VCO requirement in the GPS system?

[wccheng]

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]

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]

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

 

[Ghost Tweaker]

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]

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]

it affects, but not much

 

[jcpu]

one more question Sir:

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

 

[Wilson_yu_chen]

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 !!