Downmix of AM　signal

Hi All:
  I am studying RFID DSP, the reader's carrier frequency(Fc) is 13.56Mhz.
and the tag use load modulation to modulate the carrier with sub-carrier,
which is Fs = Fc/16. The sub-carrier is OOK modulated with symbol.
  Could this processe be written as:

S(t) = sin(2*pi*Fc*t) * sin(2*pi*Fs*t) * m(t)  (m(t) is the symbol pulse)

what I don't understand is that the demodulator use a quadrature down
mixer
to down convert the 13.56Mhz incoming signal to form an I/Q part.

Is this OOK modulation a special form of AM? if so, what's the benefit of
using quadrature mixer for down conversion? 
Would it be enough if we just multiply the incoming 13.56Mhz single with a
real signal of same frequency to get it down converted? 

Thanks
chris

On 05/18/2011 09:34 PM, ls00722 wrote:
> Hi All:
>    I am studying RFID DSP, the reader's carrier frequency(Fc) is 13.56Mhz.
> and the tag use load modulation to modulate the carrier with sub-carrier,
> which is Fs = Fc/16. The sub-carrier is OOK modulated with symbol.
>    Could this processe be written as:
>
> S(t) = sin(2*pi*Fc*t) * sin(2*pi*Fs*t) * m(t)  (m(t) is the symbol pulse)
>
> what I don't understand is that the demodulator use a quadrature down
> mixer
> to down convert the 13.56Mhz incoming signal to form an I/Q part.
>
> Is this OOK modulation a special form of AM? if so, what's the benefit of
> using quadrature mixer for down conversion?
> Would it be enough if we just multiply the incoming 13.56Mhz single with a
> real signal of same frequency to get it down converted?

What Jerry said, and, in order, Yes, and No.

Yes, OOK modulation is a special form of AM.

No, you can't just multiply the incoming signal with a carrier, because 
you don't know the amount of phase shift.  If the phase is shifted by 
plus or minus 90 degrees, your useful signal will vanish.  The closer 
your signal gets to 90 degrees shift, the worse things will be.  I/Q 
modulation corrects for that.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Thanks! That explained.
One more question, if I model this process using matlab.
Should the subcarrier modulated to 13.56Mhz carrier via adder? or
multiplier? The plot doesn't look correct if i use:
sin(2*pi*fc*t)*sin(2*pi*fsub*t).
I am expecting to see no subcarrier signal(if symbol is zero) , but carrier
signal always present.

The multiplication cause carrier signal disappears on the plot.

Thanks
>What Jerry said, and, in order, Yes, and No.
>
>Yes, OOK modulation is a special form of AM.
>
>No, you can't just multiply the incoming signal with a carrier, because 
>you don't know the amount of phase shift.  If the phase is shifted by 
>plus or minus 90 degrees, your useful signal will vanish.  The closer 
>your signal gets to 90 degrees shift, the worse things will be.  I/Q 
>modulation corrects for that.
>
>-- 
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>

On 20.5.11 1:01 , ls00722 wrote:
> Thanks! That explained.
> One more question, if I model this process using matlab.
> Should the subcarrier modulated to 13.56Mhz carrier via adder? or
> multiplier? The plot doesn't look correct if i use:
> sin(2*pi*fc*t)*sin(2*pi*fsub*t).
> I am expecting to see no subcarrier signal(if symbol is zero) , but carrier
> signal always present.
>
> The multiplication cause carrier signal disappears on the plot.

That is correct. You have created an AM signal with suppressed carrier,
also called a DSB signal.

Please follow the advice of Jerry Avins: Get a good book on modulation,
read and understand it.

An AM signal modulated with a sine wave is:

     f(t) = A * (1 + M * cos(2*pi*fm*t)) * cos(2*pi*fc*t),

where:

   A  = signal amplitude
   M  = modulation depth
   fc = carrier frequency
   fm = modulating signal frequency

-- 

Tauno Voipio