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ic_engr Senior Member Offline Posts: 135	AM Modulation. Jan 15^th, 2010, 8:20am Hello, I am trying to simulate an effective GSM signal which has GMSK modulation. What I need to set-up is 1kH AM with 80% modulation on a 900MHz carrier. Can anyone tell me if I can set this up in Spectre/Cadence using the standard analogLib sources or ahdl models ? Will greatly appreciate any suggestions. ic_engr �
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pancho_hideboo

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Re: AM Modulation.
Reply #1 - Jan 15^th, 2010, 8:39am

ic_engr wrote on Jan 15^th, 2010, 8:20am:

I am trying to simulate an effective GSM signal which has GMSK modulation.
What I need to set-up is 1kH AM with 80% modulation on a 900MHz carrier.

Is there any relation to AM regarding GMSK ?
Do you mean a generation of GMSK by QAM ?

GMSK is no more than a FSK. But it has amplitude variations due to limited bandwidth.
http://en.wikipedia.org/wiki/GMSK#Gaussian_minimum-shift_keying
http://edocs.soco.agilent.com/display/ads2009/GMSK+Mod
http://edocs.soco.agilent.com/display/ads2009/PtRF+GSM+%28Pwr+Src%2C+RF+Carrier+...

If you use Cadence Spectre, use "rfLib/GSM_xmtr" and "rfLib/IQ_modulator".

If your need is a generation of simple AM signal truely, see the following.
http://www.designers-guide.org/Forum/YaBB.pl?num=1233850018

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« Last Edit: Jan 15^th, 2010, 9:43pm by pancho_hideboo »

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ic_engr

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Re: AM Modulation.
Reply #2 - Jan 20^th, 2010, 5:59am

I did use the GSM_xmt in rfLib in Cadence. However it does not let me control the frequqeuncy. Would you know how I can use it. The only parameter controllable is the 'seed' and the amplitude ?

Notice the GSM has a frequency of 900MHz or 2GHz. I would like to try different frequencies from 900MH to 2.5GHz in my simulation.

Noticing the 'seed' parameter makes me think this is a random number generator.

ic_engr

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pancho_hideboo Senior Fellow Offline Posts: 1424 Real Homeless	Re: AM Modulation. Reply #3 - Jan 20^th, 2010, 6:03am Answer my question. Is there any relation to AM regarding GMSK ? "rfLib/GSM_xmtr" is baseband I and Q signal generator. If you use these I and Q signals as RF signal, use "rfLib/IQ_modulator" with "rfLib/GSM_xmtr".
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ic_engr Senior Member Offline Posts: 135	Re: AM Modulation. Reply #4 - Jan 20^th, 2010, 10:51am I believe its a constant envelope, therefore, you are right no AM modulation relation exists for GMSK. ic_engr
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ic_engr Senior Member Offline Posts: 135	Re: AM Modulation. Reply #5 - Jan 22^nd, 2010, 5:34am The I&Q modulator still does not allow me to specify the modulation index. ic_engr
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pancho_hideboo

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Re: AM Modulation.
Reply #6 - Jan 22^nd, 2010, 5:41am

ic_engr wrote on Jan 22^nd, 2010, 5:34am:

The I&Q modulator still does not allow me to specify the modulation index.

It seems you can't understand MSK.
MSK is CPFSK of modulation_index=0.5 constant.

Study basic modulation theory.
http://en.wikipedia.org/wiki/Continuous-phase_frequency-shift_keying#Continuous-...

The following is MSK signal.
http://www.designers-guide.org/Forum/YaBB.pl?num=1195226387

If you would like to vary modulation index, there is an another option of using "ahdlLib/vco", although you can generate any RF modulated signal by using QAM(I/Q Modulator).

See http://edocs.soco.agilent.com/display/ads2009/MSK+Mod

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« Last Edit: Jan 22^nd, 2010, 6:52am by pancho_hideboo »

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