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Design >> RF Design >> Help regarding MSK Modem data sheet
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Message started by aaron_do on May 18^th, 2012, 5:49am

Title: Help regarding MSK Modem data sheet
Post by aaron_do on May 18^th, 2012, 5:49am

Hi all,

I'm having trouble understanding this datasheet for the cmx589a. It is a GMSK modem. The datasheet can be found on this webpage,

http://www.cmlmicro.com/products/wdata/CMX589A.htm

Anyway one thing I don't understand is that it seems to require the output from a frequency discriminator which - correct me if I'm wrong - means that the FSK signal is already converted to an amplitude modulated signal. So why is it called a GSMK modem if it can't detect GSMK?

Any kind of thoughts on this kind of thing would be helpful as its a little out of my comfort zone.

thanks,
Aaron

Title: Re: Help regarding MSK Modem data sheet
Post by ACWWong on May 18^th, 2012, 9:31pm

Hi Aaron,

The link needs a register/login to get the datasheet...

Anyway without looking at the datasheet, if the chip doesn't include the frequency discriminator, then all it must be doing is clock/data/timing recovery.... as to whether you call it a "modem" is upto the manufacturer. I'm sure you could find another GMSK "modem" which does include the FM discriminator.
Welcome to the world of "spec-manship" !

Cheers
Alan

Title: Re: Help regarding MSK Modem data sheet
Post by aaron_do on May 19^th, 2012, 8:52pm

Hi Alan,

thanks for the help. OK so I get your point, but I'm wondering, if it doesn't include the discriminator, then does that mean that the modem is actually just demodulating ASK? What distinguishes the signal coming out of the discriminator from ASK? I'm guessing it has something to do with the fact that its a Gaussian-MSK modem.

P.S I guess I better not post the datasheet itself.

thanks,
Aaron

Title: Re: Help regarding MSK Modem data sheet
Post by loose-electron on May 20^th, 2012, 10:11am

post the data sheet - they want you to register to get to the data so they can spam your email

Title: Re: Help regarding MSK Modem data sheet
Post by aaron_do on May 20^th, 2012, 6:03pm

I see. Here's the first half.

Aaron

Title: Re: Help regarding MSK Modem data sheet
Post by aaron_do on May 20^th, 2012, 6:05pm

And here's the second

Title: Re: Help regarding MSK Modem data sheet
Post by ACWWong on May 22^nd, 2012, 11:11am

aaron_do wrote on May 19^th, 2012, 8:52pm:

I wouldn't say demodulation of AM, as for FSK(MSK) you are looking +ve and -ve frequency excursions, and so at discrimator output +ve and -ve voltages. It says that in section 4.2. Its more of a threshold crossing detect.
GMSK just means gaussian data filter has been applied to MSK. And basically MSK is just a special case of FSK with index=0.5. So nothing too special about that with regards to the demode design... just BT=0.3 will mean SNR needed will be higher, in fact that is shown in fig5.
Anyway, this chip is quite old, can you even order it ? is there any particular reason you want to use it ?

Title: Re: Help regarding MSK Modem data sheet
Post by aaron_do on May 22^nd, 2012, 5:48pm

Hi Alan,

For MSK, there is a specific relationship between the frequency separation and the data-rate. But at the output of the discriminator, to my understanding, there will be no difference in the signal between an FSK input or an MSK input (except for the SNR as you pointed out). Does that sound right? Anyway thanks a lot for the help.

Quote:

I don't really follow you here. The datasheet says +ve and -ve voltages wrt a bias point. Isn't that the same as OOK or ASK? So if I had OOK with gaussian filtered data, and I fed it into this receiver, could I expect it to work? Assume I level the signal properly.

Quote:

Anyway, this chip is quite old, can you even order it ? is there any particular reason you want to use it ?

Its a long story but basically some students got their hands on it to do a project, some questions found their way to me, and I said there were some things that the people on this forum would know better.

best regards,
Aaron