Re: AM/FM on one carrier
- From: Rich Grise <richgrise@xxxxxxxxxxx>
- Date: Tue, 14 Jun 2005 21:47:00 GMT
On Tue, 14 Jun 2005 15:43:18 -0500, John Fields wrote:
> On Tue, 14 Jun 2005 11:27:31 -0700, Don Bowey <dbowey@xxxxxxxxxxx>
>
>>Getting to the bottom line, if you use a selective frequency voltmeter to
>>observe an unmodulated carrier, and then you (correctly) AM modulate the
>>carrier, you will observe that the carrier amplitude does not change.
>
> You'll notice that the _average_ carrier amplitude doesn't change, but
> if you look at it with something a little more fine-grained, like a
> scope, you'll notice that it does, I believe.
>
>>And the modulation power is all in the sidebands.
>
> OK, but if the carrier never moves how do you suppose they got there?
They got injected through the miracle of modulation. :-)
Look at your standard class A, B, or C plate-modulated transmitter.
You've got the carrier going in through the grid and coming out the
plate going into the tank circuit.
In series with B+, you interpose a source of the modulation signal,
like the secondary of the modulation transformer.
Now, it has been said that the instantaneous amplitude of the
carrier hits zero at the most negative excursion of the modulating
signal, at 100% modulation. Well, yeah, maybe, the instantaneous
value in the time domain, but _that does not make the fundamental
go away!_ What we are seeing is the instantaneous value of the
entire modulated wave, at which point the phase of the carrier
and the phase of what's on the sidebands cause a resultant of
zero - at that instant.
In fact, if the AM carrier goes away, why do they go to such great
lengths to suppress it for SSB or DSBSC? ;-)
Now, if you try to frequency-modulate it simultaneously, well, you'd
get the set of sidebands that that particular frequency/deviation
cause - the way this was explained to me is "It's simply the Fourier
transform." OK. ;-)
But demodulating them - well, Frank Raffaeli reports in another
branch, that he's done it with DSP:
----<quote>----
Here is my 0.02 - worth every penny ....
I thought this would work so I decided to try it in the lab:
1) Racal-Dana model 9087 AM/FM RF generator
2) "Ideal" demodulator - see below
The demodulator consists of an I.F. AGC amplifier feeding a Linear tech
14-bit ADC running at 50.4 Ms/S, feeding an FPGA computing amplitude
(sqrt(I^2+Q^2)) and arctangent. Arctangent is differentiated to give FM
- without pre-emphasis or de-emphasis.
fc = 10.8 MHz, Rx Analog BW = 500 kHz, digital filtering to 22 kHz
Modulation: 1kHz sine AM, 1.5 kHz sine, FM. FM = 5kHz deviation, AM =
90% modulation.
Result: at 10 mV carrier input, the demodulated signal looks ok in the
time domain, both FM and AM. When I decrease the signal level to 40uV
(-75 dBm), I see a little "fuzz" on the FM waveform that seems
synchronous with the AM modulating waveform negative peaks.
Now, a question for the OP: Why? there are experts here (somewhere) who
can help you do this a better way (if it's not homework). Tell us more
about the application.
Frank Raffaeli
http://www.aomwireless.com/
----<end quote>---
Cheers!
Rich
.
- References:
- AM/FM on one carrier
- From: pdrunen
- Re: AM/FM on one carrier
- From: John Fields
- Re: AM/FM on one carrier
- From: Don Bowey
- Re: AM/FM on one carrier
- From: Larry Brasfield
- Re: AM/FM on one carrier
- From: Don Bowey
- Re: AM/FM on one carrier
- From: John Fields
- Re: AM/FM on one carrier
- From: Don Bowey
- Re: AM/FM on one carrier
- From: John Fields
- AM/FM on one carrier
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