. Also, different ships (or shore
> stations) had different power levels, and so different
> range that they could reach.
>
> Detection by coherer ( http://en.wikipedia.org/wiki/Coherer )
> and no vacuum tubes yet for amplification.
>
> Amazing that anyone came to help at all!

I built a coherer detector when I was a kid, using silver and nickel
filings in a glass tube. A coherer is a bit like an SCR in that once
it's triggered, it stays on until it's shaken. We fastened it to the
clapper of a buzzer, so it would go open circuit as soon as the RF
stopped. Spark-gap transmitters were illegal by then.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
```
______________________________

# Re: frequency domain multilexing - glen herrmannsfeldt - 2012-10-15 03:11:00

```Jerry Avins <j...@ieee.org> wrote:

(snip)
>> Among others, it was before FDM. Only one could transmit at once,
>> so they learned to take turns. Also, different ships (or shore
>> stations) had different power levels, and so different
>> range that they could reach.

>> Detection by coherer ( http://en.wikipedia.org/wiki/Coherer )
>> and no vacuum tubes yet for amplification.

>> Amazing that anyone came to help at all!

> I built a coherer detector when I was a kid, using silver and nickel
> filings in a glass tube. A coherer is a bit like an SCR in that once
> it's triggered, it stays on until it's shaken. We fastened it to the
> clapper of a buzzer, so it would go open circuit as soon as the RF
> stopped. Spark-gap transmitters were illegal by then, but so what?

As well as I remember it from the book (already returned to
the library) some ships used a decoherer that rotated the tube
continuously. That one other ship might have gotten the message,
but the (not usual operator) didn't know to turn on the
decoherer, and so didn't hear anything.

-- glen
```
______________________________

# Re: frequency domain multilexing - manishp - 2012-10-15 08:39:00

```>You multiply each signal by a _different_ carrier.
>

Let me know if my question was confusing. I am re-stating again below:

I have 10 independent signals at base band. The first step would be to
convert each of these into carrier frequency by frequency modulation.

But having now got multiple signals each containing individual signals at
carrier frequency, how can this be converted into single signal for
transmission. Or in FDM, signals are transmitted as independent signals?

Again, my fundamental doubt is how to combine multiple signals. What is the
mathematical process to achieve this from a designer's perspective.

Thanks, Manish

```
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# Re: frequency domain multilexing - 2012-10-15 08:45:00

```You can also combine the signals at the "baseband", and then do just one
modulation with desired carrier frequency.

- Kalvin
```
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# Re: frequency domain multilexing - DougB - 2012-10-15 09:01:00

```>>You multiply each signal by a _different_ carrier.
>>
>
>Let me know if my question was confusing. I am re-stating again below:
>
>I have 10 independent signals at base band. The first step would be to
>convert each of these into carrier frequency by frequency modulation.
>
>But having now got multiple signals each containing individual signals at
>carrier frequency, how can this be converted into single signal for
>transmission. Or in FDM, signals are transmitted as independent signals?
>
>Again, my fundamental doubt is how to combine multiple signals. What is
the
>mathematical process to achieve this from a designer's perspective.
>
>Thanks, Manish
>

You should use a polyphase channelizer to take baseband signals and create
an FDM stack, then upconvert the stack to your carrier frequency.

-Doug
>
```
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