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VCO requirement in the GPS system?

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wccheng

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Dear all,

Could anyone give me an information about the VCO requirement in the GPS system? I means what is the phase noise or required output power do they need usually?

Thanks

wccheng
 

vfone

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Because GPS is a spread spectrum system, phase noise is not a critical parameter, and even a value around 80–85 dBc/Hz at 100 kHz offset is still acceptable.

Normal values for LO phase noise of the GPS receivers on the market are better than –95dBc/Hz @ 100kHz.

Output level of the LO is dictated by the mixer type, and in this case could be somewhere between –10dBm and 0dBm.
 

wccheng

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very thanks for your reply.
then how about the frequency band of the VCO need to vary?
 

Ghost Tweaker

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GPS is spread spectrum modulation (there's no bands, just one common band with all the signals on top of each other) so you just design for a fixed frequency which ensures downconversion of the GPS signal to the IF you have chosen.
 

jcpu

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Dear Sir:

Allow me to add one question:

Seems to me frequency resoluation (accuracy) of
carrier and code tracking loops critical to GPS receiver sensitivity.

Does LO (VCO+PLL) close-in phase noise affecting accuracy of
this carrier and code tracking loops?

Thanks.
 

pk3316

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it affects, but not much
 

jcpu

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one more question Sir:

For the reference clock source of RF PLL in GPS receiver,
do we need TCXO?
 

Wilson_yu_chen

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Hi Jcpu,

GPS need both TCXO (25M or 16M) for RF section as well as RTC (32K).

Hi PK:

Could u please explain us more how much degrade with poor VCO PN in terms
of quantity of PN and C/I ...

Thanks !!
 

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