divider

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nxing

Joined: 10 May 2004
Posts: 446
Helped: 13
Location: China

11 Jul 2007 2:19 divider

Hi everybody,

I have a question regarding the divider: say I have a divider working at 2GHz, for some reason, there is another signal is added to this 2GHz, (say 2.1GHz), I am wondering whether or not this divider will divide this 2.1GHz? or can I see a signal of 1.05GHz at the output? what's the reason for that?

Thanks

nxing

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biff44

Joined: 24 Dec 2004
Posts: 1835
Helped: 244
Location: New England, USA

11 Jul 2007 6:27 Re: divider

It has been a while since I did the experiment, but as I recall the divider will lock up only on the signal that is higher power, even by a few dB, and ignore the other signal except to treat it as a high frequency phase modulation on the signal it locked up on.

nxing

Joined: 10 May 2004
Posts: 446
Helped: 13
Location: China

11 Jul 2007 17:57 Re: divider

Hi biff44,

First of all, thanks for your reply and I think you are right. That's what I got from my experiment. However, what I can not understand is that if we have two signal being added up in time domain, will the zero-crossing be changed so that the divider will work at other frequency (somewhere between the two frequency)?

Thanks

nxing

biff44

Joined: 24 Dec 2004
Posts: 1835
Helped: 244
Location: New England, USA

12 Jul 2007 19:37 Re: divider

It does seem to defy logic, but my experience seems to say no. The divider locks up onto the bigger signal and ignores the smaller one. Obviously, if you were designing a system and had a choice, you would want the desired signal to be tens of db above the spurious input--just to make sure.

psmon

Joined: 04 Jun 2007
Posts: 173
Helped: 22

15 Jul 2007 0:23 divider

You should see both 1 and 1.05 GHz signals from the spectrum analyzer. How strong are the signals depend on the source and the sensitivity of the divider...

echo47

Joined: 07 Apr 2002
Posts: 4206
Helped: 566

15 Jul 2007 2:14 Re: divider

Here's a MATLAB simulation. Add two sinewaves, 2.0 GHz at 0dB and 2.1 GHz at -3dB, and pass it through a zero-crossing detector (no divide-by-2). The vertical axes are linear.
If I decrease the amplitude of the 2.1 GHz sinewave, the square wave's spectrum cleans up more rapidly.

Code:

clear;
N = 32000; % number of points
fs = 400.0; % sample rate
f1 = 2.0; % signal 1 frequency
f2 = 2.1; % signal 2 frequency
a1 = 1.0; % signal 1 amplitude
a2 = 0.7071; % signal 2 amplitude
t = (0 : N-1) / fs;
%
y1 = a1 * sin(2 * pi * f1 * t) + a2 * sin(2 * pi * f2 * t);
subplot(4,1,1); plot(t,y1); xlabel('ns'); xlim([0 20]);
h1 = fftshift(fft(y1));
freq = fs * (-N/2 : N/2-1) / N;
subplot(4,1,2); plot(freq, 2/N*abs(h1)); xlabel('GHz'); xlim([0 5]);
%
y2 = sign(y1);
subplot(4,1,3); plot(t,y2); xlabel('ns'); xlim([0 20]); ylim([-1.5 1.5]);
h2 = fftshift(fft(y2));
subplot(4,1,4); plot(freq, 2/N*abs(h2)); xlabel('GHz'); xlim([0 5]);

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